Retroperitoneum

Abdominal Retroperitoneal Anatomy

The abdominal retroperitoneum is bounded anteriorly by the posterior parietal peritoneum, posteriorly by the transversalis fascia, and extends craniocaudally from the diaphragm to the pelvic brim. It is divided by fascial planes into three compartments: the anterior pararenal space, the perirenal (or perinephric) space, and the posterior pararenal space ( Figs. 55-1 to 55-4 ).

The anterior pararenal space contains the retroperitoneal portions of the colon, duodenum, and pancreas, is continuous with the transverse mesocolon and root of the small bowel mesentery, and is confined by the posterior parietal peritoneum anteriorly, the anterior renal fascia posteriorly, and the lateroconal fascia laterally (see Fig. 55-4 ). The junction of the anterior renal fascia, posterior renal fascia, and lateroconal fascia is most often located lateral to the kidney and tends to be located more posteriorly and medially at the craniocaudal level inferior to the renal hilum. Although the anterior pararenal space is potentially continuous across the midline, fluid collections are generally confined to their side of origin without crossing the midline, with exception of pancreatic fluid collections related to the presence of pancreatic enzymes.

The perirenal space contains the kidney, renal vessels, adrenal gland, renal pelvis, proximal ureter, perirenal lymphatics, and perirenal fat and is confined by the anterior renal fascia and posterior renal fascia that comprise the renal fascia (also called Gerota's fascia ). It has the shape of a cone superiorly and an inverted cone inferiorly. Medially the anterior renal fascia blends into the dense connective tissue surrounding the great vessels in the root of the mesentery posterior to the pancreas and duodenum (which may leave a potential communication between the perirenal space and the central prevertebral space), and the posterior renal fascia fuses with the psoas or quadratus lumborum fascia. Laterally the anterior and posterior renal fascia fuse posterior to the ascending or descending colon to form the lateroconal fascia, which continues anteriorly around the flank to merge with the peritoneal reflection to form the paracolic gutter. Superiorly the posterior renal fascia fuses with the posterolateral hemidiaphragm, and the right anterior renal fascia merges with the right inferior coronary ligament. The anterior renal fascia also extends cranially to the diaphragm but may be deficient superiorly on the right side posterior to the bare area of the liver. As a result, lacerations of the liver involving the hepatic capsule adjacent to this bare area may be associated with hemorrhage that extends posteriorly into the right perirenal space. Furthermore, perirenal fluid collections may extend from the right perirenal space anteriorly to the bare area of the liver ( Fig. 55-5 ). Similarly the splenorenal ligament may serve as a conduit for hemorrhage from the bare area of the spleen to the left anterior pararenal space in the setting of splenic trauma.

The perirenal space is contiguous superiorly with the mediastinum through splanchnic foramina of the diaphragmatic crura and through small transdiaphragmatic perforations and lymphatic vessels, providing conduits of potential disease spread between the thorax and abdomen. The perirenal space is further divided into multiple compartments by fibrous lamellae, the bridging septa, which are of three types: those that arise from the renal capsule and extend to the renal fascia, those that are attached only to the renal capsule and are arranged more or less parallel to the renal surface, and those that connect the anterior renal fascia to the posterior renal fascia. These septa can exert an important influence on pathologic processes by either limiting the spread of disease or by serving as bidirectional conduits for spread of disease between the renal fascia, perirenal space, and central prevertebral space ( Fig. 55-6 ; also see Fig. 55-2 ).

The posterior renal fascia can further be separated into two layers: the anterior and posterior laminae. The anterior lamina extends anteriorly and is continuous with the anterior renal fascia, the posterior lamina extends anterolaterally and is continuous with the lateroconal fascia, and multiple septa extend between the lateroconal and renal fascia. As such, a potential space contiguous with the anterior pararenal space exists between these two laminae of the posterior renal fascia that can accumulate fluid collections, especially when related to pancreatitis.

The posterior pararenal space almost always contains only fat, is confined by the posterior renal fascia anteriorly, by the transversalis fascia posteriorly, by the psoas muscle medially, and continues laterally external to the lateroconal fascia as the properitoneal fat of the abdominal wall. Inferiorly the posterior pararenal space is open to the pelvis, and superiorly it continues as a thin subdiaphragmatic layer of extraperitoneal fat.

The retroperitoneal fascia are not composed of single membranes but are laminated owing to variable embryonic fusion of discrete layers of dorsal mesenteries. As a result, whereas most slowly accumulating fluid collections are confined to the retroperitoneal compartment in which they originate, these potentially expansile fascial planes may serve as routes of decompression for rapidly accumulating retroperitoneal fluid or gas collections, or as conduits for the spread of inflammatory or neoplastic disease to other abdominal or pelvic compartments ( Figs. 55-7 and 55-8 ; also see Figs. 55-1 and 55-6 ). In addition, retroperitoneal fascial layers may not be intact in all patients owing to congenital variation, traumatic or iatrogenic disruption, or dissolution due to infected fluid or enzymatic digestion, allowing for fluid collections and other pathologies to involve multiple retroperitoneal or peritoneal compartments. On CT and MRI, when a fluid collection abuts a fascial plane, the margin tends to be linear or curvilinear and sharply defined, whereas the interface between fluid and retroperitoneal fat usually has an uneven and feathery or spiculated appearance.

The anterior interfascial retromesenteric plane is a laminated potentially expansile interfascial plane between the anterior pararenal space and the perirenal spaces. It is continuous across the midline and communicates with two other potentially expansile interfascial planes, the posterior interfascial retrorenal plane and the lateroconal plane, at the fascial trifurcation, providing potential routes for disease spread. At a level inferior to the origin of the superior mesenteric artery, retroperitoneal fluid collections can occasionally cross the midline anterior to the abdominal aorta and inferior vena cava (IVC) within the anterior interfascial retromesenteric plane. The posterior interfascial retrorenal plane is a complex potential compartment located between the perirenal space and the posterior pararenal space. Posterior interfascial fluid, peritoneal fluid, or posteriorly dissecting fluid in the anterior pararenal space may reside within this plane. The lateroconal interfascial plane is the potentially expansile space between layers of the lateroconal fascia.

The normal thickness of the retroperitoneal fascial planes is 1 to 3 mm. On CT and MRI, retroperitoneal fascial planes are more frequently detected when there is an abundance of retroperitoneal fat. In general the posterior renal fascia is more often seen than the anterior renal fascia, and the anterior renal fascia is more commonly seen on the left side than on the right. Fascia that is focally thickened or greater than 3 mm in width is considered abnormal and can be caused by a large variety of pathologic conditions involving the retroperitoneal organs or nonparenchymal retroperitoneum.

The anterior and posterior renal fascia fuse inferiorly in the iliac fossa to form a single multilaminar fascia called the combined interfascial plane. The combined interfascial plane continues in the pelvis along the anterolateral margins of the psoas muscles, contiguous with the pelvic extraperitoneal perivesical and presacral spaces. There is controversy in the literature as to whether the inferior aspect of the perirenal space communicates openly with the pelvis; it is uncommon for perirenal fluid collections to extend into the pelvis or vice versa. However, the combined interfascial plane can serve as a conduit for disease extension from the abdominal retroperitoneum into the pelvis (see Figs. 55-4, 55-6, and 55-7 ). Fluid can also extend posteriorly from the posterior interfascial retrorenal plane through a cleft at the lumbar triangle between the medial aspect of the posterior pararenal space and the lateral aspect of the quadratus lumborum fat pad to the transversalis fascia, which accounts for the occurrence of flank discoloration (Grey Turner's sign) in patients with acute pancreatitis. Furthermore, fluid within the posterior interfascial retrorenal plane can extend posteriorly through a narrow communicating passageway into the subfascial plane between the posterior pararenal space anteriorly and the transversalis fascia posteriorly at a level separate from that of the lumbar triangles, also potentially leading to Grey Turner's sign (see Figs. 55-6 and 55-7 ).

At the level of the iliac crest, inferior to the inverted cone of the renal fascia, the anterior and posterior pararenal spaces may communicate where the lateroconal fascia disappears as a distinct boundary. As a result, there is communication of these abdominal retroperitoneal spaces with the extraperitoneal pelvic spaces inferiorly and the properitoneal fat anterolaterally, serving as a multidirectional conduit for the spread of retroperitoneal pathology. In patients with large amounts of fluid in the infrarenal retroperitoneal space, extension into the pelvic extraperitoneal space is frequent. The most common pathways are posterior extension medial to the iliac vessels and/or a more medial extension into the prevesical space. Posterior extension lateral to the iliac vessels is less common.

Pelvic Extraperitoneal Anatomy

The pelvic extraperitoneal connective tissue is organized into groups of fascia that are fused together to demarcate the pelvic extraperitoneal spaces ( Fig. 55-9 ). The parietal pelvic fascia (PPF) is the variable dense fascial system that covers the structures limiting the pelvic cavity, including the levator ani, obturator, coccygeus, and piriformis muscles, the anterior surfaces of the sacrum and coccyx, and structures contiguous to the pelvic walls such as internal iliac vessels and sacral roots.

The visceral pelvic fascia (VPF) is the fascial system derived from the visceral reflection of the PPF. It envelops the pelvic organs and attaches them to the pelvic walls. The VPF is composed of two symmetric layers oriented in the sagittal plane and courses along the lateral borders of the pelvic organs and medial borders of the vessels, thinning as it extends anteriorly. On CT and MRI it is best seen posteriorly where it is the thickest; the appearance is that of thin continuous lines enveloping the perirectal fat ( Fig. 55-10 ). An additional group of fascia is oriented in the coronal plane that originates from the genital artery sheaths and from the inferolateral extensions of the peritoneal layer of the cul-de-sac. The anterior layer separates the urinary bladder and genital tract, and the posterior layer separates the genital tract from the rectum. These fascial planes are not well seen on CT or MRI owing to a paucity of surrounding fat.

The extraserosal pelvic fascia is the variably dense connective tissue between the PPF and VPF in which the pelvic organs are embedded, which provides support to the vascular and neural structures of the pelvis and permits independent physiologic volume changes of the pelvic organs. The umbilicovesical fascia (UVF) is located anterior to the parietal peritoneum and posterior to the transversalis fascia, has a triangular configuration with its apex at the umbilicus, courses around the anterior aspect of the urachus and urinary bladder, and inferiorly blends with the pubovesical ligament medially and the VPF and PPF laterally ( Fig. 55-10 ; also see Fig. 55-9 ). Its superior edges are occupied by the obliterated umbilical arteries that extend anteriorly from the internal iliac arteries, are covered by parietal peritoneum, and are visible as thin linear structures on CT and MRI in the location of the medial umbilical folds (see Fig. 55-10 ). The obliterated urachus is also often visible in the axial plane in the location of the median umbilical fold. Inferior to the peritoneal reflection the UVF extends to the pelvic floor on each side of the urinary bladder but is not well visualized on CT or MRI.

The extraperitoneal fat of the pelvis is divided by the UVF into the prevesical space and the perivesical space. The prevesical space lies anterior and lateral to the UVF, medial to the thin anterior portion of the VPF and posterior to the pubic bones where it is also called the retropubic space or space of Retzius (see Fig. 55-9 ). It is a large potential compartment that has multiple potential extensions and communications, allowing for bidirectional spread of fluid collections or other pathologies. It has posterior extensions on the lateral aspects of the urinary bladder and communicates with the presacral space. Superiorly it becomes thinner as it approaches the umbilicus and communicates with the abdominal retroperitoneal spaces; it is continuous with the rectus sheath (inferior to the semilunar line) and femoral sheaths and has extensions along the round ligaments, vasa deferentia, and spermatic cords. It may also communicate with the perivesical and perirectal spaces.

Prevesical fluid collections typically have a “molar tooth” configuration in the axial plane on CT and MRI because of the orientation of the UVF ( Fig. 55-11 ; also see Fig. 55-9 ). The “crown” of the tooth lies anterior to the urinary bladder between the UVF and transversalis fascia of the anterior abdominal wall and displaces the urinary bladder posteriorly. The “roots” extend posteriorly between the UVF and either the peritoneum superiorly or the PPF inferiorly, displacing the urinary bladder medially if the roots are symmetric or laterally if the roots are asymmetric in size. As much as 2500 mL of fluid may occupy the prevesical space and its extensions without evidence of a palpable abdominal wall mass. Rectus sheath hematomas inferior to the arcuate line can also extend inferiorly into the prevesical space to displace and compress the pelvic viscera, and postpuncture hematomas in the femoral sheath can also expand rapidly to fill the prevesical space and subsequently spread to any communicating compartments.

The perivesical space is a thin space bounded by the UVF and contains the urinary bladder, urachus, obliterated umbilical arteries, and a little fat (see Fig. 55-9 ). It is continuous with the seminal vesicles and prostate gland in males, and the supravaginal portion of the cervix and lower uterine segment in females. Perivesical fluid collections are typically small owing to compression by the UVF and may occasionally be confused with thickening of the bladder wall on CT or with fluid collections in the cul-de-sac when localized posteriorly in the perivesical space; in general, they are better characterized by MRI.

The perirectal space, which contains the rectum, hemorrhoidal vessels, and fat, is limited by the posterior portion of the VPF, continues superiorly into the abdominal extraperitoneal space, and is continuous with the sigmoid mesocolon ( Fig. 55-12 ). The Denonvilliers' fascia comprises the anterior portion of the perirectal fascia and the posterior layer of the prostatic fascia.

Liposarcoma

Liposarcoma is the most common retroperitoneal sarcoma, accounting for approximately 40% of such cases, and is the second most common adult soft tissue sarcoma. UPS is the most common adult soft tissue sarcoma, accounting for 14% to 18% of all soft tissue sarcomas. Most liposarcomas occur in deep soft tissue, in contrast to lipomas, which occur more commonly in superficial soft tissue.

Liposarcomas originate from primitive mesenchymal cells and not from adipocytes. The World Health Organization (WHO) divides liposarcomas into four subtypes: well-differentiated, myxoid, dedifferentiated, and pleomorphic. Liposarcomas can be conceptually divided into three subgroups. Well-differentiated and dedifferentiated liposarcomas comprise one subgroup, since over time a subset of well-differentiated liposarcomas histologically can progress to dedifferentiated sarcomas, which have metastatic potential. A second subgroup is composed of the myxoid liposarcomas, with the continuum of lesions ranging from pure myxoid liposarcoma at one extreme to the more poorly differentiated round cell liposarcoma at the other. Finally, a third subgroup exhibits unusual features or combined patterns not accounted for in the other classifications.

Retroperitoneal liposarcoma has a mean diameter of 20 cm and is associated with local recurrence, which is responsible for most patient morbidity and mortality. The rate of distant metastasis to both the liver and lung is less than 10%. In comparison, patients with nonliposarcoma retroperitoneal tumors have a fourfold higher risk of metastases compared with liposarcoma. In patients with unresectable retroperitoneal liposarcoma, particularly primary disease, incomplete or partial resection or debulking can provide improvement in survival and palliate symptoms, in contrast to patients with other types of retroperitoneal sarcoma. Partial resection increased survival as compared with exploration or biopsy alone (median survival, 26 months vs. 4 months), and successful palliation of symptoms was achieved in 75% with preoperative symptoms.

Well-Differentiated Liposarcoma.

Well-differentiated liposarcoma is the most common subtype of retroperitoneal liposarcoma, reaching a peak incidence during the sixth and seventh decades of life, with men and women affected equally. Together the well-differentiated and dedifferentiated subtypes account for approximately 35% to 40% of liposarcomas. Grossly, well-differentiated liposarcomas are large multilobular lesions that could be mistaken for lipomas except for their extremely large size and their tendency to have more fibrous bands, gelatinous zones, or punctate hemorrhage. On CT, well-differentiated liposarcomas contain components that are similar in attenuation to macroscopic fat ( Fig. 55-13 ). On T1- and T2-weighted images, well-differentiated liposarcomas are usually isointense to subcutaneous fat, with loss of SI on fat-suppressed imaging sequences. However, subtle differences in the attenuation or SI of normal abdominopelvic fat compared to areas of fat-containing liposarcoma may be seen, and in patients who have undergone surgical resection of liposarcoma, such findings must be viewed with suspicion for possible tumor recurrence. Streaky zones of fibrous or sclerotic components within a well-differentiated liposarcoma show SI similar to that of skeletal muscle ( Fig. 55-14 ; also see Fig. 55-13 ). Well-differentiated liposarcoma demonstrates little or no contrast enhancement, which reflects the paucity of intratumoral vessels. However, thickened irregular septa and minor nodular components within lipoma-like components may enhance, and sclerosing components may also enhance homogeneously. Areas of necrosis and calcification tend to be uncommon in all liposarcomas. Imaging features that favor well-differentiated liposarcoma over lipoma include large lesion size (>10 cm), presence of thick septa (>2 mm), nodular and/or globular areas, nonadipose masslike areas, and a decreased percentage of fat composition (<75% fat in mass). The presence of thick septa and associated nonadipose masslike areas increase the likelihood of a well-differentiated liposarcoma over a lipoma by 9- and 32-fold, respectively. Large exophytic renal angiomyolipomas may mimic the appearance of perirenal well-differentiated liposarcomas, but presence of a renal parenchymal defect, enlarged vessels within the lesion, and presence of other renal angiomyolipomas favor an angiomyolipoma.

Well-differentiated liposarcomas are for the most part nonmetastasizing lesions that are of low-grade histology, but their rate of local recurrence in the retroperitoneum approaches 100%. With local recurrence, cachexia and intestinal obstruction are common. Approximately 10% of well-differentiated retroperitoneal liposarcomas can dedifferentiate after an average of 7 to 8 years.

Dedifferentiated Liposarcoma.

Dedifferentiated liposarcomas develop in approximately the same age group as well-differentiated liposarcomas, reaching a peak during the early seventh decade of life, with men and women affected equally. Histologically the lesions have areas of well-differentiated liposarcoma and a nonlipogenic (dedifferentiated) component that has an appearance of a high-grade fibrosarcoma or UPS. On CT and MRI, dedifferentiated liposarcomas have areas with attenuation and SI characteristic of fat-containing well-differentiated liposarcoma, but they have more masslike areas of nonfatty tissue with higher attenuation, as well as lower T1 and higher T2 SI ( Fig. 55-15 ). Calcification or ossification may be seen in up to 30% of cases. The biological behavior of dedifferentiated liposarcomas is similar to that of other pleomorphic high-grade sarcomas in adults, with a 5-year survival rate of less than 25%. Up to 41% of patients may experience local recurrences, and 17% may experience metastases, with a 5-year survival rate as low as 16%. Overall, dedifferentiated retroperitoneal liposarcomas have the worst prognosis.

Myxoid Liposarcoma.

Myxoid liposarcoma is the second most common subtype of retroperitoneal sarcoma. Unlike well-differentiated liposarcomas, myxoid liposarcoma occurs in a younger age group, with a peak incidence during the fifth decade of life. The myxoid subtype has an intermediate prognosis between those of the well-differentiated and dedifferentiated subtypes. Histologically, myxoid liposarcomas are composed of a myxoid matrix containing mucopolysaccharides with small amounts of mature fat. Frequently the extracellular mucoid material forms large pools and often shows a lacelike pattern. Grossly the lesion is frequently lobulated, may be multiloculated or oval, and may be accompanied by surrounding soft tissue edema.

A spectrum of features on CT and MRI may be present in myxoid liposarcomas, related to fat content, amount of myxoid material, degree of cellularity and vascularity, and presence of necrosis. On CT and MRI the myxoid dominant component of these tumors shows low attenuation, low SI on T1, and very high SI relative to muscle on T2 ( Fig. 55-16 ). Foci of fat- and/or soft tissue–attenuation/low-SI septa may be revealed within the predominately myxoid containing mass, and calcifications may also be seen. A potential pitfall is that a fatty component in the margin of a mass may simulate extraperitoneal fat and be missed.

As many as 25% of myxoid liposarcomas may not exhibit foci of attenuation or SI typical of a fatty tumor, but may instead appear as completely cystic lesions on unenhanced images. The absence of foci with attenuation and SI similar to macroscopic fat makes a myxoid liposarcoma indistinguishable from most other soft tissue masses. In such cases the lesion can mimic a cyst, cystic degeneration, or necrosis on unenhanced CT (due to presence of water attenuation) as well as on T2-weighted images because of the presence of very high SI. However, the myxoid stroma will enhance to a variable degree depending on its degree of vascularity, and thus enhanced images can distinguish cystic and myxoid components of the neoplasm. Myxoid tissue may also be present within UPS, and in the absence of fat, the two lesions have a similar appearance.

Leiomyosarcoma

Leiomyosarcoma is the second most common retroperitoneal sarcoma (≈30% of retroperitoneal sarcomas) and accounts for about 8% of soft tissue sarcomas overall. Most leiomyosarcomas present in the fifth or sixth decade of life, with two thirds of all retroperitoneal leiomyosarcomas occurring in women. Approximately half of all soft tissue leiomyosarcomas develop in the retroperitoneum, making it the single most common soft tissue site. Retroperitoneal leiomyosarcomas are typically well circumscribed, have a mean diameter of 16 cm, and often contain intratumoral necrosis and hemorrhage. Histologically a typical pattern of interlacing sweeping bundles of spindle-shaped cells with elongated blunt-ended nuclei is seen. Initially, adjacent organs are displaced without direct invasion but eventually do become involved by direct extension. Two thirds of retroperitoneal leiomyosarcomas are located external to the lumen of the IVC, whereas approximately one third have both intraluminal and extraluminal components. Leiomyosarcoma is the most common intraluminal venous neoplasm and is the most common primary tumor of the IVC. The finding of a retroperitoneal mass that has both intraluminal and extraluminal components is very suggestive of a leiomyosarcoma.

Purely intraluminal caval leiomyosarcomas account for 5% of leiomyosarcomas, occur primarily in women (80%-90% of patients), and present at a younger age (mean age, 50 years). On imaging, intracaval leiomyosarcomas are seen as polypoid or nodular masses that are firmly attached to the vessel wall. They are smaller than those that are entirely extravascular, are less likely to show intratumoral hemorrhage and necrosis, and are most frequently located between the diaphragm and renal veins. Leiomyosarcomas with an intraluminal component are more likely to produce early symptoms than those that are completely extraluminal. Patients with upper segment IVC involvement may develop symptoms and signs of Budd-Chiari syndrome. Patients with middle segment IVC involvement may develop right upper quadrant pain and tenderness, frequently mimicking biliary tract disease, and extension into the renal veins may cause a variable degree of renal insufficiency. Lower extremity edema occurs when there is lower segment IVC involvement. Tumors that do not extend into or above the intrahepatic IVC are resectable, whereas those tumors that infiltrate the intrahepatic IVC, hepatic veins, right atrium, and beyond are often unresectable.

On CT and MRI, leiomyosarcomas usually have attenuation similar to muscle, being low to intermediate SI on T1 and heterogeneous intermediate to high SI on T2 ( Figs. 55-17 and 55-18 ). Central liquefactive necrosis is more common and extensive than in other sarcomas and is seen as foci with low attenuation, low T1 SI, and high T2 SI. Fat and calcification are not typically seen. Intratumoral hemorrhage typically appears as high attenuation and high T1-weighted SI relative to muscle. Smaller tumors may present as a solid nonnecrotic mass, and uncommonly, the tumor may present as a predominantly cystic mass with extensive necrosis. The enhancement of leiomyosarcomas is variable and depends on their muscular and fibrous components; it is usually delayed compared to enhancement of the surrounding skeletal muscles. Intraluminal bland thrombus and tumor thrombus can be differentiated with contrast-enhanced CT and contrast-enhanced T1-weighted imaging; findings that favor tumor thrombus include enlargement of the vascular lumen and enhancement of the thrombus (see Fig. 55-17 ).

As with other retroperitoneal sarcomas, complete surgical removal is the treatment of choice for retroperitoneal leiomyosarcoma and is the most important factor affecting patient survival. Retroperitoneal leiomyosarcomas have a poor prognosis, with a 5-year survival rate of approximately 15%. Many patients have unresectable disease at the time of presentation secondary to infiltration of the intrahepatic IVC, hepatic veins, or right atrium, or metastatic disease, which is present in 40% of patients. Patients with resectable IVC leiomyosarcoma have the best prognosis, with two thirds of patients surviving beyond 5 years. Local recurrence after resection is seen in 40% to 77% of patients.

Undifferentiated Pleomorphic Sarcoma

UPS, previously known as malignant fibrous histiocytoma, is the most common soft tissue sarcoma in adults, with most patients presenting between the sixth and eighth decades of life. Two thirds of these tumors occur in men. UPS accounts for 25% of all soft tissue sarcomas, but only 16% arise in the retroperitoneum. Nevertheless it is the third most common retroperitoneal sarcoma after liposarcoma and leiomyosarcoma.

On gross examination, retroperitoneal UPS appears as a solitary, multilobulated, large mass, often with hemorrhage and necrosis, sometimes with intratumoral calcification or ossification. Histologically there is typically a highly variable mixture of storiform and pleomorphic cellular elements.

On CT and MRI, UPS appears as a large relatively well-circumscribed mass that spreads along fascial planes and between muscle fibers, with attenuation similar to muscle, low to intermediate SI on T1-weighted images, and heterogeneously increased SI on T2-weighted images relative to muscle ( Fig. 55-19 ). Intratumoral fat is absent. Areas of low attenuation due to cystic degeneration or necrosis may be present. The “bowl of fruit” sign is a mosaic of mixed low, intermediate, and high SI on T2 that correlates to the presence of intratumoral solid components, cystic degeneration, hemorrhage, myxoid stroma, and fibrous tissue. Although the bowl of fruit sign is commonly revealed in UPS, it is not specific and has been described in other tumors such as synovial sarcoma and Ewing's sarcoma; these latter subtypes only very rarely occur in the retroperitoneum, however. Extensive intratumoral hemorrhage frequently occurs in UPS, but most masses of UPS have nonhemorrhagic solid components, and thus most tumors can be differentiated from a bland benign hematoma. Intratumoral calcifications may be seen in up to 20% on CT, often peripherally with a lumpy or ringlike configuration or sometimes more centrally with a speckled or amorphous configuration. Calcification is more difficult to detect prospectively on MRI.

Paraganglioma

Paragangliomas, sometimes called extraadrenal pheochromocytomas, are rare neurogenic tumors that arise from highly vascularized specialized neural crest cells called paraganglia that are symmetrically distributed along the aortic axis in close association with the sympathetic chain in the neck, chest, abdomen, and pelvis. The largest collection of paraganglia includes the paired organs of Zuckerkandl that overlie the aorta at the level of the inferior mesenteric artery and have an uncertain physiologic role. They are prominent during early infancy and regress after 12 to 18 months. From 10% to 20% of pheochromocytomas are extraadrenal in location, and most often arise in the retroperitoneum from the organs of Zuckerkandl. Only a few tumors develop at other locations along the aorta or its branch vessels.

On gross examination, paragangliomas are partially encapsulated solid and/or cystic brown masses that usually measure several centimeters in diameter and are commonly hemorrhagic. Histologically, paragangliomas are composed of small polygonal or slightly spindled cells with an amphophilic or eosinophilic cytoplasm, arranged in short irregular anastomosing sheets around a delicate vasculature. Well-defined cell nests (“Zellballen”) may also be seen, composed of chief cells with a peripheral thin layer of sustentacular cells separated by highly vascularized fibrous septa.

Patients with paragangliomas present in the fourth and fifth decades of life, although malignant paragangliomas may sometimes arise in younger patients. Men and women are affected equally. Paragangliomas may be multicentric, particularly if there is a family history of paraganglioma, or may be associated with other tumors such as gastric malignant gastrointestinal stromal tumors (GISTs) and pulmonary chondromas as a component of Carney's triad. Some 10% of paragangliomas are familial, occurring in conditions such as multiple endocrine neoplasia (MEN) types IIA and IIB, NF1, von Hippel-Lindau syndrome, and hereditary paraganglioma-pheochromocytoma syndrome (HPPS) related to genetic mutations in succinate dehydrogenase (SDH).

Up to 40% of paragangliomas are malignant, as compared to 10% of adrenal pheochromocytomas. Malignant nature is recognized by metastatic spread or locally aggressive behavior; approximately 10% of patients initially present with metastatic disease. The malignant potential and biological behavior of a paraganglioma cannot be determined from its histologic appearance. Paragangliomas may spread both via the lymphatics and hematogenously, and the most common sites of metastatic disease are lymph nodes, bone, lung, and liver. Occasionally, spontaneous rupture of a retroperitoneal paraganglioma may occur with associated life-threatening retroperitoneal hemorrhage.

Paragangliomas may be considered to be functional or nonfunctional depending on whether catecholamines are secreted or not. Nonfunctional abdominal paragangliomas present with nonspecific symptoms and signs and are rarely diagnosed as such prior to surgery. Paragangliomas may be functional in up to 60% of patients and may cause chronic or intermittent hypertension, headaches, or palpitations. In affected patients, detection of elevated urinary catecholamines is the most efficacious way of characterizing an abdominal mass as a paraganglioma. There is poor correlation between the functional activity of the tumor and the degree of malignancy, but functional paragangliomas are usually smaller than nonfunctional paragangliomas when detected (mean size, 7 cm vs. 12 cm, respectively).

On CT, enhancing, well-circumscribed, lobular or round soft tissue–attenuation masses are seen that may be homogeneous in attenuation when small or heterogeneous when large. Central areas of low attenuation may be due to central necrosis or cystic change, which may be seen in up to 40% of cases. Punctate calcification may be seen in up to 15% of cases, and focal areas of high attenuation due to acute hemorrhage may also be seen in some tumors. Anecdotal reports in the literature report the induction of hyperadrenergic symptoms or hypertensive crises in patients with paraganglioma or pheochromocytoma after IV administration of older ionic contrast agents for CT. However, more recent reports in the literature suggest that IV administration of nonionic contrast material for CT is safe in patients with paraganglioma and pheochromocytoma, and that α-blocking medication is unnecessary in conjunction with contrast administration.

On MRI, SI and enhancement characteristics of paragangliomas are similar to those of adrenal pheochromocytoma. The masses generally have low to intermediate SI on T1-weighted images, moderately high SI on T2 relative to skeletal muscle, and are commonly heterogeneous secondary to foci of intratumoral necrosis or hemorrhage ( Fig. 55-20 ). Low-SI intratumoral septa and a low-SI peripheral capsule are also commonly seen on T2-weighted images. Progressive enhancement is present on parenchymal/venous and delayed phases of enhancement, but arterial-phase enhancement is variable. The size, attenuation and SI characteristics, and degree of heterogeneity cannot be used to differentiate between benign and malignant paragangliomas, the only definitive imaging criterion of malignancy being the presence of metastatic disease. CT and MRI also cannot differentiate between functional and nonfunctional paragangliomas.

¹²³ I or ¹³¹ I-metaiodobenzylguanidine (MIBG) scintigraphy may be useful to detect clinically suspected tumor that cannot be localized, multiple primary tumors, tumors in unusual locations, or metastatic disease. The specificity of MIBG scintigraphy approaches 100% and is greater than that of CT and MRI, but its sensitivity is generally lower (range, ≈58% to 94%). Thus imaging with CT or MRI is recommended if there is a high index for suspicious for paraganglioma and the tumor is not found with scintigraphy. PET using other radiolabeled tracers such as ¹⁸ F-fluorodopamine (FDOPA) or ¹¹ C-hydroxyephedrine is currently being studied and is promising in improving the diagnosis, staging, and therapeutic monitoring of such tumors.

Complete surgical resection is the treatment of choice for abdominal paraganglioma; adjunctive therapies including radiotherapy, ¹³¹ I-MIBG, and chemotherapy are considered palliative. Surgical resection of localized metastatic disease may also be performed, especially if solitary. If an abdominal paraganglioma is suspected clinically, assessment of the functional activity should be performed prior to surgery. Patients with functional tumors are premedicated with α-blockers to avoid intraoperative hypertensive crises during surgical manipulation of the tumor. Overall the 5-year survival rate may approach 82% for those with abdominal paraganglioma. The limitations of histopathologic criteria in predicting malignant behavior, the long natural history of the disease, and a high propensity for subsequent metastasis make extended follow-up necessary.

Ganglioneuroma

Ganglioneuromas are uncommon benign neurogenic tumors that arise from sympathetic ganglia and represent 1% to 2 % of all primary retroperitoneal tumors, outnumbering neuroblastomas by about 3 to 1. They occur slightly more commonly in women than in men (1.5;1), most commonly in the first through fifth decades of life, with a mean age at diagnosis of 7 years. They are most often located in the posterior mediastinum (39%-43%), followed by the retroperitoneum (32%-52%), and the neck and pelvis (8%-9%). Although some ganglioneuromas result from maturation of neuroblastomas and ganglioneuroblastomas, the majority arise de novo.

Most patients with ganglioneuromas are asymptomatic and have normal levels of urinary catecholamines. When symptomatic, abdominal pain and a palpable abdominal mass are the most frequent symptoms and signs. Patients with hormonally active ganglioneuromas may clinically present with episodic hypertension, sweating, flushing, or diarrhea due to the excess catecholamine production. Secretion of androgenic hormones may lead to virilization with some tumors.

On gross inspection, ganglioneuromas are well-circumscribed encapsulated benign neoplasms that have a mean size of 8 cm. Histologically they are composed of uniform bundles of longitudinal and transversely oriented mature Schwann cells that crisscross each other in an irregular fashion, along with scattered mature ganglion cells, variable amounts of myxoid stroma and collagen, and rarely stromal fat. Extremely rarely, ganglioneuromas may undergo malignant transformation.

Retroperitoneal ganglioneuromas are typically well-defined longitudinally oriented masses that are lobulated or oval in shape, have a tendency to surround major blood vessels with little or no luminal narrowing, usually do not result in osseous changes, and only infrequently extend into the neural foramina. On CT and MRI, ganglioneuromas have relatively homogeneous attenuation similar to or lower than that of skeletal muscle, with low SI on T1-weighted images and heterogeneous intermediate to high SI on T2-weighted images ( Fig. 55-21 ). Intratumoral hemorrhage or fatty components may cause mixed intermediate to high SI on T1. SI on T2 is influenced by the proportion of myxoid stroma to cellular components and collagen fibers. Intermediate SI on T2-weighted images occurs when there is an abundance of cellular components and collagen fibers relative to myxoid stroma, whereas high SI on T2 occurs when there is a large amount of myxoid stroma with relatively few cellular components and collagen fibers. A whorled appearance may be present on T2-weighted images and sometimes on T1-weighted images and is due to interlacing bundles of Schwann cells and collagen fibers. In contrast to schwannomas, cystic degeneration is not present. On both T1 and T2 a low-SI peripheral rim may be visualized but is more often identified on contrast-enhanced images. Calcifications are seen in 42% to 60% of tumors on CT, tend to be discrete and punctuate, and appear as low-SI foci on T1- and T2-weighted images. In contrast the calcifications in neuroblastoma are amorphous and coarse in appearance.

After contrast administration, ganglioneuromas show gradual progressive enhancement, although linear septa may enhance early. The CT and MRI imaging characteristics of ganglioneuroma overlap with those of ganglioneuroblastoma and neuroblastoma, and distinguishing between these lesions is not possible unless metastatic disease is present. Approximately 57% of ganglioneuromas may be functional, produce increased amounts of catecholamines, and accumulate ¹²³ I-MIBG.

Management of retroperitoneal ganglioneuromas involves complete surgical resection when possible, especially when symptomatic due to large size or hormonal activity. After surgical resection, the prognosis is excellent and recurrence is rare, but periodic radiologic surveillance is advised.

Ganglioneuroblastoma and Neuroblastoma

Ganglioneuroblastoma most often develops in children aged 2 to 4 years. It is rare in adults and affects men and women equally. It occurs most often in the posterior mediastinum, retroperitoneum, and adrenal gland. Neuroblastoma most commonly occurs in the first decade of life, with 80% occurring in children younger than 5 years and 42% occurring in children younger than 1 year. It is slightly more common in boys than girls and rare in adults. Neuroblastoma is the third most common pediatric malignancy after leukemia and central nervous system tumors, accounting for close to 15% of childhood cancer fatalities.

Two thirds of neuroblastomas are located in the abdomen, of which two thirds are located in the adrenal gland and the remainder in the extraadrenal retroperitoneum. The distribution of ganglioneuroblastomas and neuroblastomas follows the distribution of the sympathetic ganglia in a paramedian location, with the adrenal medulla and organ of Zuckerkandl being additional sites of involvement. Locally advanced tumors or distant metastases may be present at the time of diagnosis in 50% of patients with ganglioneuroblastoma and over 70% of patients with neuroblastoma, with involvement of bone, bone marrow, liver, lymph nodes, and skin. Serum or urinary catecholamines and their metabolites, vanillylmandelic acid (VMA) and homovanillic acid (HVA), may also be elevated in some cases.

On gross examination, ganglioneuroblastomas may be partially or totally encapsulated, whereas neuroblastomas are unencapsulated with variable amounts of hemorrhage, calcification, and necrosis present. Histologically, ganglioneuroblastoma is composed of both mature ganglion cells and Schwann cells in addition to immature neuroblasts and has intermediate malignant potential. Neuroblastoma is composed of small neuroepithelial cells that may show glial or ganglionic differentiation, contains nests of primitive round cells with dark-staining nuclei and scant cytoplasm, and is frankly malignant. Homer Wright rosettes, circular or ovoid columns of tumor cells arranged around a central core of neuropil, if present, are characteristic of neuroblastoma but are not always present.

There is remarkable heterogeneity observed in neuroblastoma phenotype, ranging from spontaneous regression to relentless progression, and there are dozens of clinical and biological markers that have been proposed as being predictive of disease outcome. Children who present with neuroblastoma earlier than age 12 months have a better prognosis than those who present after age 12 months. Even with metastatic disease, infants younger than 12 months can have a favorable outcome with treatment. The majority of children older than 12 months with advanced neuroblastoma at presentation will die from progressive disease.

The most common staging system used for neuroblastoma is the International Neuroblastoma Staging System (INSS) ( Box 55-1 ), which takes into account the clinical stage, imaging results, surgical findings, and bone marrow examination. Patients with stage 1 and 2 disease have a good prognosis, and there is a high rate of spontaneous remission of neuroblastoma in infants with stage 4S disease, as well as high survival rates in patients with stage 4S disease without MYCN amplification. Patients older than age 18 months with stage 4 disease and unresectable disease have a worse outcome.

On CT and MRI, ganglioneuroblastomas and neuroblastomas are less than 10 cm in size and have similar attenuation and SI characteristics to ganglioneuromas. They tend to have poorly defined margins with invasion of adjacent organs and encasement or occlusion of vessels. There may be cystic change or foci of hemorrhage. MRI is the preferred imaging modality for demonstrating intraspinal extension of tumor, which is seen with 10% of abdominal neuroblastomas, as well as for detecting metastatic involvement of bone marrow. Foci of amorphous and coarse calcification are commonly seen on CT in up to 90% of patients with neuroblastoma, in contrast to the discrete and punctate calcification seen in ganglioneuroma. These calcific foci are seen as very-low-SI areas on T1- and T2-weighted imaging. Heterogeneous arterial-phase enhancement is also seen. Only 70% of ganglioneuroblastomas and neuroblastomas are MIBG positive, but FDG PET may be useful to evaluate patients with poorly differentiated tumors that are MIBG negative.

Treatment of retroperitoneal ganglioneuroblastoma and neuroblastoma may include locally aggressive surgical excision, adjuvant or neoadjuvant chemotherapy, and external beam or targeted radiation therapy, depending on the age at diagnosis, stage of disease, tumor histopathology, MYCN amplification, tumor cell ploidy, and extent of involvement of vital anatomic structures by tumor. Patients with distant metastases have a poorer prognosis, while patients without distant metastases who undergo radical resection may have a favorable prognosis. The prognosis and response to therapy for patients with retroperitoneal ganglioneuroblastoma are better than those for patients with retroperitoneal neuroblastoma. The 5-year survival rate for adult patients (aged 21 or older) with neuroblastoma is about 36%, compared to 85% for infants.

Schwannoma

Schwannomas are benign tumors of nerve sheaths of peripheral nerves that account for up to 4% of all retroperitoneal tumors; they are most frequently found in the head and neck region and flexor surfaces of the extremities. They occur in the third through sixth decades of life, usually as solitary lesions that arise sporadically, and are twice as common in women than in men. Most patients are asymptomatic and present with a slowly growing painless soft tissue mass. Malignant transformation is rare. Schwannomas are usually less than 5 cm in diameter at presentation, but retroperitoneal schwannomas are typically larger than 8 cm at the time of presentation.

Schwannomas are solitary fusiform masses derived from Schwann cells. They are surrounded by a fibrous capsule and are eccentrically located in relation to the parent nerve. Histologically the schwannoma is composed of a highly ordered cellular component of compact spindle cells in short bundles or interlacing fascicles (Antoni A area) and a less organized and less cellular loose myxoid component (Antoni B area). Larger retroperitoneal schwannomas are more likely to undergo degenerative changes, including cyst formation (in up to 66%), calcification, hemorrhage, and hyalinization. Ancient schwannomas refer to long-standing lesions with advanced degenerative changes such as cyst formation, hemorrhage, calcification, and hyalinization.

On CT and MRI, schwannomas are sharply circumscribed, fusiform, round or oval masses, usually located in the paravertebral or presacral portions of the retroperitoneum. They are of low to intermediate SI on T1, high SI on T2, and have solid enhancing components. A low-SI capsule may sometimes be seen on T1- and T2-weighted images ( Fig. 55-22 ). Heterogeneous attenuation and SI are much more common in schwannomas than in neurofibromas and may be due to the mixture of Antoni A and B areas, along with hemorrhage, cystic degeneration, or punctate, mottled, or curvilinear calcification that is often difficult to visualize on MRI prospectively. Myxoid components have low attenuation and high SI on T2 and show variable enhancement. A “target sign” may be present in both schwannomas and neurofibromas on T2-weighted imaging, which consists of a central area of low-to-intermediate-SI fibrous tissue surrounded by peripheral high-SI myxoid tissue. A target-like enhancement pattern with greater enhancement centrally than peripherally may be seen on contrast-enhanced CT or T1-weighted imaging. If the parent nerve of a schwannoma can be identified, the schwannoma can be seen to have an eccentric position in relation to the nerve.

Schwannomas are treated by surgical excision or enucleation, particularly if large or symptomatic. The adjacent nerve can usually be spared because the schwannoma is separable from the underlying nerve fibers.

Neurofibroma

Neurofibromas are benign tumors of nerve sheaths of peripheral nerves that represent 5% of all benign soft tissue neoplasms. They occur sporadically in the third through fifth decades of life, are more common in men, and may develop in up to 10% of patients with NF1 in a younger age range. Approximately one third of patients with a solitary neurofibroma have NF1, and almost every patient with multiple or plexiform neurofibromas has NF1. Neurofibromas commonly occur in deep anatomic locations in patients with NF1 (especially in retroperitoneal and paraspinal locations) and are commonly associated with neurologic symptoms. Other nonspecific symptoms and signs may also be seen with abdominal neurofibroma.

Sporadic neurofibromas are usually smaller than 5 cm, whereas neurofibromas in those with NF1 tend to be multifocal and larger. Neurofibromas are not encapsulated and may occur as localized, plexiform, or diffuse types. The latter type is typically found in the subcutaneous tissues of patients with NF1 and rarely undergoes malignant transformation. Histologically, neurofibromas are composed of nerve sheath cells, thick wavy collagenous bundles, and variable amounts of myxoid degeneration. Often, longitudinal bundles of residual nerve fibers are centrally situated in the neurofibromas. Unlike schwannomas, neurofibromas are not encapsulated and lack a clear partition into Antoni A and B areas.

Neurofibromas usually have a fusiform shape that is oriented longitudinally in the distribution of the nerve and has tapered ends that are centered upon and contiguous with the parent nerve. Associated muscle atrophy in a particular nerve distribution may also be seen. The “split-fat sign” may be seen when a rim of fat surrounds the tumor, originating from a nerve in an intermuscular location. A characteristic dumbbell shape may sometimes be seen with spinal nerve root neurofibromas that extend through and enlarge the neural foramen. This morphology may also be seen with other nerve and nerve sheath tumors involving the spinal nerve roots.

When multiple or of the plexiform type, which is pathognomonic of NF1, large conglomerate infiltrative masses of innumerable neurofibromas diffusely thicken a parent nerve and extend into multiple nerve branches, resulting in a characteristic “bag of worms” appearance. Plexiform neurofibromas in the retroperitoneum are typically bilateral and symmetric in a parapsoas or presacral location and follow the distribution of the lumbosacral plexus. Parapsoas neurofibromas tend to be long and cylindrical in configuration, whereas confluent presacral neurofibromas often extend into the pelvis in a sheetlike fashion. Other findings of NF1 such as scoliosis, dural ectasia, or anterior and lateral meningoceles may be present.

On CT, neurofibromas are of low soft tissue attenuation. On MRI, they are of low SI on T1 and high SI on T2 ( Fig. 55-23 ). The SI on T2-weighted images may be homogeneous or may be heterogeneous with either a target sign, as described with schwannomas, or with a whorled appearance consisting of linear or curvilinear low-SI Schwann cell bundles and collagen fibers in a background of high SI. The whorled appearance may also be seen on CT, with high-attenuation linear or curvilinear foci seen in a background of low attenuation.

After contrast administration, enhancement may be homogeneous or heterogeneous. Areas of fluid attenuation may be seen and are due to regions of myxoid degeneration; this may cause confusion with necrotic lymphadenopathy or other cystic lesions in the retroperitoneum. Target-like central enhancement may also be encountered on CT or MRI.

Because neurofibromas are intimately associated with their underlying parent nerves, surgical resection requires removal of the nerves of origin. Therefore surgical treatment is performed only when neurofibromas are plexiform, symptomatic, or suspicious for malignant degeneration.

Malignant Peripheral Nerve Sheath Tumor

Malignant peripheral nerve sheath tumors (MPNSTs) are malignant tumors that arise from or differentiate toward cells of the nerve sheaths of peripheral nerves. They represent 5% to 10% of soft tissue sarcomas, occur in 2% to 5% of patients with NF1, and arise from preexisting neurofibromas, usually after a latent period of at least 10 years. Conversely, about 50% of patients with MPNST have NF1. From 10% to 20% of MPNSTs are related to prior radiation exposure, occurring after a latent period of more than 15 years. Most MPNSTs occur during the third through sixth decades of life, which is earlier than other retroperitoneal sarcomas. MPNSTs that develop in association with NF1 tend to present within the third decade of life; they are of higher histologic grade, larger size, and greater aggressivity, and have a very poor prognosis compared to those that are sporadic.

MPNSTs that arise from major nerves such as the sciatic nerve or sacral plexus may cause sensory and motor symptoms such as radiating pain, paresthesias, and weakness. Unexpected growth of a preexisting neurofibroma, especially a plexiform neurofibroma, or associated pain, particularly if spontaneous and unremitting, should raise clinical suspicion for MPNST, although benign neurofibromas may also grow and become painful. In patients with NF1, pain associated with a mass is the greatest risk factor for MPNST development, with a relative risk of 30-fold.

On CT and MRI, benign and malignant neural tumors cannot be reliably differentiated; attenuation, SI, and enhancement characteristics overlap. MPNSTs tend to be larger than 5 cm, may exhibit poorly defined margins suggesting infiltration of surrounding tissues, may have associated edema, and may expand neural foramina. These features may also be seen with benign neural tumors. Heterogeneity with central necrosis is common in MPNST, but benign tumors with cystic degeneration may also appear heterogeneous. Calcification is more commonly associated with MPNST but may also be seen in ancient schwannomas. The most important imaging finding that should raise suspicion of MPNST is rapid enlargement of a tumor mass, particularly if associated with pain ( Fig. 55-24 ).

Complete surgical resection is the treatment of choice for retroperitoneal MPNST, as with other retroperitoneal sarcomas, and is the most important factor influencing patient survival. Local recurrence and distant metastatic disease are common complications of MPNST. The overall 5-year survival rate for patients with MPNST is 34% to 52%, and the 5-year survival rate of MPNST in patients with NF1 is 15%.

Retroperitoneal Fibrosis

Retroperitoneal fibrosis (RPF) is a rare fibrotic reactive process considered a member of a family of disorders referred to as chronic periaortitis. Two thirds of all cases of RPF are considered idiopathic (also called Ormond's disease ), and approximately one third of cases develop in response to various medications, malignancies, or other etiologies.

The exact etiology of idiopathic RPF is unclear, although chronic fibrosing periaortitis, possibly due to an immune reaction to a component of ruptured atherosclerotic plaque such as ceroid (a lipoproteic polymer that results from low-density lipoprotein oxidation within plaque macrophages), may play a role. The observation that RPF tends to occur in areas where an arterial wall (usually of the abdominal aorta) has atherosclerotic plaque and attenuation of the media supports this theory. However, this theory has been challenged over the past decade, and an underlying systemic autoimmune process is believed to be responsible instead. In up to 15% of individuals with RPF, associated fibrotic processes outside the retroperitoneum may be present, including fibrosing mediastinitis, sclerosing mesenteritis, orbital pseudotumor, primary sclerosing cholangitis, Reidel's thyroiditis, and immunoglobulin G4 (IgG4)-related disease. Other autoimmune or inflammatory disease processes such as systemic lupus erythematosus, rheumatoid arthritis, ankylosing spondylitis, small and medium-sized vessel vasculitides, and asbestos exposure also have an association with RPF. Up to 15% of abdominal aortic aneurysms (AAAs) may have an association with perianeurysmal fibrosis and are considered by some to be an early or mild form of RPF. These are discussed later under “Inflammatory Abdominal Aortic Aneurysm” (IAAA).

One percent of patients who take methysergide, an ergot derivative used to treat migraine headaches, develop RPF, with symptomatic relief commonly occurring after drug withdrawal. This known side effect of methysergide (as well as pleural and valvular fibrosis) has led to decreased use of this drug to treat migraines and consequently fewer drug-related cases of RPF. Other ergot derivatives such as bromocriptine, used in the treatment of parkinsonism, may also be associated with RPF.

Malignant RPF is an unusual subtype of RPF and is clinically difficult to distinguish from RPF due to benign or idiopathic causes. Malignant RPF occurs when small metastatic foci to the retroperitoneum (usually from lymphoma) elicit a desmoplastic response. Most commonly, Hodgkin's disease and other lymphomas, retroperitoneal sarcomas, carcinoid tumors, and carcinomas from the breast, lung, thyroid, stomach, colon, kidney, bladder, prostate, and cervix are the culprits, although any primary malignancy may be involved. Malignant RPF is distinct from malignant retroperitoneal lymphadenopathy. Infections due to tuberculosis (TB), syphilis, actinomycosis, or fungi; nonspecific gastrointestinal (GI) inflammation including appendicitis, Crohn's disease, or diverticulitis; and retroperitoneal hemorrhage, urine extravasation, or prior irradiation or surgery may also lead to RPF.

RPF typically originates below the aortic bifurcation at the level of the sacral promontory or lower lumbar vertebrae and then extends superiorly along the anterior spinal surface in a periaortic and pericaval distribution toward the renal hila, where it may rarely surround the renal pelves. Typically one or both ureters, usually in the middle third, may be encased, often resulting in hydronephrosis. However, the fibrotic process may also spread inferiorly to involve the iliac and gonadal vessels, rectosigmoid colon, urinary bladder, and other pelvic organs, or anteriorly along the celiac and superior mesenteric arteries. Infrequently, involvement of the lungs, mediastinum, pericardium, pleura, spine, epidural space, spinal cord, gonadal vessels, pancreas, spleen, duodenum, colon, small bowel mesentery, urinary bladder, unilateral renal sinus or ureter, uterus, and cervix have also been reported. Symptoms and signs are related to entrapment and compression of retroperitoneal structures, including the ureters, IVC, abdominal aorta and its branches, and gonadal vessels. The ureters are the most frequently compressed structures, and oliguria, anuria, and eventual renal failure may occur. Compression of the IVC can result in lower extremity edema, whereas aortic and common iliac arterial compression may lead to lower extremity claudication. Renal artery compression may result in renovascular hypertension, and mesenteric artery compression may cause bowel ischemia. Rarely, pancreatic involvement, biliary obstruction, portal vein occlusion, GI obstruction, or spinal cord compression due to extension of RPF through the neural foramina may occur. Laboratory abnormalities may include abnormal renal function tests in those with ureteral obstruction, and there may be elevations in erythrocyte sedimentation rate (ESR) and C-reactive protein levels.

Idiopathic RPF occurs more commonly in men by a ratio of 2-3 : 1, which has been attributed to the higher incidence of symptomatic atherosclerotic disease in men. However, RPF due to methysergide occurs twice as commonly in women as in men, and RPF associated with malignancy occurs equally in men and women. Most patients with RPF present during the fifth through seventh decades of life.

On gross examination a dense grayish-white plaquelike mass surrounds the inferior aspect of the abdominal aorta and often encases or compresses the aortic branches and the ureters. Histologically, early RPF is characterized by immature fibrosis with numerous fibroblasts, inflammatory cells, capillary proliferation, and a loose network of collagen fibers with a high fluid content due to leaky vascular interendothelial junctions. Malignant RPF shows a similar histologic pattern, with the addition of scattered malignant cells in the stroma. As RPF matures, the cellular and inflammatory elements decrease and hyalinized collagen becomes more prominent. This maturation process progresses laterally from the midline such that the lateral edges of RPF tend to be inflammatory, whereas the central portion tends to be more fibrotic.

The anterior margin of RPF is sharply marginated with the posterior peritoneum, whereas the posterior margin is poorly defined. The fibrotic plaque may be midline or eccentric, well circumscribed or poorly defined, localized or extensive, and may appear similar morphologically in both benign and malignant RPF.

On CT, RPF has homogeneous soft tissue attenuation similar to or slightly greater than that of skeletal muscle. On MRI the T2-weighted SI of RPF is dependent on the activity of the disease. RPF has low to intermediate SI on T1-weighted images, and on T2, mature fibrotic plaque in benign RPF has low SI, whereas immature fibrotic plaque in benign RPF and malignant RPF has higher SI due to inflammatory edema or hypercellularity ( Fig. 55-25 ). Chronic RPF tends to have a higher ADC value (i.e., less restricted diffusion) compared to that of active or malignant RPF. Enhancement of RPF on CT and MRI is variable and depends on the maturity of the fibrous process, with immature plaque enhancing to a greater degree owing to greater vascularity. After corticosteroid therapy and with maturation, there is a decrease in inflammatory reaction, with a subsequent decrease in T2-weighted SI as well as decreased enhancement on dynamic contrast-enhanced T1-weighted imaging.

Imaging findings that suggest malignant RPF include the presence of other lymphadenopathy or metastatic disease, adjacent osseous destruction, a heterogeneous soft tissue mass with poorly defined margins, a longitudinal extent from above the renal arteries to below the aortic bifurcation, and associated high-SI changes of the adjacent psoas muscles on T2 ( Fig. 55-26 ). Uptake of FDG on PET may be seen in RPF and is due to the increased metabolic activity of inflammatory cells; this may play a role in differentiating benign from malignant RPF, discriminating active from inactive disease, and monitoring disease activity following therapeutic intervention.

Malignant retroperitoneal lymphadenopathy may mimic RPF, since it can occasionally become confluent and encase the great vessels. However, metastatic disease most commonly appears as lobulated paraaortic and paracaval masses that are due to enlarged lymph nodes. Lymphoma and other tumors that result in malignant lymphadenopathy typically displace the aorta anteriorly and the ureters laterally, whereas benign RPF usually does not cause significant anterior aortic displacement or lateral ureteral displacement.

The goals of RPF treatment are to stop progression of the fibroinflammatory reaction, inhibit or relieve obstruction of the ureters or other retroperitoneal structures, switch off the acute-phase reaction and its systemic manifestations, and prevent disease recurrence or relapse. Although spontaneous regression of RPF has been reported rarely, most patients require some form of medical and/or surgical treatment. In patients with methysergide- or bromocriptine-related RPF, discontinuation of the drug results in regression of both symptoms and fibrosis. In other patients with RPF, biopsies are obtained to exclude a malignant or infectious etiology before therapy is instituted. If RPF is secondary to infection, corticosteroids are contraindicated and specific antimicrobial therapy is instituted.

Corticosteroids are used to decrease inflammation in early idiopathic disease, relieve the symptoms as well as shrink the RPF mantle on imaging, and relieve the ureteral obstruction within 1 to 2 weeks. Tamoxifen and immunosuppressants have also been used successfully to treat nonmalignant RPF. If medical therapies are not effective in relieving the ureteral obstruction, ureteral stenting or ureterolysis (ureteral dissection from the surrounding fibrotic plaque) is successful. After ureterolysis, the ureters may be transposed laterally (after which retroperitoneal fat may be interposed between the ureters and the RPF), the ureters may be wrapped in omental fat, or the ureters may be transplanted into the peritoneal cavity, with the posterior peritoneal flap closed behind them to protect the ureters from future encasement and obstruction by the fibrotic process.

The prognosis for patients with malignant RPF is poor, with a mean survival of 3 to 6 months after diagnosis. The prognosis for those with idiopathic RPF is generally favorable. Long-term success in prevention of recurrent urinary obstruction and maintaining renal function may be achieved with ureterolysis and medical therapy in over 90% of patients. However, because these patients usually have atherosclerotic disease with resultant complications such as myocardial infarctions and cerebrovascular accidents, the 10-year survival rate is usually reported as less than 70%.

Inflammatory Abdominal Aortic Aneurysm

From 5% to 15% of patients with AAAs have associated asymptomatic perianeurysmal fibrosis, which is similar morphologically and histologically to RPF. IAAA is also considered a member of the family of disorders referred to as chronic periaortitis. The pathogenesis of IAAA is thought to be similar to that of idiopathic RPF, with the perianeurysmal inflammatory response representing an immune reaction to antigens such as ceroid that leak from the aortic wall. Recent reports suggest that it may also be a local manifestation of a systemic autoimmune process, in particular IgG4-related disease.

The aortic wall is thickened up to 3 cm with intense perianeurysmal and retroperitoneal fibrosis and extensive adhesions involving the surrounding organs. The anterior and lateral walls of IAAAs are covered with a thick white layer of fibrous tissue that fixes the duodenum and other adjacent structures to the aneurysm sac. In severe cases the perianeurysmal inflammatory/fibrous tissue may extend to involve the ureters, IVC, left renal vein, small bowel mesentery, or transverse mesocolon, with potential obstruction of the involved structures. Histologically, most of the aortic wall thickening results from expansion of the adventitia by a marked inflammatory reaction.

Both atherosclerotic AAAs and IAAAs occur much more commonly in men compared to women, and in patients with IAAA, the mean age at presentation is in the seventh decade of life, 5 to 10 years younger than the mean age for atherosclerotic AAA. However, 75% of patients with IAAAs present with abdominal or back pain, compared to only 13.5% with noninflammatory AAA. Patients with IAAA are more likely to have a family history of aneurysms and tend to be current smokers. IAAAs tend to be larger than noninflammatory AAAs at presentation (mean size, 8.0 cm vs. 6.4 cm, respectively), and the ESR is elevated in the majority of patients with IAAAs. The triad of chronic abdominal pain, weight loss, and elevated ESR in a patient with an AAA is highly suggestive of an IAAA.

On CT, typical findings of an IAAA include a dilated abdominal aorta (diameter > 3 cm), thickening of the aortic wall (most marked anteriorly and laterally, with relative sparing posteriorly), and periaortic inflammation and fibrosis (which is of soft tissue attenuation with a variable amount of enhancement). The periaortic inflammation and fibrosis may be well defined or infiltrative in appearance. In a study of 355 patients with AAA, CT was 83.3% sensitive, 99.7% specific, and 93.4% accurate in diagnosis of IAAA. On MRI the typical appearance of an IAAA consists of a dilated aorta with concentric rings of high SI in the wall that are most prominent on T2 but are also present on T1. These mural concentric rings represent alternating layers of fibrosis and inflammation. IAAAs tend to have three or more concentric layers of high SI, whereas noninflammatory AAAs show at most two rings of high SI. On T1-weighted images, the surrounding rim of inflammatory tissue shows intermediate SI with poor differentiation from intraluminal thrombus and surrounding structures. On contrast-enhanced CT and T1-weighted imaging the inflammatory cuff usually enhances homogeneously, similar to that in patients with RPF who do not have an aneurysm. In contrast to retroperitoneal malignancy, IAAA rarely displaces the aorta anteriorly from the spine. Restricted diffusion may be seen on DWI and ADC map images. Uptake of FDG on PET may be seen in IAAA and is due to the increased metabolic activity of inflammatory cells.

Preoperative treatment of IAAA patients with corticosteroids may be useful to control the inflammatory process and facilitate surgical repair of the IAAAs, particularly when the inflammatory process is severe or if there is associated involvement of adjacent retroperitoneal structures, and corticosteroids or other antiinflammatory immunosuppressive therapies may also be used after surgical repair if inflammation persists or is exacerbated. Aneurysm resection is the traditional treatment for IAAAs, owing to the risk of aortic rupture, especially when the maximal aortic diameter exceeds 5.5 cm. Over 75% of patients undergo spontaneous remission of perianeurysmal fibrosis following surgical repair. Perianeurysmal fibrosis is not protective against aneurysm rupture but often makes operative repair of such aneurysms more difficult and hazardous. Endovascular repair of IAAAs is also an option, reportedly with a high success rate. However, the perianeurysmal fibrosis may not resolve but actually worsen over time in some patients, leading to or exacerbating ureteral obstruction. Endovascular repair is associated with a lower 1-year all-cause mortality compared to open surgical repair, although open surgical repair may be preferred in those patients who have hydronephrosis and are deemed low risk. The 5-year survival rates for IAAAs and noninflammatory AAAs are similar, ranging between 67% to 80%.

Fibromatosis (Desmoid Tumor)

The fibromatoses are divided into superficial (fascial) and deep (desmoid tumor) groups, and the deep group is further subdivided into extraabdominal, abdominal wall, and intraabdominal (mesenteric, mesocolic, omental, retroperitoneal) subgroups. In this chapter we will focus predominantly on the intraabdominal subgroup of desmoid tumors.

Desmoid tumors are uncommon neoplastic lesions, comprising 0.1% of all tumors and 3.5% of fibrous tumors, occurring either sporadically or in association with familial adenomatous polyposis (FAP). FAP is inherited in an autosomal dominant fashion, and patients with a phenotypic variant of FAP known as Gardner's syndrome may develop desmoid tumors in addition to polyposis coli and colon carcinoma. The incidence of desmoid tumor in FAP ranges from 3.6% to 34%, and patients with FAP are at an approximately 1000-fold increased risk compared to the general population. Sporadic desmoid tumor occurs more commonly in women by a ratio of 2 to 5 : 1, whereas FAP-associated desmoid tumor occurs with equal gender frequency. Both types of desmoid tumor have a peak incidence in the fourth decade of life.

Sporadic desmoid tumors are solitary in over 90% of patients; they are often present in the retroperitoneum, pelvis, and anterior abdominal wall and tend to be large, with a mean diameter of 13.8 cm. In contrast, FAP-associated desmoid tumors are multiple in 40% of patients, are more likely to involve the mesentery and the abdominal wall, and tend to be smaller, with a mean diameter of 4.8 cm. Retroperitoneal desmoid tumors are rare and principally originate from the connective tissue of the muscles and their overlying fascia or aponeuroses. They tend to be infiltrative, may be asymptomatic, or may have a variable clinical presentation due to ureteral obstruction or bowel invasion. Minor bone malformations such as exostoses, enostoses, and incomplete spinal segmentation may be seen on imaging in 80% of sporadic desmoid tumor patients compared to 5% of the general population.

The exact etiology of desmoid tumors is not known. Most cases are idiopathic, but genetic abnormalities, trauma (including iatrogenic trauma), and estrogenic hormones are potential etiologic factors. Histologically, desmoids are composed of uniform, elongated, slender, highly differentiated spindle-shaped fibroblasts that are surrounded by abundant collagen in interlacing bundles. Cellularity is variable, with some portions of tumor showing complete replacement by dense fibrous tissue and other areas revealing stromal myxoid change. When large, desmoid tumors may have cystic degeneration or necrosis. No peripheral capsule is present, but instead an infiltrative margin is seen that extends several centimeters beyond the palpable edge of the lesion.

No correlation between clinical behavior and histologic appearance has been observed. Interestingly the growth rate of desmoid tumors parallels the level of endogenous estrogen, which has more effect than progesterone, and between 25% and 75% of desmoid tumors have estrogen and antiestrogen binding sites.

On CT and MRI, desmoid tumors are typically infiltrative and cross fascial boundaries, have homogeneous low to intermediate attenuation and low to intermediate SI on T1- and T2-weighted images relative to skeletal muscle, are uncommonly calcified, and tend to appear aggressive although they are not malignant ( Fig. 55-27 ). However, because desmoid tumors may have a variable degree of cellularity, fibrosis, myxoid stroma, vascularity, and infiltration, they may be either well defined or poorly defined and may have variable SI. Slightly high SI components on T1-weighted imaging relative to skeletal muscle are frequently seen. The presence of high SI on T2 does not exclude a diagnosis of a desmoid tumor, because immature lesions with higher cellularity and less mature fibrosis can be of higher SI than skeletal muscle on T2. These immature high-SI desmoid tumors are associated with rapid growth on follow-up imaging. Occasionally a low-SI fibrous capsule may be partially or completely visualized on T1, and low-SI bands of collagen may be present on T1- and T2-weighted images. DWI may be useful to differentiate a desmoid tumor from a malignant soft tissue tumor; the former tends to have a higher mean ADC compared to the latter.

Both mature and immature desmoid tumors tend to enhance, and the enhancement can be variable. Associated encasement of the bowel or mesenteric vessels or hydronephrosis may also be seen.

Imaging features of intraabdominal desmoid tumor that suggest a poor prognosis include a mesenteric mass greater than 10 cm, multiple mesenteric masses, extensive small bowel involvement, and bilateral hydronephrosis. Overall, the mean survival rate of patients with intraabdominal desmoid tumors is 5 years. The prognosis of patients with FAP-associated intraabdominal desmoid tumors is significantly worse than those without them.

Surgical removal remains the mainstay of treatment of desmoid tumors, despite frequent local recurrence. Optimally, wide-field surgical resection is performed for small intraabdominal tumors so that tumor-free margins may be achieved. Large intraabdominal desmoid tumors cannot be completely removed without sacrificing vital structures such as the mesenteric vessels. Retroperitoneal desmoid tumors in particular are rarely completely resectable and often recur locally. Thus surgical removal of intraabdominal desmoids has a perioperative mortality rate and major morbidity rate of up to 60%, with recurrence in up to 90% of cases, and is not generally recommended unless there is GI or GU tract obstruction, fistula formation, or abscess formation.

Postsurgical radiation therapy may help reduce the risk of recurrent disease. After surgery, recurrent desmoids have MRI characteristics similar to those of the original lesion, and the site of recurrence is frequently at the margin of the original lesion. MRI is also useful in evaluating the effectiveness of nonsurgical therapy; effective therapy decreases the size of the mass and increases the amount of low SI on T2-weighted imaging, reflecting increased intratumoral collagen.

Noncytotoxic drugs, cytotoxic drugs, radiation therapy, and observation are nonsurgical treatment options for desmoid tumors, although no single approach has been proven consistently effective. Noncytotoxic medications including nonsteroidal antiinflammatory drugs (NSAIDs) are considered by some as first-line treatment for desmoid tumors, particularly if they are small, asymptomatic, and unlikely to cause bowel obstruction. A trial of watchful waiting may also be implemented, especially in patients with FAP or with lesions that are intimately associated with the mesenteric vessels, because spontaneous regression may occur in 5% to 15% of cases. Cytotoxic medications may be used to induce variable degrees of remission in symptomatic, clinically aggressive, recurrent, nonresectable, or unresponsive desmoid tumors and may achieve a good initial response for inoperable desmoid tumors that have caused progressive bowel or ureteral obstruction.

Lipoma

Lipomas are benign mesenchymal tumors composed of mature fat and represent the most common mesenchymal neoplasm. Even though retroperitoneal lipomas are rare, they are the most common benign tumor of the retroperitoneum. Variants of lipoma include angiolipoma, myolipoma, angiomyolipoma, myelolipoma, chondroid lipoma, spindle cell/pleomorphic lipoma, and lipoblastoma and are found much less commonly in the abdomen. Lipomatous tumors account for half of all soft tissue tumors in surgical series.

Solitary lipomas occur in the fifth through seventh decades of life at a significantly younger peak age range compared to that of well-differentiated liposarcomas. They usually appear during periods of weight gain. Deep lipomas occur more commonly in men. Abdominal lipomas are usually not treated, although surgical removal may be performed if they are large or symptomatic. Lipomas are composed of mature adipocytes, similar to normal adult fat, although the cells are slightly larger and slightly more variable in size and shape. Admixed mesenchymal elements such as fibrous connective tissue may be present. Cartilaginous or osseous metaplasia may occur, particularly in large chronic lipomas. Lipomas do not undergo malignant transformation into liposarcomas, although they may occasionally grow to a large size.

On CT and MRI, lipomas typically have homogeneous attenuation and SI identical to that of macroscopic fat on all pulse sequences, without enhancing components, in contrast to well-differentiated liposarcomas. In up to 49% of cases, however, a few thin (<2 mm) septae may be seen that have minimal to moderate enhancement, and mild enhancement of a thin fibrous capsule may also be seen. Many lipomas also have prominent nonadipose areas that may demonstrate enhancement, and the imaging appearance then overlaps with that of well-differentiated liposarcomas. These nonadipose components may be seen in up to 31% of lipomas and are secondary to fat necrosis and associated calcification, fibrosis, inflammation, and areas of myxoid change. Lipomas tend to be smaller than well-differentiated liposarcomas, and most remain stable in size.

Extrarenal Angiomyolipoma

Angiomyolipomas (AMLs) are benign neoplasms composed of ectopic rests of normal tissue in an abnormal arrangement containing fat, abnormal blood vessels, and smooth muscle in varying relative proportions and are most often found in the kidneys. They are rarely encountered in the extrarenal retroperitoneum. Approximately 90% of AMLs occur sporadically in the kidney, most frequently in middle-aged women. Whereas 20% of patients with renal AMLs have tuberous sclerosis (TS), 80% of TS patients have AMLs, which tend to be multiple and bilateral. Up to 57% of patients with lymphangioleiomyomatosis (LAM) have AMLs. Although benign, some AMLs exhibit aggressive features that may mimic malignancy. These include vascular invasion, lymph node involvement, nonrenal organ involvement, and local recurrence after resection. Multicentric or extrarenal retroperitoneal AMLs are thought to be caused by the congenital presence of cell precursors in multiple sites or by a form of benign metastases similar to that in benign metastasizing leiomyoma. Similar to renal AMLs, extrarenal AMLs have a propensity to undergo spontaneous hemorrhage, with a risk that correlates with the size of the lesion.

On CT and MRI, extrarenal AMLs typically appear as masses that contain areas of macroscopic fat. Soft tissue components and vascular components are frequently present, and there is a variable amount of enhancement. The major differential diagnosis is retroperitoneal liposarcoma, but preoperative cross-sectional imaging combined with tissue sampling may permit a nonoperative diagnosis in lesions that pose a diagnostic problem. Surgical resection or arterial embolization may be performed for symptomatic lesions.

Extraadrenal Myelolipoma

Myelolipomas are uncommon benign well-circumscribed tumors that are composed of mature adipose cells and hematopoietic tissue. They are of uncertain etiology but usually arise in the adrenal gland. Extraadrenal retroperitoneal myelolipomas are rare, occurring most often in the presacral space. They are more common in women than in men by a ratio of 2 : 1, with a peak occurrence in the seventh decade of life.

Extraadrenal myelolipomas can range in size from 2 to 26 cm, with a mean size of 8.2 cm. Smaller lesions are typically asymptomatic, whereas larger lesions are frequently symptomatic owing to mass effect or hemorrhage. Associated hemorrhage is more common with larger lesions than with smaller lesions. The natural history of extraadrenal myelolipoma is unknown, but adrenal myelolipomas can enlarge with time and become symptomatic. Surgical removal may be performed when they are symptomatic, large (increased risk of hemorrhage), or when they increase in size over time. Histologically, mature fat and focal collections of normal hematopoietic elements are seen in a well-encapsulated lesion.

On CT and MRI, myelolipomas contain areas with attenuation and SI characteristics similar to macroscopic fat, often with enhancing soft tissue components due to mixed hematopoietic tissue, and less commonly may have foci of calcification. A thin peripheral rim of mild enhancement is often seen on contrast-enhanced CT and T1-weighted imaging. Associated retroperitoneal hemorrhage may occasionally be seen as well. Extraadrenal myelolipomas tend to have smaller amounts of macroscopic fat compared to adrenal myelolipomas. ^99m Tc sulfur colloid scintigraphy may be useful for establishing the diagnosis by demonstrating the presence of radiotracer-avid erythropoietic elements within such masses.

Primary Retroperitoneal Teratoma

Primary retroperitoneal teratomas are rare lesions that represent 6% to 11% of primary retroperitoneal tumors. The majority occur in children. Less than 20% of lesions occur in adults, and they are 2 to 4 times more common in women than in men. Teratomas in adults have a greater chance of being malignant than those in children (14%-26% vs. 6%-7%, respectively). Teratomas are most common in the ovaries, testes, and anterior mediastinum. Primary retroperitoneal teratomas tend to be asymptomatic but may cause nonspecific symptoms, as seen with other retroperitoneal masses when large. AFP levels are normal in patients with benign teratomas but may be elevated with malignant teratomas.

Primary retroperitoneal teratomas develop from totipotential germ cells that have failed to migrate to their normal gonadal locations. They may be cystic and/or solid, and frequently contain mature tissues including skin and dermal appendages, cartilage, bone, teeth, or fat. Cystic teratomas are benign well-defined lesions that contain multiple solid and cystic areas along with mature tissues, sebaceous material, and/or mucoid fluid. Solid teratomas are frequently malignant and contain immature embryonic tissue in addition to the mature components. Retroperitoneal teratomas tend to be located near the upper poles of the kidneys, with preponderance on the left side. From 60% to 83% of all primary retroperitoneal teratomas have calcifications that are visible on CT; up to 74% of benign teratomas may contain calcifications, and up to 25% of malignant teratomas may also contain calcifications. Therefore the presence of calcifications does not predict benignity.

On CT and MRI, teratomas are usually heterogeneous well-defined solid and/or multiloculated cystic lesions that may contain fatty components, calcifications, ossifications, or teeth. Other tumoral components have less specific CT and MRI features. Occasionally a low-SI fibrous pseudocapsule may be visualized on T1- and T2-weighted images, and a dermoid plug or Rokitansky nodule may also be seen along the inner surface of a cystic component, with variable attenuation and SI depending on components present within it. An almost pathognomonic finding of a mature cystic teratoma is an internal fat-fluid level, with the nondependent fat layer following the attenuation and SI of fat on all pulse sequences. This finding is usually seen in ovarian teratomas but may rarely be present in teratomas developing at other anatomic sites. Other fatty lesions such as a retroperitoneal liposarcoma may extremely rarely demonstrate an internal fat-fluid level. The demonstration of fat-fluid levels in the peritoneum may be a sign of intraperitoneal rupture of an abdominal teratoma.

Retroperitoneal teratomas are treated by surgical removal, regardless of whether benign or malignant, because even histologically benign lesions may result in significant morbidity from continued growth. Patients have an excellent prognosis after the resection of benign retroperitoneal teratomas.

Fetus in Fetu

Fetus in fetu (FIF) is a rare entity, infrequently occurring in adults, in which a vestigial vertebrate fetus is enclosed within the abdomen of a normally developing fetus and may mimic a teratoma on cross-sectional imaging. It is thought to arise from abnormal embryogenesis in a diamniotic monochorionic twin pregnancy, in which a malformed monozygotic twin lies within the body of its fellow twin owing to unequal division of totipotential cells of a blastocyst, although others consider it to represent a highly mature teratoma. Approximately 75% present in infancy, and men are affected up to two times more commonly than women. FIF may be asymptomatic or cause nonspecific symptoms due to mass effect or hemorrhage when large.

In most cases, FIF is found in the upper retroperitoneum or in the nonretroperitoneal portion of the abdomen, often with a capsule and a vascular pedicle, whereas teratomas most commonly occur in the gonads or anterior mediastinum. The fetus is usually single and is always anencephalic, although up to 5 fetuses have been reported. Symptoms are mainly related to mass effects and include abdominal distention, feeding difficulty, emesis, jaundice, and dyspnea. Serum AFP levels may be normal or elevated. Malignant degeneration occurs very rarely in FIF, but surgical excision is still the recommended treatment.

On CT and MRI a heterogeneous cystic mass containing mature fat and central osseous structures is typically seen, with imaging features similar to a teratoma, as described earlier. Identification of a fetal vertebral column or fetal long bones is essential to make the diagnosis, although 9% and 17.5% have no well-defined vertebral axis or limbs, respectively, on pathologic examination.

Hibernoma

Hibernomas are very rare benign, painless, slow-growing tumors consisting primarily of brown fatty tissue. They are usually seen in locations where normal brown adipose tissue is present, such as the thigh, shoulder, back, neck, chest, arm (in descending order of frequency), and rarely in the retroperitoneum. They occur most commonly during the third and fourth decades of life. A slight female predominance has been noted in the literature by some investigators, and a slight male predominance by others.

Hibernomas are well-circumscribed, encapsulated, soft, lobulated masses that usually measure 5 to 10 cm in maximal diameter but can grow to larger sizes, particularly in the retroperitoneum. Histologically they are composed of multivacuolated fat cells with variably eosinophilic cytoplasm and small central nuclei, small granular eosinophilic cells with little or no lipid, and abundant capillaries. Univacuolated fat cells may be seen, often with abundant mitochondria. Morphologic variants of hibernoma (in descending order of frequency) are typical, myxoid, lipomalike, and spindle cell variants. The typical variant is composed primarily of brown fat cells and is the most common variant (82% of cases). Surgical resection is usually performed for symptomatic lesions, and local recurrence is unlikely with complete resection.

On CT, hibernomas have attenuation that varies between fat and skeletal muscle. They are well circumscribed and may have a peripheral soft tissue attenuation capsule. They enhance slightly and may contain prominent enhancing vessels. On MRI, hibernomas typically have high SI on T1-weighted images and heterogeneously high SI on T2-weighted images relative to skeletal muscle, may variably lose SI on fat-suppressed T1, and demonstrate variable enhancement, which can be marked. Prominent branching and serpentine enhancing vessels may also be seen on both CT and MRI. Retroperitoneal hibernomas are treated by surgical excision, particularly if large or symptomatic. Intense FDG uptake with PET has been reported as well.

Fat Necrosis

Retroperitoneal fat necrosis is another fatty process that is characterized by the coalescence of fat cells into fat cysts, sometimes bordered by foreign body giant cell granulomata, foam cells, chronic inflammatory infiltrates, and fibrosis. The most common cause is acute pancreatitis. Other etiologic factors that have been described include trauma, surgery, infection, sclerosing mesenteritis, and chronic hemorrhage or urine leaks. It may also be idiopathic. Fat necrosis can present as a palpable abdominal mass, mimic other abdominal masses including retroperitoneal well-differentiated liposarcoma, and rarely lead to ureteral obstruction when located in the retroperitoneum.

On CT and MRI a lesion of predominantly fat attenuation or SI lesion is typically seen that contains internal foci of soft tissue attenuation or SI that may enhance. Foci of calcification may also be seen on CT. Fat necrosis tends to remain stable in appearance or decrease in size with time. Treatment is generally conservative. Percutaneous biopsy may be necessary to differentiate from retroperitoneal liposarcoma in an occasional case.