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CT and MRI of the Retroperitoneum
1
What are the boundaries of the retroperitoneum?
The retroperitoneum is bounded anteriorly by the posterior parietal peritoneum and posteriorly by the transversalis fascia and extends from the diaphragm to the pelvic brim.
2
What are the compartments of the retroperitoneum and their contents?
The anterior pararenal space, perirenal space, and posterior pararenal space are the main retroperitoneal compartments, which are separated from each other by fascial planes ( Figure 37-1 ). The anterior pararenal space, which contains the ascending colon, descending colon, second through fourth portions of the duodenum, and pancreas, is located anterior to the anterior renal fascia and medial to the lateroconal fascia and is contiguous across the midline. The bilateral perirenal (or perinephric) spaces contain the kidneys, renal pelves and proximal ureters, adrenal glands, and fat and are located between the anterior and posterior renal fascia. The posterior pararenal spaces contain fat are located posterior to the posterior renal fascia and external to the lateroconal fascia, and are contiguous with the properitoneal fat of the abdomen. The retroperitoneal fascial planes are not composed of single membranes but are laminated. As such, although they may serve as barriers to disease spread, these potentially expansile fascial planes may alternatively serve as conduits of disease spread to other compartments in the abdomen and pelvis.
3
What are the extraperitoneal spaces of the pelvis?
The prevesical space, also called the retropubic space or the space of Retzius, is an extraperitoneal space located anterior and lateral to the umbilicovesical fascia (UVF) and posterior to the pubic bones, extends posteriorly to communicate with the presacral space, and contains fat. It is a large potential space with multiple potential extensions and communications, allowing for bidirectional spread of fluid collections and other disease processes into the retroperitoneum and thighs. Prevesical fluid collections typically have a “molar tooth” configuration in the axial plane and can be large in size ( Figure 37-2 ). The perivesical space is a small extraperitoneal space bounded by the UVF which contains the urinary bladder, urachus, obliterated umbilical arteries, and fat. Perivesical fluid collections are typically small in size. The perirectal space is surrounded by the perirectal fascia and contains the rectum, hemorrhoidal vessels, and fat.
4
Are the psoas muscles located within the retroperitoneum?
No. The psoas muscles are located within the retrofascial space posterior to the transversalis fascia (the posterior boundary of the retroperitoneum).
5
Which tumors most commonly occur in the retroperitoneum?
Most retroperitoneal tumors are malignant and are due to lymphoma, retroperitoneal sarcoma, or metastatic disease. Neurogenic tumors such as schwannoma, neurofibroma, and paraganglioma are less commonly encountered in the retroperitoneum.
6
What are the major types of retroperitoneal sarcoma?
Liposarcoma is the most common retroperitoneal sarcoma (40%), followed by leiomyosarcoma (30%) and malignant fibrous histiocytoma (MFH) (15%). Malignant peripheral nerve sheath tumor (MPNST) is more frequently encountered in patients with neurofibromatosis type 1 (NF-1).
7
How are computed tomography (CT) and magnetic resonance imaging (MRI) useful in patients with retroperitoneal sarcoma?
CT and MRI are useful to stage the extent of disease in patients with retroperitoneal sarcoma, to determine whether the tumor is fully resectable, and to detect presence of residual or recurrent tumor after treatment. The main pretreatment imaging features that indicate tumor unresectability include extensive vascular involvement by tumor, peritoneal spread of tumor, and presence of distant metastatic disease. Complete surgical resection is the key prognostic factor of clinical outcome in patients with retroperitoneal sarcoma.
8
What are the CT and MR imaging features of a retroperitoneal sarcoma?
Retroperitoneal sarcoma typically presents as a large retroperitoneal mass that may contain fat, soft tissue, myxoid tissue, cystic/necrotic change, hemorrhage, and/or calcification ( Figure 37-3 ). Fatty components have attenuation and signal intensity properties similar to macroscopic fat located elsewhere in the body and when present indicate presence of a liposarcoma. Soft tissue components have soft tissue attenuation and in general low-intermediate T1-weighted and intermediate-high T2-weighted signal intensity relative to skeletal muscle with variable amounts of enhancement. Myxoid tissue and cystic/necrotic change both have fluid attenuation and very high T2-weighted signal intensity, although the former enhances, whereas the latter does not. Hemorrhagic areas typically show soft tissue attenuation (30 to 70 HU) and high T1-weighted signal intensity but do not enhance. Calcifications appear as very high attenuation foci on CT and are sometimes seen as very low signal intensity foci although they are much more difficult to visualize on MRI.
9
What CT and MR imaging features favor a liposarcoma over a lipoma?
Large size (>10 cm), thick septa (>2 mm), nodular and globular areas, nonadipose masslike areas, and decreased percentage of fat composition (<75% fat) favor liposarcoma.
10
What clinical and imaging features favor presence of a retroperitoneal leiomyosarcoma?
Retroperitoneal leiomyosarcoma occurs more commonly in women, whereas other retroperitoneal sarcomas occur more commonly in men or with similar frequencies in men and women. In addition, retroperitoneal leiomyosarcoma is often found in a location adjacent to or within the lumen of the inferior vena cava. Presence of these features may therefore favor this particular type of retroperitoneal sarcoma.
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