Clinical Features

Most patients with ES are adolescents or young adults, the majority younger than 30 years. In previous studies that attempted to distinguish ES from PNET, there tended to be a broader age range in PNET, with a significant number of patients older than 40, although the mean ages were similar. There is a slight male predilection, and the disease is rare in non-Caucasians. There is no evidence of familial predisposition or an association with environmental factors. Although some patients treated for ES develop secondary neoplasms, such as radiation-induced osteosarcoma or therapy-related acute myeloid leukemia, ES rarely occurs as a second neoplasm after therapy for another tumor.

ES may arise at virtually any anatomic site but usually occurs in the deep soft tissues of the extremities, most often the upper thigh and buttock, followed by the upper arm and shoulder. Tumors that are intimately attached to a major nerve may give rise to signs and symptoms related to diminished neurologic function. Less frequently, the tumor arises in the paravertebral soft tissues or the chest wall, often in close association with the vertebrae or the ribs ( Fig. 33.1 ). Well-characterized examples of ES, often with molecular confirmation, have been reported in virtually every anatomic site.

Gross photograph of Ewing sarcoma from chest wall of 10-year-old boy. Foci of necrosis are apparent.

In general, the tumor presents as a rapidly growing, deeply located mass measuring 5 to 10 cm in greatest diameter. Superficially located cases occur but are uncommon. The tumor is painful in about one-third of cases. If peripheral nerves or the spinal cord are involved, there may be progressive sensory or motor disturbances. As with other round cell sarcomas, the preoperative duration of symptoms is usually less than 1 year. Unlike neuroblastoma, catecholamine levels are within normal limits. Computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) are a routine part of the evaluation to determine anatomic relationships, the presence of distant disease, and the extent of therapeutic response to (neo) adjuvant therapy.

Pathologic Findings

The gross appearance of the tumor varies. In general, it is multilobulated, soft, and friable, rarely exceeding 10 cm in greatest diameter. The cut surface has a gray-yellow or gray-tan appearance, often with large areas of necrosis, cyst formation, or hemorrhage. Despite the extensive necrosis, calcification is rare ( Fig. 33.1 ).

There is a spectrum of histologic change in the Ewing family of tumors; in the older literature, criteria distinguishing ES, so-called atypical ES (large cell variant), and PNET were varied. In a large clinicopathologic study of this family of tumors, based on morphologic criteria alone, Llombart-Bosch et al. classified 280 cases as conventional ES, 53 as PNET, and 80 as atypical ES. Because these lesions comprise a spectrum of histogenetically related tumors, the precise criteria for designating a tumor as an extraskeletal ES, atypical ES, or PNET are less critical. Therefore, cases across the spectrum can simply be classified as ES. The morphologic spectrum of this family of tumors is now known to include adamantinoma-like cases, keratin-positive tumors, and rare desmin-positive cases. As discussed later, cases with classic morphologic features can be accurately diagnosed using light microscopy with ancillary immunohistochemistry (IHC). However, given the wide morphologic spectrum, genetic confirmation is essential for the diagnosis of unusual morphologic variants, as well as in select circumstances (e.g., unusual immunophenotype, entry into clinical trial).

The histologic features of classic ES include a solidly packed, lobular pattern of strikingly uniform round cells ( Figs. 33.2 to 33.10 ). The individual cells have a round or ovoid nucleus with a distinct nuclear membrane, fine powdery chromatin, and one or two inapparent or small nucleoli. Multinucleated giant cells are not seen. The cytoplasm is poorly defined, scanty, pale staining, and in many cases irregularly vacuolated as a result of intracellular deposits of glycogen ( Figs. 33.5 and 33.6 ). Intracellular glycogen is present in most cases, but the amount varies from tumor to tumor and sometimes in different portions of the same neoplasm. Glycogen droplets may compress and indent the nucleus ( Fig. 33.6 ). The number of mitotic figures varies, and, in many cases, the paucity of mitotic figures contrasts with the immature appearance of the neoplastic cells.

A, Low-power view of extraskeletal Ewing sarcoma characterized by lobular round cell pattern of striking uniformity. B, Typical low-power view of the monotonous appearance of this tumor.

Extraskeletal Ewing sarcoma composed of sheets of relatively uniform round cells.

A and B, Primitive cells in Ewing sarcoma with finely stippled chromatin pattern.

Ewing sarcoma with clear cytoplasm secondary to glycogen deposits.

A and B, Cytoplasmic vacuoles secondary to deposition of intracellular glycogen often indent the nuclei in Ewing sarcoma.

Hemorrhagic zone in extraskeletal Ewing sarcoma resembling a vascular neoplasm.

Alveolar pattern in Ewing sarcoma caused by degeneration.

Necrosis and crushed cells in Ewing sarcoma.

Zonal necrosis in Ewing sarcoma.

Although the tumor is richly vascular, the thin-walled vessels are compressed and obscured by the closely packed tumor cells; this rich vascularity is often discernible only in areas of degeneration and necrosis ( Fig. 33.8 ). The association of distinct vascular structures with degenerated or necrotic ghost cells is a common, striking feature ( filigree pattern ) ( Fig. 33.9 ). On occasion, ES may show a pseudovascular or pseudoalveolar pattern caused by small, fluid-filled pools or blood lakes amid the solidly arranged tumor cells ( Fig. 33.7 ). This feature occasionally is misinterpreted as evidence of angiosarcoma or alveolar rhabdomyosarcoma by those unfamiliar with this secondary change. Approximately 75% show these classic morphologic features. A minority of cases display moderate nuclear enlargement, irregular nuclear contours, and frequently prominent nucleoli, corresponding to the atypical or large cell variant of ES ( Figs. 33.11 and 33.12 ).

A and B, Ewing sarcoma composed of sheets of larger cells, so called large cell or atypical Ewing sarcoma.

High-power view of large cell Ewing sarcoma.

Perhaps the most difficult subtype of ES to recognize is the adamantinoma-like variant, described originally by Bridge et al. Adamantinoma-like tumors have accounted for up to 5% of cases in some consultation-based series of ES but are much less common in routine practice. These tumors show a distinctly nested, epithelioid growth pattern with striking stromal desmoplasia ( Fig. 33.13 ). The nests of tumor cells may display prominent peripheral nuclear palisading and contain large polygonal cells with irregularly contoured, hyperchromatic nuclei, prominent nucleoli, and moderate amounts of cytoplasm. Rarely, squamous pearls may be seen. Immunophenotypically, in addition to expressing markers of conventional ES (e.g., CD99, NKX2.2), adamantinoma-like ES typically show strong, uniform expression of pankeratins, including high-molecular-weight (HMW) keratins, as well as the basal cell/squamous cell–associated marker p40. Adamantinoma-like tumors most often harbor the EWSR1-FLI1 gene fusion.

A, Adamantinoma-like Ewing sarcoma with keratin immunoreactivity ( B ).

Approximately 15% of cases correspond to the previous designation of PNET and are composed of sheets or lobules of small round cells containing darkly staining, round or oval nuclei ( Figs. 33.14 and 33.15 ). The cytoplasm is indistinct, except in areas where the cells are more mature and the elongated, hairlike cytoplasmic extensions coalesce to form rosettes. Most of the rosettes are similar to those seen in neuroblastomas and contain a central solid core of neurofibrillary material ( Homer Wright rosette ). Rarely, the rosettes resemble those of retinoblastoma and contain a central lumen or vesicle ( Flexner-Wintersteiner rosette ). Rare cases of ES with extensive neural differentiation, including neuropil and ganglion cells resembling ganglioneuroblastoma, have been described. Some tumors are composed of cords or trabeculae of small round cells. These areas resemble a carcinoid tumor or a small cell undifferentiated carcinoma. However, histogenetically, they are properly compared to primitive neuroepithelium. Rarely, ES may show evidence of cartilaginous or osseous differentiation.

A and B, Ewing sarcoma with distinctive lobular architecture and numerous Homer Wright rosettes apparent at low magnification.

A and B, Ewing sarcoma with Homer Wright rosettes.

Immunohistochemical Findings

For many years, the diagnosis of ES was essentially an immunohistochemical diagnosis of exclusion. Beginning in the early 1990s, however, numerous studies confirmed the utility of the product of the MIC2 gene (CD99) in recognizing this group of tumors, confirming the high sensitivity of this marker for ES ( Fig. 33.16 ). The MIC2 gene is a pseudoautosomal gene located on the short arms of the sex chromosomes; its product is a membranous glycoprotein (CD99) that can be detected on IHC using a variety of antibodies, including 12E7, HBA71, and O13. Although initially believed to be highly specific for ES, it is now well recognized that most other round cell tumors in the differential diagnosis occasionally show membranous immunoreactivity for CD99 ( Table 33.1 ). These include lymphomas (particularly T-lymphoblastic lymphoma and precursor B-lymphoblastic lymphoma ), Merkel cell carcinoma, small cell carcinoma, rhabdomyosarcoma, small cell osteosarcoma, desmoplastic small round cell tumor, mesenchymal chondrosarcoma, and CIC -rearranged sarcomas. Notably, childhood neuroblastomas do not express CD99. Given the high sensitivity but poor specificity of CD99, it is best considered a “screening marker” for ES and should always be used as part of a panel of immunostains. The finding of patchy (or absent) CD99 expression in a “Ewing-like” round cell sarcoma should always raise concern for a CIC- or BCOR -rearranged tumor (see later).

Strong membranous CD99 immunoreactivity in Ewing sarcoma.

Many ES also express neural markers, including neuron-specific enolase (NSE), CD57, S-100 protein, synaptophysin, and protein gene product 9.5 (PGP9.5). However, these markers lack specificity and are not particularly helpful in arriving at a diagnosis. The degree of immunohistochemical expression of neural markers has not been found to be predictive of clinical behavior, nor has their expression been found to be related to the specific EWSR1 gene fusion type.

Expression of epithelial markers in ES, in particular keratins, is a recognized diagnostic pitfall. Approximately 25% of tumors show aberrant keratin expression, usually confined to a small number of cells, and often showing a dotlike pattern of immunoreactivity. Typical ES expresses only low-molecular-weight (LMW) keratins. In contrast, the adamantinoma-like variant may show diffuse expression of keratins, including HMW isoforms, probably reflecting the complex epithelial differentiation seen in this variant. Adamantinoma-like ES may also express p40. Aberrant desmin expression is much less common in ES, present in only about 2% of cases. Desmin-positive Ewing sarcomas lack expression of MyoD1 and myogenin, distinguishing them from alveolar rhabdomyosarcoma.

As detailed later, FLI1, a member of the ETS family of DNA-binding transcription factors, is involved in the t(11;22) (EWSR1-FLI1) translocation frequently observed in ES. Polyclonal and monoclonal antibodies to FLI1 protein have been reported to show nuclear immunoreactivity in 70% to 94% of ES, including cases showing the less common EWSR1-ERG fusion gene. FLI1 immunoreactivity in EWSR1-ERG –positive cases likely reflects homology between the ERG and FLI1 proteins. IHC for FLI1 is not specific for ES and is also frequently positive in lymphoblastic and other non-Hodgkin lymphomas as well as in rare examples of melanoma, Merkel cell carcinoma, and neuroblastoma. Antibodies to ERG are more specific for tumors with ERG rearrangement, showing only rare positivity in cases with FLI1 rearrangements.

More recently, a number of studies have confirmed the utility of NKX2.2 as a marker of ES. NKX2.2 is a transcription factor that plays a role in neuronal development and glial or neuroendocrine differentiation and appears to be a downstream target activated by the EWSR1-FLI1 fusion. Immunoreactivity with NKX2.2 has been reported in up to 93% of ES cases, including those with EWSR1-FLI1 and EWSR1-ERG fusions, with most cases showing strong and diffuse nuclear staining. However, many other tumors in the differential diagnosis are also occasionally positive for this marker, including most mesenchymal chondrosarcomas and olfactory neuroblastomas, as well as rare CIC -rearranged sarcomas, poorly differentiated synovial sarcomas, neuroblastomas, and small cell carcinomas.

Given the therapeutic success of imatinib mesylate (Gleevec) in the treatment of gastrointestinal stromal tumor (GIST), there has been considerable interest in evaluating the expression of CD117 in other tumors, including ES. The frequency of CD117 expression in this tumor varies considerably, ranging from 20% to 71%. Very few cases of ES have been evaluated for KIT mutations, and the significance of CD117 expression in this tumor type is unclear.

Cytogenetic and Molecular Genetic Findings

The defining feature of ES is the presence of nonrandom translocations leading to the fusion of the EWSR1 gene on 22q12 with one of several members of the ETS family of transcription factors ( Fig. 33.17 ). The EWSR1-ETS gene fusions encode chimeric transcription factors that activate or repress target genes, as well as a number of epigenetic events that are central to the pathogenesis of ES. The most frequent of these translocations is t(11;22)(q24;q12), which is detected in approximately 85% of cases, and results in fusion of the 3′ end of the FLI gene on 11q24 with the 5′ end of the EWSR1 gene on 22q12. The second most common translocation, which is found in up to 10% of cases, is t(21;22)(q22;q12), leads to the fusion of EWSR1 to ERG at 21q22. Less common alterations (<5% of cases) result in the fusion of EWSR1 to ETV1 at 7p22, ⁷⁶ ETV4 (also known as E1AF ) at 17q12, ⁷⁷ FEV at 2q33, ⁷⁸ and ZSG , resulting in an inv(22), among others. Very rarely, ES may show an EWSR1 fusion with non- ETS family members, including SMARCA5 , SP3 , NFATC2, and PATZ1. A small minority of ES show FUS gene rearrangements instead of EWSR1 rearrangements; such tumors are morphologically and immunohistochemically identical to EWSR1 -rearranged ES. The translocation breakpoints are restricted to introns 7 to 10 of the EWSR1 gene and introns 3 to 9 of the ETS -related genes. Fusion of EWSR1 exon 7 to FLI1 exon 6 ( type 1 fusion) and EWSR1 exon 7 to FLI1 exon 5 ( type 2 ) account for about 85% of EWSR1-FLI1 fusions. Detection of these fusions by molecular genetic techniques (FISH or RT-PCR) using fixed, paraffin-embedded tissues has greatly facilitated the diagnosis of ES. One should keep in mind that EWSR1 and FUS gene rearrangements occur in many other sarcomas (see Chapter 4 ), and these molecular tests should always be interpreted in combination with other morphologic and immunohistochemical findings.

Critical role of EWSR1 gene (22q12) in molecular genesis of sarcomas.

Secondary cytogenetic abnormalities may occur in ES, including trisomy 8, trisomy 12, ⁸⁹ and an unbalanced t(1;16) leading to gain of 1q and loss of 16q. These cytogenetic abnormalities lack sufficient sensitivity and specificity for diagnostic purposes, but some have suggested a prognostic role. In addition, alterations in TP53 and p16/p14ARF are detected in up to 25% of Ewing sarcomas; these alterations are detected in a subset of chemotherapy-refractory tumors, and are associated with an aggressive clinical course.

Clinical Behavior and Therapy

Until the introduction of modern therapy, the outlook for patients with ES was bleak, and only a small percentage of patients with this tumor survived. For example, in the series of extraskeletal ES reported by Angervall and Enzinger in 1975, 22 of the 35 patients with follow-up information died of metastatic disease, usually to the lung and skeleton. Similarly, Jürgens et al. cited a survival rate of approximately 50% at 3 years, whereas Kushner et al. found that only 25% of patients with tumors larger than 5 cm were alive at 24 months. Although several older studies suggested that patients with PNET had a worse prognosis than those with extraskeletal ES, others have not found this to be the case. Parham et al. studied 63 Ewing sarcomas from patients who were treated uniformly to determine the prognostic significance of neuroectodermal differentiation. Tumors were classified as PNET if they showed rosettes or immunohistochemical expression of at least two neural markers (or both). Using another classification scheme, tumors were classified as PNET if they showed rosettes or immunohistochemical expression of at least four neural markers (or both). Finally, using a third classification scheme, tumors that showed ultrastructural evidence of neural differentiation were classified as PNET. Using any of these classification schemes, no significant differences in clinical outcome for patients with or without neuroectodermal differentiation were noted. As such, distinction of ES from PNET is no longer clinically relevant.

The prognosis for patients with ES has steadily improved. About 75% of patients present with localized disease, along with the combination of surgery and/or radiotherapy and systemic chemotherapy results in a cure rate of almost 75% in this group. However, little progress has been made for patients who present with metastatic disease. The role of megatherapy (myeloablative high-dose chemotherapy, with or without total body irradiation followed by stem cell infusion) in the treatment of metastatic disease remains unclear. A number of clinical trials through the Children’s Oncology Group (COG) and European Cooperative Groups continue to assess new potentially more efficacious protocols. With increased understanding of the molecular pathways, opportunities for targeted therapy have emerged. Potential targets can be broadly classified into those related to the EWSR1-ETS fusion (e.g., trabectedin, lurbinectedin ), receptor tyrosine kinases and associated signaling pathways (e.g., IGFR, PDGF ), the TP53 and retinoblastoma pathways, angiogenesis, and apoptosis. Minimal residual disease can be detected in peripheral blood or bone marrow by RT-PCR, and detection of fusion transcript-positive cells in the blood seems to predict disease progression.

Key prognostic factors that adversely influence the outcome of the disease are the presence of metastatic disease at the initial diagnosis, large tumor size, extensive necrosis ( filigree pattern ), central axis tumors, and poor response to initial chemotherapy. Several studies have found that the type of EWSR1-FLI1 fusion may be prognostically relevant; patients with type 1 fusions have been reported to have longer disease-free survival, lower proliferative rates, and better chemotherapeutic response than those with other fusion types. However, another study found no significant clinical differences between those with EWSR1 – FLI1 and those with EWSR1 – ERG fusions. Several studies have identified gene expression profiles associated with the presence of metastases, prognosis, and response to therapy.

Clinical Findings

These tumors generally arise in young patients, with a peak in the third decade of life, although there is a wide age distribution. In the largest study (115 patients) published to date, patients ranged in age from 6 to 81 years (mean: 32). CIC -rearranged sarcomas have a slight male predilection and most often occur in the deep soft tissues of the extremities, followed by the trunk and head/neck. About 12% of tumors arise in visceral organs, especially the kidney. Very few cases arise as primary lesions in bone. Patients typically present with a slowly enlarging, painless soft tissue mass.

Pathologic Findings

CIC -rearranged sarcoma most closely resembles ES and generally is composed of small round cells with vesicular nuclei, fairly prominent nucleoli, and amphophilic to slightly eosinophilic cytoplasm. The cells are often arranged into distinct lobules ( Fig. 33.18 ), and extensive areas of geographic necrosis are typical ( Fig. 33.19 ). Mitotic figures, including atypical mitotic figures, are easily identified. The cells tend to show more nuclear pleomorphism than is seen in classic ES ( Figs. 33.20 to 33.22 ). Less common features include cytoplasmic clearing, myxoid stroma, spindle cell areas, and occasionally large epithelioid or rhabdoid cells.

Low-magnification view of a CIC-DUX4 –translocated sarcoma arising in a thigh of 33-year-old woman. Note lobular growth pattern with fibrous septa.

Photomicrograph courtesy of Dr. Darya Buehler, University of Wisconsin, Madison, WI.

CIC-DUX4 –translocated sarcoma with central area of necrosis surrounded by malignant round cells.

Case provided by Dr. David Lucas, University of Michigan, Ann Arbor, MI.

A to D, CIC-DUX4 –translocated sarcoma showing range of patterns.

CIC-DUX4 –translocated sarcoma showing sheets of large rounded cells.

CIC-DUX4 –translocated sarcoma with vesicular nuclei ( A ) and rhabdoid features ( B ).

Immunohistochemical Findings

On IHC, most cases (up to 85%) express CD99. However, unlike ES, the pattern is typically patchy rather than diffuse ( Fig. 33.23 ). As previously noted, absent or patchy CD99 expression in a Ewing-like sarcoma should suggest CIC -rearranged sarcoma. Unlike ES, this tumor is usually negative for NKX2.2. However, both tumors are often FLI1 positive. ¹³² Some CIC -rearranged sarcomas are ERG positive. A role for calretinin (expressed in 70% of CIC-rearranged sarcomas but not in ES) has been suggested.

Focal membranous CD99 immunoreactivity in CIC-DUX4 –translocated sarcoma.

Courtesy Dr. David Lucas, University of Michigan, Ann Arbor, MI.

Gene expression studies of CIC -rearranged sarcomas have consistently noted upregulation of the PEA3 family of genes, including ETV1, ETV4, and ETV5 . Subsequently, several immunohistochemical studies have shown a high degree of sensitivity and specificity for ETV4 IHC, with diffusely nuclear immunoreactivity in CIC -rearranged sarcomas but only occasional weak reactivity in ES. More recently, RNA in situ hybridization for ETV1/4/5 has also been found to have a high degree of sensitivity and specificity (perhaps even better than IHC). IHC for WT1 protein, using both amino- and carboxy-terminus antibodies, has also been shown to be a highly sensitive, if imperfectly specific, marker of CIC-rearranged sarcomas. There may also be a role for DUX4 IHC in this differential diagnosis; DUX4 expression has been shown to be a consistent feature of CIC -rearranged sarcomas, but not other round cell tumors.

Cytogenetic and Molecular Genetic Findings

This family of tumors is defined by the presence of a CIC gene rearrangement, either with 4q35 or 10q26.3. The CIC-DUX4 gene fusion can be detected by either RT-PCR or FISH, using either frozen or paraffin-embedded tissues. Several studies have emphasized a small but definite false-negative rate (up to 15%) using a CIC break-apart FISH probe. Rarely, alternate fusion partners have been reported, including a CIC-NUTMTA fusion and CIC-FOXO4 fusion. Antonescu et al. found the CIC-DUX4 fusion in 57% of tested cases, with either DUX4 on 4q35 (35%) or 10q26 (22%); no FOXO4 rearrangements were identified.

CIC aberrations have recently been identified in a small number of very unusual, nonvasoformative, epithelioid malignant neoplasms. They show expression of endothelial markers (CD31 and FLI1) and occur in the soft tissues of young adults. These tumors are discussed in Chapter 22 .

Discussion

CIC -rearranged sarcomas have now been clearly delineated as the most common type of “Ewing-like” sarcoma, with histologic, immunohistochemical, and molecular genetic features that distinguish it from ES and all other round cell sarcomas ( Table 33.2 ). Its recognition is important because it appears to be more clinically aggressive than ES. Before the study of Antonescu et al. in 2017, clinical data on this tumor were limited to small retrospective series. These tumors usually follow an aggressive clinical course with high rates of metastasis and a lower sensitivity to ES chemotherapeutic protocols, with a 5-year survival of only 43% (vs. 77% in ES). CIC -rearranged sarcomas also reportedly more often present with disseminated disease, show greater chemoresistance, and have a poorer overall survival than conventional ES.

Clinical Findings

BCOR -rearranged sarcoma mainly affects adolescents and young adults, although the age distribution is wide. In the largest study to date, patients ranged in age from 2 to 44 years, with a mean and median age of 15 years, with a striking male predominance (31 of 36 patients; 86%). Although other studies showed a strong predilection for this tumor to arise in both long and flat bones, more recent studies report that up to 44% of tumors arise in the soft tissues or viscera, particularly the kidney. Within the soft tissues, BCOR -rearranged tumors have been reported in the trunk, extremities, and head/neck.

Pathologic Findings

Grossly, the tumors most often measure between 8 and 15 cm at excision, usually with a fleshy appearance, and often with grossly identifiable areas of necrosis. BCOR -rearranged sarcomas show a wide range of morphologic features, resulting in a broad differential diagnosis that includes both round cell and spindle cell sarcomas, depending on the predominant cytologic features in a given case ( Figs. 33.24 and 33.25 ). Many show an admixture of uniform, medium-sized round cells admixed with spindled cells, but some cases show an exclusively round cell or spindle cell appearance. The cells may be arranged in sheets, whorling fascicles, or even long fascicles imparting a herringbone appearance. The nuclei have fine chromatin and small, indistinct nucleoli. Mitotic figures usually are easily seen. The stroma can range from myxoid to fibrous, and areas of hemorrhage and necrosis are common. A rich capillary network is usually present.

BCOR -rearranged sarcoma.

BCOR -rearranged sarcoma showing variation from rounded ( A ) to spindled ( B ) areas.

Immunohistochemical Findings

CD99 expression is variable from case to case and even within the same tumor. Overall, about 40% of cases express this antigen. Many also express SATB2, which potentially can cause confusion with small cell osteosarcoma. TLE1 expression is present in 80%, potentially resulting in confusion with synovial sarcoma. These tumors typically do not express keratins, S-100 protein, SOX10, melanocytic, or myoid markers. As discussed next, most of these tumors harbor a BCOR-CCNB3 fusion, resulting in expression of both BCOR and CCNB3 , which can be detected on IHC. Nuclear CCNB3 immunoreactivity is found in most BCOR -rearranged sarcomas and appears to be highly specific, although cytoplasmic staining may be seen in some tumors, including ES, alveolar rhabdomyosarcoma, and synovial sarcoma. Nuclear BCOR immunoreactivity is also a highly sensitive, but not entirely specific, marker of this tumor; although present in all BCOR -rearranged sarcomas it is also in some Ewing, CIC -rearranged, and synovial sarcomas.

Molecular Genetic Findings

The majority of these tumors harbor a BCOR-CCNB3 fusion that can be detected by FISH or RT-PCR. Both genes are located on chromosome X, and inversions of both with fusion of the 5′ centromeric region of BCOR (exon 15) with the 3′ centromeric region of CCNB3 (exon 15) are usually seen. Several alternate fusions have been described, including BCOR-MAML3 , ZC3H7B-BCOR, and KMT2D-BCOR . Interestingly, the same ZC3H7B-BCOR fusion has been described in endometrial stromal sarcoma and ossifying fibromyxoid tumor.

The relationship among a number of tumors has yet to be fully elucidated, including clear cell sarcoma of kidney, primitive myxoid mesenchymal tumor of infancy, infantile soft tissue undifferentiated round cell sarcoma, and BCOR-CCNB3 fusion sarcomas. However, there is clearly a considerable morphologic, immunohistochemical, and molecular genetic overlap between these entities. In particular, many of these tumors have been reported to harbor recurrent BCOR exon 16 internal tandem duplications (ITDs) and YWHAE-NUTM2B fusions, suggesting that these are all part of a spectrum of histogenetically related tumors.

Discussion

BCOR -rearranged sarcomas seem to have a clinical course not too dissimilar from ES but less aggressive than CIC -rearranged sarcoma. In the study by Kao et al., follow-up on 22 patients showed a 5-year overall survival of 72%, similar to patients with ES (79%), but significantly better than patients with CIC -rearranged sarcoma (43%). Six patients developed local recurrences, and four developed metastatic disease (lung, pancreas, bone, and soft tissue sites). Seven of nine patients treated with an ES chemotherapeutic protocol showed a significant posttherapeutic response in their resection specimen. Others have reported similar results.

Clinical Findings

This tumor is quite uncommon and accounts for less than 3% of all soft tissue sarcomas. EMC usually arises in patients older than 35 years, and only a few cases have been encountered in children and adolescents. Most series have found a peak incidence during the fifth or sixth decade. Men are affected about twice as often as women. The clinical signs and symptoms are nonspecific. Most patients present with a slowly growing, deep-seated mass that causes pain and tenderness in approximately one-third of cases. Complications such as ulceration and intratumoral hemorrhage may be encountered with large tumors. The duration of symptoms varies considerably, ranging from a few weeks to several years. Some patients have a history of trauma before discovery of the tumor, but as with other sarcomas, the significance of this finding remains uncertain and is, in all likelihood, coincidental.

More than two-thirds of cases occur in the proximal extremities and limb girdles, especially the thigh and popliteal fossa similar to myxoid liposarcoma. Most are deep seated, although occasional tumors are confined to the subcutis; the latter may be difficult to distinguish from myxoid forms of chondroma or myoepithelial tumors. Rare examples have been described in unusual locations, including the lung, heart, and vulva. Radiography, CT, and MRI simply show a soft tissue mass with no distinctive radiologic features that would set the tumor apart from other types of soft tissue sarcoma.

Pathologic Findings

Macroscopically, the neoplasm is a soft to firm, ovoid, lobulated to nodular, circumscribed mass surrounded by a dense fibrous pseudocapsule. On section, it has a gelatinous, gray to tan-brown surface. However, its color largely dependent on the extent of hemorrhage, a frequent feature of the tumor ( Fig. 33.26 ). Occasionally, hemorrhage is so prominent that the tumor is mistaken for a hematoma. Hematoidin pigment is often identified and may be a good clue to look closely for small nodules of tumor, especially in masses otherwise resembling an old, fibrotic hematoma. Highly cellular higher-grade tumors often have a fleshy consistency. The size of the tumor varies from a few centimeters to 15 cm or more, although most are 4 to 7 cm in greatest diameter at excision (range: 1.1-25.0 cm).

Gross appearance of extraskeletal myxoid chondrosarcoma with its characteristic gelatinous appearance and multinodular growth pattern. Dark appearance of some nodules is the result of hemorrhage.

Microscopically, a characteristically multinodular pattern is clearly evident at low magnification ( Fig. 33.27 ). The individual tumor nodules consist of round or slightly elongated cells of uniform shape and size separated by variable amounts of mucoid material ( Figs. 33.28 and 33.29 ). The individual cells have small hyperchromatic nuclei and a narrow rim of deeply eosinophilic cytoplasm reminiscent of chondroblasts ( Fig. 33.30 ). Occasional cells show cytoplasmic vacuolization. Unlike chondrosarcoma of bone, differentiated cartilage cells with distinct lacunae are not seen in EMC. Mitotic figures are usually rare but may be numerous in less well differentiated and more cellular forms of the tumor.

Low-power view of extraskeletal myxoid chondrosarcoma with characteristic nodular arrangement.

A to C, Extraskeletal myxoid chondrosarcoma. Cords of eosinophilic cells are deposited in an abundant myxoid stroma.

Characteristic alignment of tumor cells in strands and cords separated by large amounts of mucoid material in extraskeletal myxoid chondrosarcoma.

A and B, Extraskeletal myxoid chondrosarcoma composed of cells with thin rim of eosinophilic cytoplasm. Note vacuolization of matrix, a consistent artifact in these tumors.

Characteristically, the individual cells are arranged in short anastomosing cords, strands, or pseudoacini, often creating a lacelike appearance. Less frequently, the cellular elements are organized in small, loosely textured whorls or aggregates, reminiscent of an epithelial neoplasm. Rarely, cellular foci composed of fibroblastic/myofibroblastic spindle-shaped cells are present. Indeed, if these features prevail throughout the tumor, a definitive diagnosis of EMC may be exceedingly difficult. Although most EMCs are highly myxoid tumors, a distinct subset is hypercellular with less myxoid stroma between the neoplastic cells and composed of sheets of large cells with vesicular nuclei and prominent nucleoli (i.e., cellular variant of extraskeletal myxoid chondrosarcoma ) ( Fig. 33.31 ). These tumors are best diagnosed by identifying typical less cellular areas of EMC or by cytogenetics/molecular genetics (discussed later). Some tumors are composed of a cellular proliferation of relatively small round cells closely resembling Ewing sarcoma. Even more rarely, typical EMCs are associated with or progress to a high-grade pleomorphic sarcoma ( dedifferentiated extraskeletal myxoid chondrosarcoma ), and still others may have rhabdoid features characterized by cells with large, paranuclear hyaline inclusions. These tumors often show loss of SMARCB1 ¹⁹⁵ and tend to have variant fusions (non– EWSR1-NR4A3 ). Secondary changes such as fibrosis and hemorrhage are common, but calcification or bone formation is rare.

A to C, Cellular variant of extraskeletal myxoid chondrosarcoma composed of large cells with vesicular nuclei and deeply eosinophilic cytoplasm. Other areas of typical myxoid chondrosarcoma were present in this tumor.

Immunohistochemical Findings

The cells of extraskeletal myxoid chondrosarcoma stain strongly for vimentin, but this is the only marker that is consistently positive. In contrast to true chondroid neoplasms, the majority of EMCs show an absence or only focal staining for S-100 protein; SOX10 is typically negative. As with many other types of sarcoma, rare cases show focal immunoreactivity for keratins. In addition, almost 30% of EMCs show scattered cells that are EMA positive, potentially resulting in confusion with a soft tissue myoepithelial tumor. Some authors have found evidence of neuroendocrine differentiation (via expression of NSE, chromogranin, or synaptophysin), and identification of dense-core granules on ultrastructural examination. Interestingly, the presence of neuroendocrine features has been associated with the relatively uncommon t(9;17)(q22;q11). Others have found these cells to express microtubule-associated protein-2 (MAP2) and class III β-tubulin, which are components of microtubules and are specifically localized in neurons and their derivatives. This has been taken as further evidence of neuroendocrine differentiation in at least some examples of EMC. More recently, the majority of EMCs have been found to express INSM1, a zinc-finger transcription factor that plays an important role in neuroendocrine differentiation, providing further evidence for this line of differentiation. Tumors with rhabdoid features typically show loss of SMARCB1 (INI1). Up to 30% of cases show positivity for CD117.

Cytogenetic and Molecular Genetic Findings

Extraskeletal myxoid chondrosarcoma is characterized most frequently (up to 75% of cases) by a balanced t(9;22)(q22;q12), which fuses EWSR1 with NR4A3 (previously known as NOR1, CHN, or TEC ). Variant translocations include a t(9;17)(q22;q11) that results in a TAF15 (also known as RBP56 or TAF2N ) fusion with NR4A3, a t(9;15)(q22;q21) resulting in a TCF12-NR4A3, or a t(3;9)(q12;q22) resulting in an TFG-NR4A3 fusion. Recently, the HSPA8-NR4A3 fusion has also been reported. As found with other EWSR1 -rearranged sarcomas, rarely an FUS rearrangement can be detected ( FUS-NR4A3 ). These genetic events are not found in conventional myxoid chondrosarcoma of bone. Tumors morphologically and genetically identical to EMC, however, may occur in bone and have been termed “osseous myxochondroid sarcoma.” Molecular assays using paraffin-embedded tissues (RT-PCR or FISH) can be extremely helpful in confirming this diagnosis using probes to NR4A3 and EWSR1 .

Interestingly, non– EWSR1-NR4A3 variant fused tumors have been associated with high-grade morphologic features, including high cellularity, increased cytologic atypia, and rhabdoid morphology. In contrast, EWSR1 -rearranged tumors usually show low cellularity, minimal cytologic atypia, low mitotic counts, and indolent clinical behavior.

The molecular consequences of these translocations are now being unraveled. Expression profiling studies have identified potential downstream targets, including DKK1, NMB, DNER, CLCN3, RET, and DEF6. Several studies, including an expression profiling study of genetically confirmed cases, have found high levels of expression of peroxisome proliferator-activated receptor-γ ( PPARG ), a potential therapeutic target.

Differential Diagnosis

Probably the most difficult tumor to distinguish from extraskeletal myxoid chondrosarcoma is the family of benign and malignant myoepithelial tumors ( myoepithelioma and myoepithelial carcinoma ). These tumors display a curious modulation between epithelioid and spindled areas. The immunophenotype of deeply situated myoepithelial lesions usually involves coexpression of several markers, including keratins and epithelial membrane antigen (EMA), S-100 protein, calponin, and sometimes p63 and glial fibrillary acidic protein (GFAP) ( Table 33.3 ). However, it must be kept in mind that up to 50% of these tumors harbor EWSR1 aberrations that can add further confusion in its distinction from EMC. Thus, detection of NR4A3 aberrations can be more helpful in separating these tumors.

Myxoma and myxoid liposarcoma must also be considered in the differential diagnosis ( Table 33.4 ). Myxoma displays a similar paucity of vascular structures, but it is less cellular than EMC; the cytologically bland cells of myxoma are separated by abundant myxoid stroma. Myxoid liposarcoma , on the other hand, displays a strikingly plexiform vascular pattern and contains lipoblasts, especially at the margin of the tumor lobules. S-100 protein is found in approximately 40% of myxoid liposarcomas and does not help distinguish this tumor from EMC. In difficult cases, molecular genetic analysis (RT-PCR or FISH) to evaluate for aberrations of EWSR1, NR4A3, FUS, and DDIT3 can be quite helpful. However, it must be kept in mind that FUS rearrangements (common in myxoid liposarcoma) may rarely be seen in EMC. In addition, EWSR1 rearrangements (common in EMC) may occasionally be identified in myxoid liposarcoma.

EMC may be difficult to distinguish from a number of benign or malignant chondroid or myxoid lesions, including the myxoid variant of extraskeletal (soft part) chondroma. Extraskeletal chondromas usually occur in the soft tissues of the hands or feet—unusual locations for EMC. They tend to be smaller, less cellular lesions without evidence of EWSR1 or NR4A3 aberration. Chondromyxoid fibroma rarely occurs as a periosteal tumor or in soft tissue as secondary tissue implantations. It can be recognized by its greater degree of cellular pleomorphism, as well as condensation of the tumor cells underneath a narrow, richly vascularized fibrous band that borders the individual tumor nodules. In addition, there may be multinucleated giant cells and foci of calcification or ossification, features rarely seen in EMC.

Juxtacortical (parosteal) chondrosarcoma lacks a myxoid component and shows a broad attachment to the perichondrium or periosteum of the involved bone, sometimes with invasion of the underlying cortex and cortical irregularities on radiographs. Chordoma , especially its myxoid form, enters the differential diagnosis, but this diagnosis is unlikely if the tumor occurs outside its usual locations (i.e., the sacrococcygeal region, the base of the skull, or the cervical spine). EMC shows no radiographic evidence of bone involvement and lacks multivacuolated, physaliphorous tumor cells. On IHC, chordoma coexpresses S-100 protein, brachyury, and markers of epithelial differentiation (EMA and keratins, particularly keratins 8 and 19).

Myxopapillary ependymoma can be distinguished by its characteristic location in the sacrum, perivascular growth, positivity for GFAP, and the presence of glial-type microfilaments.

Lastly, the recently described primary pulmonary myxoid sarcoma can be extremely difficult to distinguish from metastatic EMC. Typically lobulated, the former is composed of cords of cells with polygonal, stellate or spindle-shaped cells deposited in a myxoid matrix. In their original description of this entity, Thway et al. emphasized its close resemblance to EMC. As with EMC, the immunophenotype is not distinctive, but this tumor does have a characteristic EWSR1-CREB1 fusion. Thus, FISH for EWSR1 aberrations will not distinguish these entities, but determination of the fusion partner ( CREB1 vs. NR4A3 ) can be exceedingly helpful.

Discussion

Generally, extraskeletal myxoid chondrosarcoma is a relatively slow-growing tumor that recurs and eventually metastasizes in many cases. Of the 31 patients in the series by Enzinger and Shiraki, 20 were alive at last follow-up, but 6 of these patients developed recurrence, and 4 died of metastatic disease. In the much larger study by Meis-Kindblom et al., local recurrences and metastasis developed in 48% and 16% of patients, respectively. Estimated 5-, 10-, and 15-year survival rates were 90%, 70%, and 60%, respectively. Ten-year survival rates ranging from 78% to 88% have been reported in more recent studies, but 10-year disease-free survival rates are much lower, ranging from 14% to 36%.

Late recurrence and metastasis are common. In the series from the Armed Forces Institute of Pathology (AFIP), one patient developed a recurrence 18 years after the initial excision. In another patient, pulmonary metastasis became evident 10 years after surgical removal of the tumor and 4 years after removal of a regional lymph node metastasis. The most frequent metastatic sites are the lungs, soft tissues, and lymph nodes. Survival may be prolonged even in the face of metastatic disease.

Radical local excision with or without adjuvant radiotherapy seems to be the treatment of choice. Good results with high-dose irradiation have been reported, but chemotherapy has not been found to be efficacious. In a study of 87 patients with EMC, Drilon et al. found that 13% of patients presented with metastases, but for those who did not, 37% developed local recurrences a median of 3.2 years after excision, and 26% developed distant recurrences. The 5-, 10-, and 15-year overall survival rates were 82%, 65%, and 58%, respectively. Twenty-one patients received chemotherapy, but no significant radiologic or clinical responses were found. Recent trials with sunitinib and trabectedin have shown promise.

Although histologic features such as high cellularity associated with high nuclear grade might suggest aggressive clinical behavior in some cases, the largest study published to date found no association between cellularity and clinical outcome. In another study, tumor size of 10 cm or greater, high cellularity, mitotic activity greater than 2 mitotic figures (MF) per 10 high-power fields (hpf), MIB-1 index greater than 10%, and anaplasia or the presence of rhabdoid cells were associated with more aggressive behavior. Increasing patient age, large tumor size, and proximal tumor location are predictive of an adverse outcome.

Clinical Findings

Radiographic Findings

Radiographic studies may be extremely helpful for suggesting a preoperative diagnosis of synovial sarcoma, largely because of the presence of calcification. Most synovial sarcomas present on routine films as round or oval, lobulated swellings or masses of moderate density, usually located close to a large joint. The underlying bone tends to be uninvolved, but in about 15% to 20% of cases, there is a periosteal reaction, superficial bone erosion, or invasion. Massive bone destruction, which is rare, is mostly caused by poorly differentiated synovial sarcomas of long duration and large size.

The most striking radiologic characteristic, found in 15% to 20% of synovial sarcomas, is the presence of multiple small, spotty radiopacities caused by focal calcification and less frequently bone formation. In most instances, these changes consist merely of fine stippling, but in some cases, large portions of the tumor are marked or even outlined by radiopaque masses ( Figs. 33.32 to 33.34 ). Confusion with other tumors is possible, but radiopacities are not observed in most other forms of sarcoma, except for extraskeletal osteosarcoma and mesenchymal chondrosarcoma.

Radiograph of synovial sarcoma originating in popliteal fossa. Note focal calcification in the tumor ( arrow ), a feature that is present in about 20% of these cases.

Massive calcification and ossification in synovial sarcoma of popliteal fossa ( arrows ). In general, tumors with extensive calcification carry a better prognosis than those without.

Radiograph of synovial sarcoma of planta pedis showing extensive calcification of the tumor ( arrow ).

CT and MRI are valuable tools for determining the site of origin and extent of the lesion. As with conventional radiographs, scans show a paraarticular heterogeneous septated mass, often with associated calcification or bone erosion.

Gross Findings

The gross appearance varies, depending on the rate of growth and location of the tumor. Slowly growing lesions tend to be sharply circumscribed, round, or multilobular; as a result of compression of adjacent tissues by the expansively growing tumor, lesions are completely or partially invested by a smooth, glistening pseudocapsule ( Fig. 33.35 ). Cyst formation may be prominent, and occasional lesions present as multicystic masses ( Fig. 33.36 ). Most are firmly attached to surrounding tendons, tendon sheaths, or the exterior wall of the joint capsule; portions of these structures can adhere to the gross specimen. On section, tumors are yellow to gray-white. They may attain a size of 15 cm or more, but most are between 3 and 6 cm at excision. Calcification is common but rarely a discernible macroscopic feature. Less well-differentiated and more rapidly growing tumors tend to be poorly circumscribed and usually exhibit a variegated and often friable or shaggy appearance, frequently with multiple areas of hemorrhage, necrosis, and cyst formation. Greatly hemorrhagic tumors may be confused with angiosarcomas or even organizing hematomas.

Gross specimen of synovial sarcoma showing fleshy gray-tan appearance with focal hemorrhage.

Multicystic synovial sarcoma of the knee region.

Microscopic Findings

Unlike most other types of sarcoma, the tumor is composed of two morphologically different types of cells: epithelial cells, resembling those of carcinoma, and fibrosarcoma-like spindle cells. Transitional forms between epithelial and spindle cells suggest a close relation, which is also supported by tissue culture, ultrastructural, immunohistochemical, and molecular genetic findings. Depending on the relative prominence of the two cellular elements and the degree of differentiation, synovial sarcomas form a continuous morphologic spectrum and can be broadly classified into the (1) biphasic type, with distinct epithelial and spindle cell components in varying proportions; (2) monophasic fibrous type; (3) rare epithelial-predominant type; and (4) poorly differentiated (round cell) type.

Biphasic Synovial Sarcoma

The classic synovial sarcoma—the biphasic type—is generally readily recognizable by the coexistence of morphologically different but histogenetically related epithelial cells and fibroblast-like spindle cells ( Figs. 33.37 to 33.41 ). The epithelial cells are characterized by large, round or oval, vesicular nuclei and abundant pale-staining cytoplasm with distinctly outlined cellular borders. The cells are cuboidal to tall and columnar; they are arranged in solid cords, nests, or glandular structures that contain granular or homogeneous eosinophilic secretions. The glandular spaces lined by epithelial cells must be distinguished from cleftlike artifacts that are the result of tissue shrinkage. Cuboidal or flattened epithelial cells also may cover small villous or papillary structures, often with spindle cells rather than connective tissue in the papillary core. A diagnosis of squamous cell carcinoma may also be suggested by focal squamous metaplasia, including the occasional formation of squamous pearls and keratohyaline granules.

Biphasic synovial sarcoma showing close apposition of epithelial structures with malignant spindle cells.

Typical biphasic synovial sarcoma with columnar epithelial cells surrounded by spindle cell elements.

Biphasic synovial sarcoma with glandular epithelial structures adjacent to a malignant spindle cell component.

Biphasic synovial sarcoma. Some cases are associated with a thickened basement membrane that separates the epithelial and spindle cell elements (A) , whereas in other cases the demarcation between the two is less distinct (B) .

Biphasic synovial sarcoma with prominent intraluminal eosinophilic secretions in epithelial elements.

The surrounding spindle cell component consists mostly of well-oriented, rather plump, spindle-shaped cells of uniform appearance, with small amounts of indistinct cytoplasm and oval, dark-staining nuclei. Generally, the cells form solid, compact sheets that are similar in many respects to adult-type fibrosarcoma ( Figs. 33.42 to 33.45 ). Mitotic figures in synovial sarcoma occur in both epithelial and spindle-shaped cells, but, as a rule, only the poorly differentiated forms of the tumor exhibit very high mitotic counts. Occasionally, there is nuclear palisading ( Fig. 33.44 ), but in contrast to leiomyosarcoma and malignant peripheral nerve sheath tumor (MPNST), this feature is confined to a small portion of the tumor.

Fibrosarcoma-like area in synovial sarcoma. Note the alternating darkly staining and lightly staining regions, imparting a marbled appearance.

Fibrosarcoma-like area in synovial sarcoma. The spindle cells are arranged in distinct fascicles.

Synovial sarcoma with focal nuclear palisading reminiscent of a neural tumor.

Monophasic synovial sarcoma with prominent perivascular hyalinization.

Typically, the cellular portions of synovial sarcoma alternate with less cellular areas that display hyalinization, myxoid change, or calcification ( Figs. 33.46 and 33.47 ). The collagen in the hyalinized zones may be diffusely distributed or may form narrow bands or plaquelike masses that are sometimes associated with a markedly thickened basement membrane that separates the epithelial and spindle cell elements (see Fig. 33.40A ). The myxoid areas are generally less conspicuous and tend to occupy only a small, poorly defined portion of the tumor, although some cases are predominantly myxoid ( Fig. 33.47 ).

Thick collagen bands separate malignant spindle cells in monophasic synovial sarcoma. This pattern of hyalinization is characteristic of this tumor.

A and B, Prominent myxoid change in synovial sarcoma.

Calcification with or without ossification is another diagnostically important and characteristic feature that is present to a varying degree in about 20% of synovial sarcomas. It may be inconspicuous and consist merely of a few small, irregularly distributed spherical concretions, or it may be extensive and occupy a large portion of the neoplasm ( Fig. 33.48 ). In general, calcification is preceded by hyalinization and is more pronounced at the periphery of the tumor than at its center. Rarely, chondroid changes are present and almost always occur in conjunction with focal calcification and ossification.

A, Calcification in synovial sarcoma, a common feature of this neoplasm. B, Note the focus of calcification in this small monophasic fibrous synovial sarcoma of the foot. C, Monophasic fibrous type of synovial sarcoma with osseous metaplasia.

Mast cells are another conspicuous feature of synovial sarcoma. They show no particular distribution but are more numerous in the spindle cell than in the epithelial portions of the neoplasm. Inflammatory elements and multinucleated giant cells are rare.

The degree of vascularity varies. In some cases it is a dominant feature, with numerous hemangiopericytomatous vascular spaces ( Fig. 33.49 ); in others there are merely a few scattered vascular structures. Some cases show prominent cystic changes ( Fig. 33.50 ). Secondary changes such as hemorrhage are most prominent in poorly differentiated tumors. Scattered lipid macrophages, siderophages, multinucleated giant cells, and deposits of cholesterol may be present but are much less conspicuous in synovial sarcomas than in synovitis.

Prominent hemangiopericytomatous vasculature in synovial sarcoma.

Cystic synovial sarcoma. Malignant spindle cells are seen in thickened fibrous septa.

Monophasic Fibrous Synovial Sarcoma

The monophasic fibrous synovial sarcoma is a relatively common neoplasm and is much more common than the biphasic form. Because this type merely represents one extreme of the morphologic spectrum, the previously mentioned morphologic features of the spindle cell portion of the biphasic type, such as cellular appearance, hyalinization, myxoid change, mast cell infiltrate, hemangiopericytoma-like vasculature, and focal calcification, apply equally to the monophasic fibrous type ( Fig. 33.51 ).

A, High-magnification view of monophasic fibrous synovial sarcoma. Malignant spindle cells are relatively uniform with respect to one another. B, Uniform spindle cells in monophasic fibrous synovial sarcoma. C, Epithelioid area in a monophasic fibrous synovial sarcoma. A small group of cells have increased amounts of eosinophilic cytoplasm and appear more cohesive.

In some tumors an obvious epithelial component can be identified by extensive sampling, in which case the tumor is more appropriately designated as a biphasic synovial sarcoma. Even in those cases without obvious epithelial differentiation, however, many monophasic fibrous synovial sarcomas have foci where the cells have a more epithelioid morphology and appear more cohesive than the surrounding spindle-shaped cells. The cells in these foci have more eosinophilic cytoplasm but otherwise have the same nuclear features as the surrounding spindle-shaped cells. Such areas often show immunohistochemical evidence of epithelial differentiation ( Fig. 33.51C ).

Epithelial-Predominant Synovial Sarcoma (so-called Monophasic Epithelial Synovial Sarcoma)

Monophasic epithelial synovial sarcoma is more of a theoretical concept than a diagnostic entity. Although it could be argued that such lesions exist as a counterpoint to monophasic fibrous-type synovial sarcomas, we are not aware of a convincing case. With the ability to identify synovial sarcomas by genetic methods, such a case might be described. In the meantime, it is more meaningful to focus on those synovial sarcomas in which the epithelial component vastly overshadows the spindled component, such that an erroneous diagnosis of carcinoma could plausibly be made.

Epithelial-predominant synovial sarcomas have large, sheetlike expanses of epithelioid cells or back-to-back glands ( Figs. 33.52 to 33.54 ). What sets the glands apart from those of classic adenocarcinomas is that they are often filled with eosinophilic debris, which contrasts with the conventional mucin of an adenocarcinoma ( Fig. 33.52 ).

Predominantly epithelial-type synovial sarcoma showing cribriform glandular pattern prominent throughout the neoplasm.

Predominantly epithelial-type synovial sarcoma showing small areas with well-developed spindle cell pattern.

Predominantly epithelial-type synovial sarcoma showing that most of tumor is composed of sheets of cohesive epithelioid cells with only small foci of spindle cell differentiation.

Although it is difficult to discount the possibility of a synovial sarcoma consisting entirely of epithelial elements, without a component of spindled cells, we have never seen such a case. Epithelial-predominant synovial sarcomas certainly exist and may closely simulate metastatic adenocarcinoma or some type of adnexal tumor. However, close inspection of these cases invariably discloses a relatively subtle spindle cell component, identical to that seen in other biphasic synovial sarcomas. Demonstration of the synovial sarcoma–specific SS18 rearrangement (see later) should allow for ready distinction of such tumors from morphologic mimics.

Poorly Differentiated Synovial Sarcoma

Poorly differentiated synovial sarcoma (PDSS) can be seen as a form of tumor progression that can be superimposed on any of the other synovial sarcoma subtypes. Recognition of this subtype of synovial sarcoma is of practical importance not only because it poses a special problem in diagnosis, but also because it behaves more aggressively and metastasizes in a significantly higher percentage of cases. The incidence of the poorly differentiated type among synovial sarcomas is difficult to estimate, but in the study by Machen et al., 21 of 34 synovial sarcomas (62%) had poorly differentiated foci, in some cases accounting for up to 90% of the neoplasm. However, this pattern predominates in fewer than 20% of all cases of synovial sarcoma.

Histologically, PDSS may have three patterns: (1) a large cell or epithelioid pattern composed of variably sized rounded nuclei with prominent nucleoli ( Fig. 33.55 ), (2) a small cell pattern with nuclear features similar to other small round cell tumors, and (3) a high-grade spindle cell pattern composed of spindle-shaped cells with high-grade nuclear features and a high mitotic rate ( Fig. 33.56 ), often accompanied by necrosis. These tumors often have a richly vascular pattern with dilated, thin-walled vascular spaces (hemangiopericytoma-like). Occasionally, cells with intracytoplasmic hyaline inclusions imparting a rhabdoid morphology may be found in poorly differentiated areas.

Poorly differentiated area of synovial sarcoma. A, Low-magnification view with prominent hemangiopericytomatous vasculature. B, Note the cytologic features of round cells in poorly differentiated synovial sarcoma.

Poorly differentiated synovial sarcoma composed of spindle cells with high-grade nuclear features.

Immunohistochemical Findings

Most synovial sarcomas display immunoreactivity for keratins and EMA ( Figs. 33.57 and 33.58 ). In 100 synovial sarcomas, Guillou et al. found focal positivity for EMA and keratin in 97% and 69% of cases, respectively; only 1 of 100 cases was negative for both these epithelial markers. Approximately 90% of all synovial sarcomas are keratin positive. In general, the intensity of staining is more pronounced in the epithelial component than in the spindled component. In some lesions of the monophasic fibrous type, only a few isolated cells express these antigens, making it necessary to stain and examine multiple sections from different portions of the tumor. Poorly differentiated variants express epithelial markers, but less often than the other subtypes. Folpe et al. found that all PDSSs expressed EMA, whereas only 30% and 50% expressed LMW and HMW keratins, respectively. Similarly, van de Rijn et al. found expression of EMA and keratins in 95% and 42% of PDSSs, respectively. In contrast to other spindle cell sarcomas, the cells of synovial sarcoma specifically express keratins 7 and 19. In fact, these markers often decorate a much larger proportion of cells than either EMA or the AE1/AE3 keratin antibody. A study of 110 synovial sarcomas of all subtypes found fairly consistent expression of K7, K19, K8/18, and K14 in the epithelial cells of biphasic tumors. However, the cells of monophasic synovial sarcoma had a more limited spectrum of keratin expression, with focal expression of K7 (79%), K19 (60%), and K8/18 (45%). Poorly differentiated cells showed even more limited expression of K7 (50%) and K19 (61%). A study of 60 t(X;18) SS18-SSX -positive cases found EMA to be the most sensitive epithelial marker. It should be emphasized that synovial sarcomas do not show diffuse immunoreactivity for keratins or EMA. Such cases invariably represent sarcomatoid carcinoma or mesotheliomas and do not require molecular genetic testing for synovial sarcoma–associated genetic events.

High-molecular-weight keratin immunoreactivity highlights epithelial elements in this biphasic synovial sarcoma.

Focal immunoreactivity for keratin 7 in monophasic fibrous synovial sarcoma.

Although not often emphasized, up to 30% to 40% of synovial sarcomas show focal immunoreactivity for S-100 protein. Most of these S-100 protein–positive synovial sarcomas coexpress epithelial markers, but the occasional synovial sarcoma expresses S-100 protein in the absence of EMA or keratins, thereby causing confusion with an MPNST. In these cases, detection of K7 or K19, diffuse and strong expression of TLE1 (discussed later) and molecular genetic studies may be useful for recognizing monophasic fibrous synovial sarcoma.

CD99, the product of the MIC2 gene, can be detected by IHC in the cytoplasm or cell membrane in 60% to 70% of synovial sarcomas. As previously noted, this can result in confusion with Ewing sarcoma. BCL2 protein is diffusely expressed in virtually all synovial sarcomas, especially in the spindled cells, but is of limited diagnostic value because many other tumors express this antigen. Unlike many other spindle cell tumors, synovial sarcoma is virtually always negative for CD34, although there are very rare exceptions. Calponin has also been frequently expressed in synovial sarcoma, which may be useful in recognizing poorly differentiated variants because other round cell tumors are negative for this antigen.

Gene expression profiling studies identified TLE1 as an excellent discriminator of synovial sarcoma from other sarcomas. Subsequently, with the emergence of an antibody to TLE1, several studies found this to be a sensitive marker of synovial sarcoma ( Fig. 33.59 ). Terry et al. found the TLE1 protein to be expressed in 91 of 94 molecularly confirmed synovial sarcomas, although it was very rarely expressed in other mesenchymal neoplasms. Similarly, all 35 molecularly confirmed synovial sarcomas tested by Jagdis et al. showed strong and diffuse TLE1 immunoreactivity, whereas only 1 of 43 MPNSTs showed focal staining; the positive and negative predictive values in this study were 92% and 100%, respectively. However, not all studies have found this marker to be as useful. For example, Kosemehmetoglu et al. found TLE1 staining in 53 of 143 nonsynovial sarcomas (37%), thereby limiting its specificity. It is now generally accepted that TLE1 is best used as a screening marker for synovial sarcoma, much like CD99 in Ewing sarcoma. Molecular genetic testing can be reserved for cases showing diffuse, strong staining with this marker.

Strong nuclear TLE1 immunoreactivity in monophasic fibrous synovial sarcoma.

Other markers of interest include SYT protein, which has been shown to be a sensitive but nonspecific marker of synovial sarcoma. Diminished expression of SMARCB1 (INI), sometimes in a “mosaic pattern,” is seen in up to 69% of synovial sarcomas, regardless of the presence or absence of rhabdoid morphology. It has been suggested that this is the result of posttranscriptional modification of SMARCB1. Several other studies have found complete or partial loss of SMARCB1 in a high percentage of synovial sarcomas.

Cytogenetic and Molecular Genetic Findings

A consistent, specific translocation, typically a balanced reciprocal translocation, t(X;18)(p11;q11), is found in virtually all synovial sarcomas, regardless of subtype. This translocation involves the fusion of the SS18 (also known as SYT ) gene on chromosome 18 and either the SSX1 or SSX2 gene on the X chromosome (both at Xp11) or, rarely, with SSX4 (also at Xp11). The function of the SS18-SSX fusion protein has yet to be fully defined but fuses transcriptional activation (SS18) and repression (SSX) domains resulting in the dysregulation of gene expression. DNA microarray expression profiling studies have shown upregulation of a number of genes, including IGFBP2 , IGF2 , and ELF3 . A consistent finding has been the upregulation of genes involved with the Wnt signaling pathway, including TLE1 . SS18-SSX1 and SS18-SSX2 appear to be mutually exclusive gene fusions, and there is concordance of fusion type between primary tumors and their metastases. Overall, approximately two-thirds of synovial sarcomas harbor an SS18-SSX1 fusion, and one-third reveal an SS18-SSX2 fusion. Interestingly, several studies have found an association between fusion type and histology. The majority of tumors with SS18-SSX2 are monophasic fibrous tumors, whereas almost all biphasic synovial sarcomas have an SS18-SSX1 fusion. The SS18-SSX fusion can be reliably detected by either RT-PCR or FISH. These techniques are particularly useful for monophasic fibrous and poorly differentiated synovial sarcomas, which may be difficult to distinguish from other spindle cell and round cell sarcomas, respectively. It is also invaluable in distinguishing the rare epithelial-predominant type of synovial sarcoma from adenocarcinoma. Amary et al. found RT-PCR and FISH to be complementary techniques, with sensitivities and specificities of 96% and 100%, respectively, when used in tandem. In general, it is not necessary to perform molecular testing in every case. However, if any question were to arise, the diagnosis of synovial sarcoma can easily be confirmed with ancillary molecular diagnostic testing, provided there is adequate tissue remaining.

Differential Diagnosis

Distinguishing synovial sarcoma from other neoplasms may be difficult, and in many instances a reliable diagnosis is not possible without ancillary diagnostic techniques. The differential diagnosis depends on the subtype of synovial sarcoma.

Discussion

Clinical Findings

This tumor occurs principally in adolescents and young adults and is most frequently encountered in patients 15 to 35 years of age. Female patients outnumber males, especially among patients younger than 25 years. Infants and children are rarely affected. The tumor has two main locations. When it occurs in adults, it is seen predominantly in the lower extremities, especially the anterior portion of the thigh. In the classic study of 102 ASPSs by Lieberman et al., 39.5% involved the soft tissues of the buttock or thigh. The tumor has also been described in a variety of unusual locations, including the female genital tract (especially the uterine cervix), lung, and penis. When the tumor affects infants and children, it is often located in the region of the head and neck, especially the orbit and tongue; tumors in the head and neck tend to be smaller, probably because of earlier detection.

Patients usually present with a slowly growing, painless mass that almost never causes functional impairment. Because of the relative lack of symptoms, it is easily overlooked; in a number of cases, metastasis to the lung or brain is the first manifestation of the disease. Headache, nausea, and visual changes are often associated with cerebral metastasis. As a rule, the tumor is richly vascular, causing pulsation or a distinctly audible bruit in some instances; massive hemorrhage may be encountered during surgical removal. Hypervascularity with prominent draining veins and prolonged capillary staining are usually demonstrable with angiography and CT scans. On MRI the tumor typically demonstrates high signal intensity on both T2- and T1-weighted images.

Pathologic Findings

The gross specimen tends to show a poorly circumscribed, soft, friable tumor; on sectioning, it consists of yellow-white to gray-red tissue, often with large areas of necrosis and hemorrhage. Frequently, the tumor is surrounded by numerous tortuous vessels of large caliber.

The microscopic picture varies little from tumor to tumor. In fact the uniformity of the microscopic picture is one of its characteristic features. Dense fibrous trabeculae of varying thickness divide the tumor into compact groups or compartments of irregular size, which in turn are subdivided into sharply defined nests or aggregates of tumor cells ( Fig. 33.60 ). These cellular aggregates are separated from one another by thin-walled, sinusoidal vascular channels lined by a single layer of flattened endothelial cells. In most cases the cellular aggregates exhibit central degeneration, necrosis, and loss of cohesion resulting in a pseudoalveolar pattern ( Figs. 33.61 and 33.62 ). This pattern should not be confused with the more irregular alveolar pattern of alveolar rhabdomyosarcoma. Less frequently, the nestlike pattern is inconspicuous or absent entirely, and the tumor is merely composed of uniform sheets of large granular cells with few or no discernible vascular channels ( Fig. 33.63 ). This more solid or compact type of ASPS occurs mainly in infants and children.

Alveolar soft part sarcoma with typical organoid arrangement of tumor cells.

A, Low-magnification view of alveolar soft part sarcoma composed of nests of large tumor cells with central loss of cellular cohesion resulting in a pseudoalveolar pattern. B, Higher-power view shows cell nests that are separated by thin-walled, sinusoidal vascular spaces.

Prominent pseudoalveolar growth pattern in alveolar soft part sarcoma.

Alveolar soft part sarcoma arising in child showing clustering and small nests of tumor cells.

The individual cells are large, rounded, or more often polygonal and display little variation in size and shape. They have distinct cell borders and one or more vesicular nuclei with small nucleoli and abundant granular, eosinophilic, and sometimes vacuolated cytoplasm. Mitotic figures are scarce. Rare pleomorphic tumors have been reported in the literature ( Fig. 33.64 ).

Range of cytologic atypia in alveolar soft part sarcomas. A, Typical cytologic appearance, with relatively uniform nuclei and prominent nucleoli. B, Scattered atypical cells in alveolar soft part sarcoma. C, Marked cytologic atypia in alveolar soft part sarcoma.

At the margin of the tumor, there are usually numerous dilated veins, probably the result of multiple arteriovenous shunts in the neoplasm. Vascular invasion is a constant, striking finding that likely explains the tendency of the tumor to develop metastasis at an early stage of the disease ( Fig. 33.65 ).

Dilated peripheral vein with tumor invasion.

Histochemical stains are useful for establishing the diagnosis. PAS preparation reveals varying amounts of intracellular glycogen and characteristically PAS-positive, diastase-resistant rhomboid or rod-shaped crystals ( Fig. 33.66 ). These crystals vary greatly in number from case to case. In some cases they can be identified in almost every cell, whereas in others they are difficult to find or absent. In our experience, the typical crystalline material is present in at least 80% of the tumors; in the remainder, there are merely PAS-positive granules, probably precursors of the crystals.

A, High-magnification view of alveolar soft part sarcoma. There is focal condensation of eosinophilic cytoplasm. B, Periodic acid–Schiff (PAS) staining with diastase reveals varying amounts of intracellular crystalline material. C, High-magnification view of crystalline material, diagnostic of alveolar soft part sarcoma.

The nature of these crystals has been elucidated, although serendipitously. While characterizing a monoclonal antibody to the monocarboxylate transporter 1 (MCT1) in a variety of tissues and tumors, Ladanyi et al. noted expression on the surface and in the cytoplasm of the cells in examples of ASPS. MCT1 is one of a family of transporter proteins that catalyzes the rapid transport of monocarboxylates across plasma membranes. The protein is normally associated with the rough endoplasmic reticulum and is transported to the plasma membrane in association with its chaperone, CD147. These investigators found an abundance of MCT1 and CD147 on the surface of the cells of ASPS, as well as within the cytoplasm in the region of the characteristic crystals. Western blot analysis confirmed the nature of the protein, and ultrastructural IHC localized MCT1 and CD147 to the cytoplasmic crystals and their precursor granules.

Immunohistochemical Findings

Numerous IHC studies have attempted to elucidate the histogenesis of this unusual tumor, often with contradictory results. The cells generally do not stain with antibodies against keratin, EMA, neurofilaments, GFAP, HMB-45, melan-A, or synaptophysin; they occasionally express S-100 protein and NSE, but these markers are of no diagnostic value in this setting. The reports in regard to staining for muscle markers differ somewhat, but most have demonstrated muscle markers in less than 50% of tumors.

In 1991, Rosai et al. detected the myogenic nuclear regulatory protein MyoD1 by IHC (confirmed by Western blot analysis) and suggested this as confirmatory evidence of its skeletal muscle nature. However, several subsequent studies have failed to identify MyoD1 or myogenin expression in ASPS, and it is now generally agreed that these tumors do not show skeletal muscle differentiation. Focal desmin immunoreactivity, however, is seen in almost 50% of cases.

As discussed later, this tumor is characterized by aberrations of the TFE3 gene on Xp11.2. As such, a number of studies have demonstrated nuclear immunoreactivity for this protein in most (but not all) ASPSs ( Fig. 33.67 ). However, TFE3 expression may be seen in a number of other “pink cell” tumors that enter the differential diagnosis of ASPS, and it is now clear that the specificity of TFE3 antibodies for TFE3 -rearranged tumors is poor. We recommend FISH for TFE3 instead of IHC in the diagnosis of ASPS. CD147 (mentioned previously) is also frequently expressed in ASPS but appears to lack specificity.

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