Mesenchymal and Miscellaneous Lesions of the Uterus

Clinical Features

Leiomyoma is the most common neoplasm in women; approximately 70%–75% of resected uteri for benign conditions harbor this tumor type. The frequency slowly increases with age: ∼20%–30% of women under 30 years have leiomyomas, compared to 40%–50% of women 40 years or older. Black women are more likely to be affected in whom they tend to be larger and more numerous. In spite of their high frequency, only a subset (∼25%) are symptomatic; large and submucosal lesions are more likely to produce symptoms. They are a common source of menorrhagia, dysmenorrhea, and infertility in reproductive-age women and postmenopausal bleeding, urinary incontinence, and prolapse in postmenopausal women. Large intramural and subserosal leiomyomas may present with pelvic pressure or bloating. Much less frequent clinical findings include secondary erythrocytosis, uterine rupture, uterine infection, and ascites (pseudo-Meigs syndrome).

Clinically, large and bulky leiomyomas can be palpated on clinical pelvic and abdominal examination. On imaging, they are well-demarcated solid nodules. Although the primary imaging modality is transvaginal ultrasound, magnetic resonance imaging (MRI) offers higher specificity as it can better identify degenerative changes (hemorrhage, pseudocystic degeneration, ischemic-type change) and features concerning for malignancy (signal heterogeneity, tumor cell necrosis, irregular borders).

Pathologic Features

Gross Findings

Leiomyomas range in size from <1 cm to >25 cm. When multiple, they are randomly distributed throughout the uterus. The conventional macroscopic appearance is of a well-demarcated, nonencapsulated spherical or ovoid mass situated in the uterine wall. Depending on its location, the lesion can bulge into the endometrium and be accompanied by mucosal ulceration and hemorrhage or toward the peritoneal cavity, where they may be pedunculated and attached to the wall by a thin stalk. In the latter scenario, the leiomyoma sometimes detaches, leaving it free within the peritoneal cavity; subsequent adhesion to bowel or other abdominal or pelvic structures leads to the phenomenon of “parasitic leiomyoma.” Leiomyomas grow under the tension exerted by the uterine wall; so, upon sectioning, such tension is released and the cut surface of the tumor protrudes from the wall and the tumor shells out quite easily from the surrounding myometrium. The cut surface is usually rubbery, with a firmer consistency compared to the normal myometrium. It has a white-to-grey color and a vaguely trabeculated or whorled visual aspect ( Fig. 11.1 ).

A spectrum of degenerative changes can be seen in uterine leiomyomata. Ischemic infarction with superimposed hemorrhage has been associated with pregnancy and hormonal treatment; it is seen macroscopically as diffuse red soft tissue (carneous, “beefy” degeneration) ( Fig. 11.2 ). Localized ischemic change and hemorrhage can also lead to pseudocystic degeneration and calcifications. Focal infarction can be soft yellow or densely firm (scar tissue), depending on the evolution of the necrotic area over time ( Fig. 11.3 ). In the context of previous arterial embolization, these focal ischemic changes can be seen in association with visible foreign material within uterine and parametrial vascular spaces.

Leiomyoma variants have certain peculiar macroscopic characteristics. Highly cellular leiomyoma and epithelioid leiomyoma are soft rather than firm or rubbery (given their hypercellularity and lesser amount of extracellular matrix) ( Fig. 11.4 ). Myxoid leiomyoma is also soft and often gelatinous (due to the abundance of myxoid stroma). Lipoleiomyoma has a heterogeneous cut surface with soft yellow areas (adipose tissue) admixed with firm rubbery areas (smooth muscle tissue) ( Fig. 11.5 ). Dissecting leiomyoma has focal worm-like protrusions into the surrounding wall, sometimes into vascular spaces (namely, leiomyoma with intravascular intrusion). The term cotyledonoid leiomyoma applies when most of the tumor outline displays a worm-like or bulbous appearance resembling the cotyledons on the maternal surface of the placenta ( Fig. 11.6 ). Diffuse leiomyomatosis is characterized by diffuse thickening of the myometrium by innumerable small (<1 cm) tumorlets.

Upon careful macroscopic examination, if all lesions show features of conventional leiomyoma as described earlier, partial representative sampling is sufficient. Popular grossing references recommend up to three representative sections. In my practice, we submit up to one-to-three sections if the largest leiomyoma is 5 cm or less in diameter and two sections per each 5 cm of the diameter of the largest leiomyoma if more than 5 cm. Any lesion with unusual findings, including degenerative-type change, soft consistency, hemorrhage, and necrosis, requires more extensive sampling (one section per centimeter of the tumor diameter is suggested) which should also include the border with the myometrium.

Microscopic Findings

Conventional leiomyomas appear as well-circumscribed lesions with cellularity closely approaching that of normal myometrium (normocellular). They are typically well demarcated from the surrounding myometrium but sometimes can have irregular interdigitation. Submucosal leiomyomas may show surface endometrial ulceration or, more commonly, thinning of the endometrial lining with paucity or absence of glands. In curettage specimens, this is seen as fragments of endometrium devoid of glands and composed only of normal stroma and surface epithelium (“aglandular functionalis”). Leiomyomata are characteristically composed of fascicles of smooth muscle that intersect each other at different angles and are closely apposed and form whorls, which is distinct from the orderly and more evenly spaced fascicle distribution of the myometrium ( Fig. 11.7A ). Each fascicle represents a bundle of fusiform cells with abundant eosinophilic cytoplasm and elongated “cigar-shaped” nuclei (based on the round, tapered ends on each side), which are classic morphologic features of smooth muscle differentiation. The spindle shape of the cells and their nuclei can be appreciated if the fascicle is cut longitudinally; fascicles sampled transversally in the histologic section contain nuclei with a round to ovoid shape (corresponding to their short axis), often with a paranuclear vacuole. Nuclei are monotonous and do not vary significantly in size or shape and the chromatin pattern is uniform, ranging from pale to hyperchromatic ( Fig. 11.7A , inset). Nuclear palisading is not uncommonly encountered, sometimes to such a degree that the tumor may resemble a Schwannoma. The blood vessels present within leiomyoma are variably sized; the presence of large and thick-walled vessels dispersed throughout is characteristic ( Fig. 11.7B ). The extracellular matrix is collagenous and usually indistinct but can be more prominent in between fascicles. Dense collagen deposition is seen in areas of resolving ischemia as eosinophilic, “hyalinized” bands or patches ( Fig. 11.7C ). Sometimes, collagen is more nodular and aggregated around vessels.

Ischemic-type necrosis, degenerative in nature, has a distinct microscopic appearance. If recent, the devitalized areas are eosinophilic but retain the outlines of the tumor nuclei as well as the vascular network. In the ischemic area, the transition occurs over a broad zone featuring a gradient progression of viable to nonviable cells, with the tumor cells increasingly separated by collagen, hemorrhage, and granulation tissue toward the infarcted zone ( Fig. 11.8A ). In old, healed infarcts, the devitalized area is largely replaced by dense collagen, and the gradient zone is more attenuated. Sprinkled histocytes with golden to yellow cytoplasmic granules are sometimes seen; the granules represent cysteine crystals and result from degradation of the infarcted tissue. Another type of degenerative change is the so-called hydropic degeneration, in which significant edema separates the muscle bundles and causes pseudocyst formation. The hydropic areas are hypocellular and often contain fibroblasts and inflammatory cells ( Fig. 11.9 ).

Changes induced by pregnancy and medical treatment: In the setting of pregnancy or high-dose exogenous progestin treatment, leiomyomas undergo hyalinization, myxoid change, and hemorrhage ( Fig. 11.10 ). The tumor cell population appears plumper due to increased amounts of cytoplasm. Hypercellularity, nuclear pleomorphism, and mitotic activity (up to 9 mitoses/10 high-power fields [HPFs]) can be seen focally, around zones of hemorrhage and/or myxoid change. Importantly, the proliferation should be restricted to the periphery of degenerated areas. The walls of large muscular vessels also undergo fibrosis and myxoid change. Hemorrhage can be quite extensive, corresponding to the so-called “carneous degeneration” seen macroscopically.

Medical treatment with gonadotropin releasing hormone agonists (GnRHa) has been associated with ischemic-type necrosis, hyalinization, inflammation, and apoptosis; increased mitotic activity has also been reported shortly after discontinuation of this treatment. Ulipristal acetate is a selective progesterone receptor (PR) modulator increasingly used in the management of symptomatic leiomyomas; however, the morphologic changes induced by this family of agents have not been extensively studied. Like GnRHa, ulipristal treatment leads to shrinkage of the tumor; it has been shown that this occurs due to induction of cell apoptosis and remodeling of the extracellular matrix. Changes observed include sclerosis, hyalinization, vascular necrosis, apoptotic bodies, and low Ki67 index.

Leiomyomas resected after previous uterine arterial embolization with polyvinyl alcohol or trisacryl gelatine microspheres display ischemic-type necrosis of varying amounts and evolution. The embolic material is often found in venous vascular spaces in the uterus, parametria, and adnexal soft tissue, associated with foreign body–type giant cell reaction ( Fig. 11.11 ). Another therapeutic technique is MRI-guided thermal ablation using ultrasound waves that induce necrosis and subsequent tumor shrinkage. This type of ablation can cause necrosis of the geographic coagulative type, which, outside of this clinical context, would be a worrisome finding. Thus, it is important to correlate this finding with the clinical history and to exclude the presence of atypia and mitoses.

Leiomyoma variants: Morphologic variants of uterine leiomyoma are defined by the presence of one unusual histologic feature. Their recognition is important, as some of these features are used in the diagnosis of leiomyosarcoma. However, in isolation, they do not portend a malignant diagnosis. These include:

• Leiomyoma with bizarre nuclei (LMBN) (discussed in the next section).

• Mitotically active leiomyoma : Tumors with mitotic counts of 5 to 15 mitoses/10 HPFs are classified as “mitotically active”. In the absence of significant nuclear atypia and tumor cell necrosis, these tumors behave indolently. The increased proliferation may be related to ischemia or progesterone effect (either endogenous or exogenous), supported by the fact that this leiomyoma variant is almost exclusively seen in reproductive-age women.

• Cellular and highly cellular leiomyoma : These tumors are defined as showing cellularity that is higher than normal myometrium (cellular) or which is reminiscent of the cellularity of a low-grade endometrial stromal neoplasm (highly cellular) ( Fig. 11.12 ). They otherwise show conventional features including fascicular growth, low to absent mitoses, no atypia (e.g. nuclear enlargement, hyperchromasia, irregular nuclear contour), and no tumor cell necrosis. Characteristic features of the highly cellular variant include large thick-walled vessels and cleft-like spaces.

  

• Dissecting leiomyoma : This variant has a focal or more extensive irregular interface with the surrounding myometrium characterized by worm-like or nodular bulbous protrusions ( Fig. 11.13A ). This permeative growth can, on occasion, extend into the parametrial soft tissue and broad ligament. When striking, the term “cotyledonoid leiomyoma” has been employed. The nodules are composed of disorganized bundles of smooth muscle which typically have a “swirled” rather than fascicular configuration as well as a prominent vascular component, which is accentuated at the periphery of the nodules due to the commonly associated hydropic change ( Fig. 11.13B ). A related phenomenon is leiomyoma with intravascular intrusion , in which an otherwise conventional leiomyoma permeates into the lumen of an adjacent vessel. In all these scenarios, the tumor has bland cytomorphology few mitoses, and lacks coagulative necrosis and infiltrative (destructive) invasion.

  

• Epithelioid leiomyoma : Epithelioid morphology (resembling an epithelial tumor) is not an infrequent finding in otherwise conventional (spindle cell) leiomyomas. Sometimes, leiomyomas have focal areas in which there are anastomosing cords of cells containing scant clear cytoplasm set within a collagenous matrix; in this scenario, the epithelioid appearance is imparted by the abundant matrix and does not represent “true” epithelioid change. Leiomyomas with this appearance have been termed “plexiform leiomyomas,” and when small, “plexiform tumorlets”; in the latter, the nodules are ≤1 cm. The term epithelioid leiomyoma should be applied if epithelioid morphology, in the form of solid aggregates, cords, and trabeculae of cells with round to ovoid shape with moderate to abundant eosinophilic cytoplasm and a central round nucleus, is present in ≥50% of tumor cells ( Fig. 11.14 ). This morphology extends beyond that expected for cells that appear rounded due to excess collagenous matrix or a cross-sectioned smooth muscle fascicle. By definition, epithelioid leiomyoma should have smooth, noninfiltrative borders and a mitotic count of less than 5 mitoses/10 HPFs. It should also lack significant nuclear atypia and geographic tumor cell necrosis.

  

• Myxoid leiomyoma : Myxoid change is defined as accumulation of an extracellular matrix rich in glycosaminoglycans (particularly hyaluronic acid), which imparts a soft gelatinous microscopic appearance and a pale blue-violet tinctorial quality in routine histologic preparations. Myxoid change often occurs in leiomyomas focally, commonly associated with ischemia and hemorrhage, and therefore believed to represent a degenerative change. The term myxoid leiomyoma should be reserved for lesions with myxoid stroma involving most of the tumor volume (an arbitrary designation of ≥50% has been proposed). This tumor category is exceedingly rare and believed to have a different histogenesis, although not much is known about it. This variant requires strict criteria for its diagnosis, which includes absence of tumor infiltration into the surrounding myometrium (after extensive sampling), and lack of significant atypia and tumor cell necrosis. Mitoses should be absent, or at most 1 in 10 HPFs, without atypical forms.

• Leiomyoma with heterologous elements : The most common is lipoleiomyoma, in which neoplastic smooth muscle is admixed with mature adipose tissue. Other heterologous tissues encountered include skeletal muscle, cartilage, bone, and bone marrow (extramedullary hematopoiesis).

• Angioleiomyoma (vascular leiomyoma): The tumor displays multiple, closely packed, often thick-walled blood vessels.

• Diffuse leiomyomatosis : In this rare condition, the uterus is extensively involved by smooth muscle tumorlets, each measuring ≤1 cm and composed of bland, but often cellular, spindled smooth muscle cells that may have an ill-defined border. Perivascular aggregation can be observed.

Other very rare findings in uterine leiomyomas include collections of inflammatory cells (lymphocytes, mast cells, eosinophils, or hystiocytes) and abscess formation (usually after traumatic or spontaneous tumor rupture).

Ancillary Studies

Differential Diagnosis

Diagnosis of conventional leiomyoma is straightforward. Distinction between conventional and variant forms is usually not problematic and is clinically inconsequential, as they all behave indolently. However, it is very important to become familiar with variant morphology in leiomyomas, in order not to classify them as leiomyosarcoma or other tumor types.

The most important difference between mitotically active leiomyoma and leiomyosarcoma is the absence of other worrisome features in the former (atypia, geographic necrosis). Of note, bland tumors with mitoses exceeding 15 per 10 HPFs are currently categorized as STUMP, since experience in this scenario is limited.

Highly cellular leiomyomas require distinction from an endometrial stromal nodule (ESN), which features similar degrees of cellularity. Both are well-demarcated tumors. The most helpful features in this differential are the presence of fascicular growth, thick-walled vessels throughout and strong smooth muscle marker expression in leiomyoma, and the evenly distributed, fine arteriolar vasculature and weak to absent smooth muscle marker staining of endometrial stromal neoplasms. Although CD10 is nonspecific, this marker is helpful if diffuse and strong in the absence of smooth muscle marker expression.

Given their peculiar growth, dissecting and cotyledonoid leiomyomas require distinction from endometrial stromal sarcoma . Dissecting leiomyoma may display high cellularity resembling low-grade endometrial stromal sarcoma (LG-ESS) and YWHAE-NUTM2 high-grade endometrial stromal sarcoma (HG-ESS); however, the tumor cells are fascicular and contain thick vessels. If the leiomyoma has superimposed myxoid or other degenerative change, BCOR -altered HG-ESS may be considered; however, these tumors lack prominent fascicular growth and typically have weak to absent smooth muscle marker expression. Leiomyosarcoma also deserves consideration; nonetheless, the pattern of infiltration of leiomyosarcoma is less often permeative and more frequently destructive, and overt malignant features (atypia, proliferation, necrosis) are present.

Epithelioid and myxoid leiomyoma variants require strict adherence to diagnostic criteria, particularly lack of infiltration and low to absent mitoses (less than 5 mitoses/10 HPFs for epithelioid and less than 2 mitoses/10 HPFs for myxoid). The presence of any worrisome feature should prompt consideration for epithelioid leiomyosarcoma or myxoid leiomyosarcoma , or at least STUMP (see corresponding sections). Perivascular epithelioid cell tumor (PEComa) can significantly overlap with epithelioid leiomyoma; it is typically cellular and contains epithelioid cells in tight aggregates and cords; moreover, it often expresses smooth muscle markers by immunohistochemistry. The presence of clear cytoplasm is a helpful clue for PEComa, and HMB45, Melan-A and cathepsin-K should be considered; importantly, HMB45 and cathepsin-K can be positive in leiomyomas, the latter not typically diffuse as is seen in PEComa. Other lesions with epithelioid morphology include epithelioid trophoblastic tumor and placental site trophoblastic tumor ; these proliferations have distinct cytomorphology and Ki67 proliferation index exceeding 5%–10%, which are helpful in excluding epithelioid leiomyoma (see Chapter 19 ). Inflammatory myofibroblastic tumor (IMT) enters in the differential of leiomyomas with myxoid stroma; there is significant overlap, as IMT often contains a smooth muscle-like component, and it is believed that many of these lesions are misinterpreted as leiomyoma. In myxoid areas, IMT features plump fusiform cells individually dispersed in a tissue culture-like pattern, different from the bundled arrangement of leiomyoma. Immunohistochemistry for anaplastic lymphoma kinase (ALK) and molecular testing for ALK rearrangements should be considered if a lesion has significant myxoid stroma, prominent inflammation, and/or spindle cells with wavy, pointed nuclear ends instead of the tapered end of the smooth muscle cell nucleus).

Prognosis and Treatment

As defined earlier, leiomyoma and leiomyoma variants are benign tumors. Definitive management of uterine leiomyomata is with simple hysterectomy. More conservative approaches, however, are attempted first in most patients and often achieve optimal symptom control. Pharmacologic treatment includes progestins, oral contraceptives, aromatase inhibitors, GnRHa, and selective ER or PR modulators. Medical interventions include uterine arterial embolization, MRI-guided ultrasonic ablation, and laparoscopic or vaginal myomectomy. The latter procedure results in a tissue specimen in which a correct diagnosis of leiomyoma (conventional or variant) and the exclusion of lesions with known or unknown malignant potential, is most critical. Unfortunately, many of these myomectomy procedures morcellate the lesion(s), precluding optimal evaluation of the tumor size, tumor border, and completeness of the excision. Moreover, morcellation with powered devices has been documented to potentially result in peritoneal dissemination of the neoplasm. It has been estimated that the 5-year recurrence rate of leiomyomas after myomectomy can be as high as 60%. Other complications of myomectomy include amenorrhea, infection, sepsis and uterine rupture (sometimes months to years after, or in the setting of pregnancy).

Gross Findings

Most tumors are macroscopically identical to a conventional leiomyoma. Soft and yellow appearance, cystic change, and hemorrhage have also been observed.

Microscopic Findings

The defining feature of LMBN is the presence of cells with atypical nuclei. The atypia needs to be appreciable from low (4× objective) magnification. Atypia is defined as nuclear enlargement, multinucleation, hyperchromasia or coarse chromatin, and conspicuous nucleoli ( Fig. 11.15A,B ). Importantly, the distribution of the atypical (“bizarre”) cells is often diffuse (30%) or multifocal (44%); only in 26% of the cases bizarre cells represent a focal finding. Mitotic activity is low, ranging from 0 to 7 mitoses/10 HPFs. Necrosis can be observed, but only of the ischemic type. Rhabdoid cell morphology has been reported in 44% of cases.

LMBNs in the context of HLRCC syndrome or abnormal FH/2SC expression (HLRCC-like; see later discussion) are associated with certain particular histologic features, including round bright eosinophilic cytoplasmic inclusions, nuclei containing prominent eosinophilic nucleoli surrounded by a halo, edema with an “alveolar” distribution pattern and staghorn vessels ( Fig. 11.16A,B ).

Immunohistochemistry

LMBN expresses smooth muscle markers and hormone receptors. Abnormal p53 staining can be seen. LMBNs with FH mutations (either germline in the context of HLRCC syndrome, or somatic) exhibit loss of FH (due to inactivating mutations) and positivity for 2SC (due to abnormal accumulation) by immunohistochemistry. Unfortunately, the latter marker is not available for routine use.

Molecular Analysis

The clinical and morphologic similarities between LMBN and conventional leiomyoma contrast with the differences in their genomic profile. LMBN often has a high index of copy number variations, resulting in a high (>10) genomic index by comprehensive genomic hybridization (similar to leiomyosarcoma). Furthermore, miRNA profiling has shown a closer resemblance of LMBN to STUMP and leiomyosarcoma than leiomyoma. Recent comprehensive analysis has shown that LMBN can be divided in two groups: one with abnormal FH/2SC expression abnormalities and FH mutations, which correlate with HLRCC-like morphology (staghorn vasculature in particular), and a second with normal FH/2SC results, absence of FH mutations, and higher prevalence of TP53 and/or RB1 alterations.

Gross Findings

Leiomyosarcoma of the uterus is often large and bulky, with an average size of 10 cm. The tumor is usually solitary, seen as an isolated uterine lesion or less commonly in a background of smaller, conventional leiomyomas. The tumor contour can be smooth, but it is more often irregular or multinodular and difficult to separate from the adjacent wall (in contrast to leiomyoma, which is easily shelled out). Upon sectioning, leiomyosarcoma is heterogeneous, with rubbery tan foci alternating with softer, yellow areas as well as hemorrhage, necrosis, and pseudocystic cavitation ( Fig. 11.17 ). Epithelioid leiomyosarcomas can be predominantly or exclusively soft and yellow. Myxoid leiomyosarcoma is typically irregular and infiltrative, soft, tan to yellow, and gelatinous in consistency.

Microscopic Findings

Leiomyosarcoma of the uterus can be divided into three main morphologic types: conventional (spindle cell), epithelioid, and myxoid, although combinations can be seen. Conventional (spindle cell) leiomyosarcoma is by far the most frequent. It is characterized by high cellularity and border irregularity at low power magnification ( Fig. 11.18A ). The interface can be frankly infiltrative or less often simulate the nodular, myopermeative growth of endometrial stromal tumors. The neoplasm is composed of cells organized in fascicles and appear elongated (spindled) or round to ovoid depending on the plane of section. The tumor cell population displays at least two of the following features, which have been proven to be associated with malignant behavior:

• Tumor cell necrosis: necrosis in the context of malignancy is a reflection of rapid tumor growth and genomic instability. The transition between viable and nonviable areas is sharp and lacks significant collagen deposition and inflammation that are common in ischemic change (which has a slower evolution over time, and tissue inflammation and repair are more often visualized). The resulting appearance of tumor cell necrosis is described as geographic (because the margin between viable and necrotic tissue is sharp and convoluted) ( Fig. 11.19A,B ) .

  

• Cytologic atypia: significant atypia (moderate to severe in degree) in the context of uterine smooth muscle neoplasia requires nuclear hyperchromasia and enlargement evident at low power magnification that is multifocal to diffuse in distribution ( Fig. 11.20 ). Minimal variation in nuclear size appreciable only at high power magnification represents only mild atypia, and is not significant. Multinucleated tumor cells are present in >50% of cases.

  

• Mitotic activity: a proliferation rate of 10 or more mitoses in 10 HPFs (400× magnification) is regarded as indicative of malignancy, if atypia and/or necrosis are also present ( Fig. 11.21A ). The mitotic count requires scanning at least 50 HPFs, choosing the areas in which proliferation appears higher, but avoiding tumor adjacent to ischemic change and hemorrhage. The highest count in 10 HPFs (“hotspot”) should be taken as the final count. The presence of atypical mitotic figures should be noted, as they also suggest malignancy. It is important to identify apoptosis, inflammatory cells, and karyorrhectic debris; misclassification of these as mitotic figures will result in overestimation of proliferation and, potentially, in misdiagnosis of malignancy ( Fig. 11.21B ).

  

Lymphatic and venous vascular invasion is identified in approximately 20% of leiomyosarcomas; occasionally, the intravascular growth exceeds in volume that of the intramural mass itself. Histologic grade of uterine leiomyosarcoma lacks established definitions and correlation with outcome and therefore is not performed; instead, all tumors are inherently considered “high grade” when they show significant cytologic atypia (defining feature as described previously).

Epithelioid leiomyosarcoma is a tumor with significant (≥50%) epithelioid cell morphology, characterized by polygonal to ovoid cells with eosinophilic (or, less often, clear) cytoplasm and central round hyperchromatic nuclei ( Fig. 11.22A ). Vacuolation with nuclear displacement occurs rarely, imparting a lipoblast-like appearance. Hyalinization with a plexiform pattern can be encountered. Definitions for cytologic atypia and tumor cell necrosis in conventional leiomyosarcoma also apply for the epithelioid variant. However, in contrast to spindled leiomyosarcoma, low mitotic counts have been associated with malignant behavior. A threshold of 5 or more mitoses/10 HPFs is accepted by most authors for this epithelioid variant. Thus, the presence of necrosis or ≥5 mitoses/10 HPFs with or without cytologic atypia, warrants a diagnosis of epithelioid leiomyosarcoma. It is important to note that, given the rarity of this tumor subset, the proposed criteria and definitions for malignancy are not well established and require further validation.

Myxoid leiomyosarcoma is also rare; by definition, the myxoid matrix occupies a significant proportion of the tumor volume ( Fig. 11.23A ). It has been hypothesized that the abundant myxoid matrix, rich in glycosaminoglycans, dissects fascicles and separates tumor cells, resulting in lower cellularity and “diluted” mitotic counts ( Fig. 11.23B ). Indeed, myxoid smooth muscle tumors with as little as 2 mitoses/10 HPFs have been associated with malignant course. Moreover, nuclear atypia can be mild and focal. Despite the abundance of myxoid matrix deposition, the tumor retains (at least focally) a fascicular arrangement, and the nuclei display the hyperchromasia and tapered ends seen in conventional tumors. The most consistent feature of uterine myxoid smooth muscle neoplasms with adverse outcomes is the presence of an infiltrative tumor border. Therefore, the diagnosis of myxoid leiomyosarcoma is warranted if the tumor has at least two of the following: infiltrative / invasive edge, moderate or severe atypia, ≥2 mitoses/10 HPFs or tumor cell necrosis. As with epithelioid tumors, the experience with myxoid leiomyosarcoma is limited, and further validation of these criteria is necessary.

Histochemistry

Trichrome and reticulin stains may be useful in the distinction between coagulative and ischemic-type necrosis. In coagulative (geographic) necrosis, there is minimal to no collagen deposition in devitalized areas compared to viable areas, and the necrotic tissue stains red with trichrome stain (instead of blue, which highlights collagen). Moreover, the pericellular reticulin network is retained in necrotic tumor areas ( Fig. 11.19C,D ). As mentioned previously, Alcian blue stain at pH 2.5 is useful to highlight myxoid stroma ( Fig. 11.23B , inset).

Immunohistochemistry

Leiomyosarcoma is positive for SMA, desmin, smooth muscle myosin, and caldesmon ( Fig. 11.18B ). Importantly, myxoid and epithelioid types tend to show less intense positivity than conventional lesions. ER and PR are expressed in most (>90%) uterine leiomyosarcomas; whereas AR is less frequent. CD10 is positive in up to 30% of tumors ( Fig. 11.18C ). C-KIT can also be positive. Keratin and EMA staining may be observed and is more common in epithelioid tumors ( Fig. 11.22B ). ALK is negative in most cases. Abnormal p53 staining (overexpression or complete absence) and p16 overexpression have been documented in ∼50% leiomyosarcomas (conventional and myxoid). Ki67 proliferation index is usually high (>10%) in conventional leiomyosarcoma, but not always in epithelioid and myxoid variants.

Molecular Analysis

Unlike leiomyoma, which has absent or at most simple karyotypic aberrations, leiomyosarcoma is frequently aneuploid with a complex karyotype. The most frequent abnormalities include gain of 1q, 17p, and Xp and loss of heterozygosity of 10q (which involves the PTEN gene) and 13q (involving the RB1 gene). The most commonly mutated genes are TP53 (65%) and VIPR2 (92%). VIPR2 deletions have been associated with poor outcome. Alterations in ATRX and DAXX , which are involved in alternative lengthening of telomeres, have been described in ∼80% of uterine leiomyosarcomas. MED12 mutations are not frequent (∼10%) and differ from those found in leiomyomas (being more frequently truncating instead of hotspot).

Recently, molecular grouping of uterine leiomyosarcoma by gene expression profiling results in two groups: one resembling normal smooth muscle (with overexpression of genes related to smooth muscle function, such as LMOD1 , SLMAP , MYLK , and MYH11 ; associated with morphology in the lower end of the spectrum and good response to chemotherapy) and one with upregulation of epithelial to mesenchymal transition and other oncogenic pathways (including CDK6 , MAPK13 , and HOXA1 upregulation; associated with more overt pleomorphism, invasion and proliferation, and less response to chemotherapy). In addition, it has been shown that clinically malignant smooth muscle tumors have a high genomic index (calculated based on the amount of global copy number variations); in contrast, indolent tumors had a genomic index less than 10 (in keeping with the lack of copy number gains and losses). These findings may open the door to the integration of molecular testing in the diagnosis and prognostic stratification of uterine smooth muscle tumors.

Regarding myxoid leiomyosarcoma, the spectrum of molecular alterations is largely unknown. This tumor type is negative for ALK rearrangements. TP53 and CDK2N mutations have been reported in ∼50 cases. Molecular alterations of epithelioid leiomyosarcoma have not been yet described.

Gross Findings

Tumors classified as STUMP vary in size. Most are grossly indistinguishable from a leiomyoma.

Microscopic Findings

As a diagnostic category, the definitions of STUMP vary among publications (and, likely, among practices). This diagnosis should be considered in the following scenarios:

• Significant cytologic atypia and mitotic activity approaching the threshold for malignancy (in the order of 8–9 mitoses/10 HPFs) in the absence of necrosis ( Fig. 11.24 ). It is recommended to expand the search for mitotic figures to more than 50 HPFs and to perform extensive sampling (at least one section per centimeter of the tumor greatest dimension). The recurrence rate of published cases in this scenario is 11.2%.

  

• Unequivocal coagulative tumor cell necrosis with no significant atypia and <10 mitoses/10 HPFs. This is a very rare scenario. The recurrence rate of tumors with these features is higher than in other STUMP scenarios (26.7%), and some consider necrosis as a sole criterion sufficient for the diagnosis of leiomyosarcoma; however, the reader is cautioned that the reproducibility for recognizing coagulative tumor cell necrosis is low.

• Tumor cell necrosis that is difficult to classify (ischemic versus coagulative). Since tumors with unequivocal coagulative necrosis can recur, the diagnosis of STUMP is warranted if the possibility of coagulative necrosis cannot be confidently excluded.

  Uterine Leiomyosarcoma—Pathologic Features

  Gross Findings

  • □n

    Large mass (mean 10 cm)

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    Solitary or dominant

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    Poorly circumscribed

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    Heterogeneous, soft and fleshy cut surface

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    Frequent necrosis and hemorrhage

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    Epithelioid and myxoid variants: soft, yellow and gelatinous (if myxoid) cut surface

  Microscopic Findings

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    High grade by definition

  Conventional leiomyosarcoma

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    Irregular, infiltrative growth

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    Hypercellular lesion composed of intersecting fascicles

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    Elongated nuclei with tapered ends (“cigar” shape)

  • □n

    Morphologic criteria for malignancy (at least two):

    • □n

      Significant cytologic atypia: moderate or severe, multifocal to diffuse; enlarged hyperchromatic nuclei with coarse chromatin and irregular nuclear membranes

    • □n

      Coagulative tumor cell necrosis: sharp demarcation (“geographic”)

    • □n

      Mitotic activity ≥10 mitoses/10 HPFs: count at least 50 HPFs, choose highest count in 10 fields (disregard apoptosis, karyorrhexis and equivocal figures)

  • □n

    Vascular invasion in 20%

  Epithelioid leiomyosarcoma

  • □n

    ≥50% of cells with epithelioid appearance (polygonal eosinophilic cytoplasm, central ovoid nucleus)

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    Sheets, nests, cords, or trabeculae; rarely pseudoglandular spaces

  • □n

    Central hyalinization common

  • □n

    Significant cytologic atypia (as defined for conventional tumors) may be present

  • □n

    Morphologic criteria for malignancy (any of following):

    • □n

      Coagulative tumor cell necrosis

    • □n

      Mitotic rate ≥5 mitoses/10 HPFs

  Myxoid leiomyosarcoma

  • □n

    Prominent myxoid extracellular matrix

  • □n

    Myxoid matrix separates fascicles and tumor cells, imparting hypocellular appearance

  • □n

    Mitotic activity and nuclear atypia may be subtle or absent

  • □n

    Fusiform hyperchromatic nuclei

  • □n

    Morphologic criteria for malignancy (at least two):

    • □n

      Infiltrative tumor border, cell necrosis, cytologic atypia or ≥2 mitoses/10 HPFs

  Histochemical and Immunohistochemical Features

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    Coagulative tumor cell necrosis lacks collagen deposition and retains pericellular reticulin network

  • □n

    Alcian blue pH 2.5 highlights myxoid matrix

  • □n

    SMA, desmin, caldesmon, smooth muscle myosin positive (strong and diffuse in conventional, less in epithelioid and myxoid variants)

  • □n

    Estrogen, progesterone, and androgen receptors often positive

  • □n

    CD10 and C-KIT can be positive

  • □n

    Keratin and EMA variably positive (more frequent in epithelioid variant)

  • □n

    Abnormal p53 staining and p16 overexpression (∼50%)

  • □n

    High Ki67 index (>10%)

  Molecular Analysis

  • □n

    Frequent complex karyotypic chromosomal aberrations

  • □n

    TP53 and VIPR2 frequently mutated (also RB1 , PTEN )

  • □n

    Molecular subgrouping by gene expression profiling and genomic index (based on copy number variation burden)

  • □n

    Absence of C-KIT mutations and ALK rearrangements

  Differential Diagnosis

  • □n

    Leiomyoma variants

  • □n

    Pleomorphic rhabdomyosarcoma

  • □n

    Low-grade endometrial stromal sarcoma

  • □n

    High-grade endometrial stromal sarcoma

  • □n

    Undifferentiated endometrial sarcoma

  • □n

    Carcinosarcoma

  • □n

    Gastrointestinal stromal tumor

  • □n

    Endometrial carcinoma (endometrioid, serous)

  • □n

    Perivascular epithelioid cell tumor

  • □n

    Placental site trophoblastic tumor

  • □n

    Epithelioid trophoblastic tumor

  • □n

    Inflammatory myofibroblastic tumor

• Mitotic activity exceeding 15 mitoses/10 HPFs, in the absence of significant atypia and necrosis. Although no reported cases in this category recurred, experience is limited, and long-term follow-up is advisable.

• Epithelioid smooth muscle tumors with significant cytologic atypia, but with no necrosis and <5 mitoses/10 HPFs. Also, in the context of a tumor with 5–10 mitoses/10 HPFs and/or cytologic atypia, if epithelioid features are either equivocal or present approaching 50% of the tumor, the diagnosis of epithelioid STUMP should be considered.

• Myxoid smooth muscle tumors with irregular borders but no other malignant feature, and tumors with well-defined borders and one of the following: atypia, ≥2 mitoses/10 HPFs, or necrosis. There is not enough experience in the literature in this scenario; thus, the default to STUMP is suggested. Likewise, if the presence of myxoid change is suspected but cannot be confirmed in a tumor with atypia and/or 2–10 mitoses/10 HPFs, the diagnosis of STUMP can be considered.

• Additional characteristics have been recently suggested. They, when seen in isolation, should warrant consideration for the diagnosis of STUMP. These include atypical mitoses, vascular involvement, and infiltrative margins.

A final diagnosis of STUMP can be made only on hysterectomy and occasionally in myomectomies that are received intact and in which completeness of excision can be assured (which is difficult and rare from the pathology standpoint). Smooth muscle tumors falling into one of the “grey zones” outlined previously in curettage, morcellation, and other fragmented myomectomy specimens should be reported as “smooth muscle neoplasm of at least unknown malignant potential.”

Immunohistochemistry

STUMPs have the same immunoprofile of leiomyomas. The expression of Ki67, p53, and p16 is variable and often not contributory.

Molecular Analysis

About 11% of STUMPs harbor MED12 mutations, much less often than leiomyomas and closer to the reported prevalence in leiomyosarcomas. It has been shown that tumors classified morphologically as STUMP can be classified using the genomic index, calculated based on the number of copy number variations detected by comparative genomic hybridization. The genomic index (with a threshold of 10) separated tumors with indolent outcome from those with recurrence and unfavorable outcome.

Gross Findings

Intravenous leiomyomatosis is characterized by numerous well-defined nodules with firm consistency and whorled appearance filling dilated vascular channels. The nodularities often acquire the shape of the vessel and become elongated and tortuous (worm-like). A leiomyoma is not identified, or if present, is anatomically separate from the intravascular lesions. Benign metastasizing leiomyoma is usually a small (<10 cm) smooth and round lesion embedded in the organ affected (usually the periphery of the lung). Lesions are often multiple.

Microscopic Findings

Intravenous leiomyomatosis : The tumors protrude into vascular channels, often with complete or near-complete occlusion; tumor plugs are covered by bland endothelium ( Fig. 11.25A,B ). Immunohistochemistry for endothelial markers may be required for confirmation. The tumor is otherwise identical to an intramural leiomyoma or leiomyoma variant. The lesions are predominantly or completely intravascular, although some attachment to the wall may be observed. However, the intravascular tumor is not solely within a leiomyoma; if a leiomyoma is seen, it is separate from the intravascular lesions.

Benign metastasizing leiomyoma: The metastatic lesion is morphologically identical to a conventional banal-appearing leiomyoma, although some may be cellular. Extensive or complete sampling is required to exclude features worrisome for malignancy.

Molecular Analysis

Intravenous leiomyomatosis has been associated with t(12;14)(q14.3;q24), which involves the HMGA2 gene. Cytogenetic analysis of cases of benign metastasizing leiomyoma has shown deletions of 19q and 22q, which are not common in uterine leiomyoma and may suggest an independent origin. However, allelic imbalance and chromosome X inactivation patterns highly mirroring uterine leiomyoma have also been documented, suggesting metastasis from a single clone.

Gross Findings

The tumor more often has a submucosal location and can protrude into the endometrial cavity, although it can be present anywhere within the myometrium. Tumor size varies, but most are 10 cm or less in greatest dimension (range 0.5–22, mean 7 cm). They are well demarcated, nonencapsulated lesions that, unlike leiomyomas, do not bulge upon sectioning and have a soft consistency and yellow coloration ( Fig. 11.26A ). Hemorrhage or pseudocystic change can be observed ( Fig. 11.26B ). The interface between the tumor and the surrounding myometrium needs to be carefully examined and sampled extensively or entirely to exclude myoinvasion.

Microscopic Findings

ESN is characterized by well demarcated and smooth borders ( Fig. 11.27A ). Bulging finger-like projections from the overall outer contour can be seen and are allowed for this diagnosis, as long as they are three or fewer in number and each measures less than 3 mm in length ( Fig. 11.27B ). Vascular space invasion is also, by definition, absent.

In conventional ESN, the neoplastic population highly resembles normal proliferative-phase endometrial stroma: it is cellular on low power magnification and composed of uniform cells with very scant cytoplasm and monotonous short-spindled to ovoid nuclei ( Fig. 11.27C ). The latter are two to four times larger than a lymphocyte nucleus; they have smooth membranes, uniform dark chromatin, and inconspicuous nucleoli. The number of mitoses varies but is usually low (less than 10 per 10 HPFs in >80% cases). The vasculature is distinctive and comprised of uniformly spaced, thin arteriole-sized vessels; thick or tortuously dilated vessels are absent, except at the tumor–myometrial interface occasionally. The stromal cells are surrounded by a scant collagenous matrix, which seldom becomes more prominent with hyalinized bands or ischemic-type change. Variant morphology is not uncommonly encountered and includes fibromyxoid, smooth muscle, skeletal muscle, sex cord stromal ( Fig. 11.27D ), and glandular differentiation (all covered in detail in the “Low-Grade Endometrial Stromal Sarcoma” section).

Immunohistochemistry

The neoplastic spindle cell component shows strong and diffuse expression of hormone receptors (ER, PR), CD10, WT1 and IFITM1. SMA is often positive. Desmin can be positive, sometimes in a diffuse distribution. Caldesmon is negative, except in areas of smooth muscle differentiation, which usually show positivity for all smooth muscle markers. Similarly, areas of sex cord differentiation may express SF1, calretinin, and inhibin. Keratin stains can show heterogeneous, nonspecific staining. Cyclin D1 and BCOR are expected to be negative, although these markers have not been tested in large numbers of ESNs.

Molecular Analysis

Approximately 50%–65% of ESNs harbor a recurrent rearrangement of the JAZF1 and SUZ12 genes, resulting in a t(7:17)(p15:q21) fusion. This alteration is also the most frequent in LG-ESSs. Other fusions involving JAZF1 , PHF1 , and other genes, characteristic of LG-ESS, can also be found in ESN.