—  SHORT COURSE #21  —

Mesenchymal Neoplasms of the Female Genital Tract

Case 9 - Desmoplastic Small Round Cell Tumor

Teri Longacre, Esther Oliva and Robert Soslow


Introduction
The desmoplastic small round cell tumor (DSRCT) was described in 1989 by Gerald and Rosai [1] and by Ordonez's group [2]; much of the subsequent work was performed by Gerald, Rosai, Ladanyi and colleagues [3, 4, 5, 6, 7] and Ordonez's group [8]. This remarkable tumor was described as disproportionately affecting adolescent males, with a predominant intra-abdominal location, a nested pattern of growth with striking desmoplasia, rhabdoid features, and coexpression of epithelial-related, neural and muscle markers. The tumor was also noted to be highly aggressive. Subsequent reports in young women, including those with ovarian disease [9, 10, 11], make this a relevant topic for us to discuss. The differential diagnosis with tumors occurring in young individuals and with tumors composed of small round cells is extensive.

Clinical Features
DSRCT is a highly aggressive neoplasm that predominates in young men, the vast majority (>95%) of whom present with disease in the peritoneal cavity [7]. The male:female is 9:2 and the mean age of presentation is 22 years (range 6-49 years). Presenting symptoms have included abdominal pain, constipation, weight loss, abdominal distension and jaundice [12]. Uncommon sites of involvement include, but are not limited to, paratesticular tissues, pleura, ovary, posterior cranium, bones and soft tissue of the hand and sinonasal tissues. Sites of metastasis have included liver, spleen, lung and lymph nodes [12]. Three-year survivals of 55% have been reported in patients who were treated with surgery, chemotherapy and radiotherapy, although this figure drops to 27% when the three treatment modalities were not administered [12].

Morphology
Grossly, tumors are large and lobulated, usually solid and grey-white on sectioning. Typically, DSRCT is composed of nests of uniform, small cells with scant cytoplasm set in a fibrous background. The nuclei are generally hyperchromatic without apparent nucleoli, although large, atypical nuclei with prominent nucleoli can be observed. Nearly every DSRCT contains such characteristic features at least focally; unfortunately, atypical patterns are seen in many examples. Some examples exhibit eccentric nuclei and ample eosinophilic cytoplasm with rounded paranuclear accentuations of filaments, reminiscent of rhabdoid cells. Variations on the nested and desmoplastic pattern include examples showing tumor cell nests with central necrosis, solid sheets of tumor cells, gland-like or papillary structures, trabeculae, cords of single cells, rosettes, and spindle cells. [7, 8]

Immunohistochemistry and Genotype
Most DSRCTs express keratins (86%), EMA (93%), vimentin (97%), desmin (90%), NSE (81%) [7] and WT1 (>90%) [7, 13, 14, 15]. Scant, weak or negative results have been reported with common muscle actin (2%), myogenin (0%), chromogranin (2%) and CD99 (O13; 20%) [7]. Paranuclear coloration is frequently seen with desmin in rhabdoid examples. Only antibodies that recognize the carboxy-terminal portion of the WT1 protein are useful for diagnosis as that is the only portion of WT1 that remains after a chimeric protein is formed.

Cytogenetic studies reveal a specific translocation between chromosomes 11 and 22 [t(11;22)(p13;q12)] in up to 95% of tumors, and molecular genetic analysis demonstrates a fusion of the EWS and Wilms tumor suppressor genes (WT1) [16, 17, 18, 19]. The most commonly identified EWS/WT1 chimeric transcript is composed of an in-frame fusion of the first 7 exons of EWS, encoding the potential transcription modulating domain, and exons 8-10 of WT1, encoding the last 3 zinc-fingers of the DNA-binding domain. [6, 7]. RT-PCR and FISH are routinely used in some centers for confirmation of the translocation.

Differential Diagnosis (Table 1)
The list of entities in the differential diagnosis is long and complex. On a positive note, the immunophenotype and genotype of DSRCT and many other entities on the list are quite specific. Unlike other tumors in the differential, DSRCT co-expresses keratins, desmin and WT1, lacks LCA, myogenin, and almost always harbors t(11;22).

De-differentiated and undifferentiated carcinoma vs DSRCT
"Dedifferentiated" endometrial carcinoma is a recently described entity that includes a well or moderately differentiated endometrioid adenocarcinoma juxtaposed with an undifferentiated carcinoma [20]. The typical patient is middle aged or older, in contrast to the typical DSRCT patient. The undifferentiated component, which could possibly be confused with DSRCT, is made of small rounded cells and characteristically shows only focal or weak keratin expression, unlike DSRCT. These tumors would not be expected to express desmin, but only anecdotal information is available regarding this.

Hypercalcemic small cell carcinoma vs DSRCT
Small cell carcinoma of hypercalcemic type is a rare ovarian tumor that affects young women predominantly. The prototypic example is composed of small, round mitotically active cells with scant cytoplasm, arranged in sheets and forming pseudofollicular spaces. Nearly two-thirds of patients are found to be hypercalcemic and most have high stage disease at presentation [21, 22]. Both the histologic appearance of a small round cell tumor and the affected patients' ages resemble DSRCT. The large cell variant of DSRCT can also demonstrate a rhabdoid appearance, potentially leading to confusion with DSRCT. Despite these superficial similarities, the tumors are immunohistochemically distinct. Hypercalcemic small cell carcinoma usually demonstrates at least focal evidence of epithelial differentiation (with keratins and/or EMA) along with expression of p53, CD10, calretinin and WT1 [23]. They are negative for desmin, inhibin, chromogranin and TTF-1 and sometimes show reactivity for synaptophysin [23]. CD99 expression has also been reported. This tumor should not be confused with the small cell carcinoma of undifferentiated, anaplastic, oat-cell or pulmonary type, a neuroendocrine neoplasm that arises primarily in the ovaries of older individuals, almost always in concert with a surface epithelial carcinoma such as serous carcinoma.

Lymphoma and leukemia vs DSRCT
With the exception of the very uncommon Burkitt lymphoma, the ovary is only rarely the primary site for lymphoma. Examples of lymphomas that may secondarily involve ovary [24, 25] include diffuse large B-cell, follicular and extranodal marginal zone (MALT) types. T-cell lymphomas, such as anaplastic large cell lymphoma and precursor T-lymphoblastic lymphoma are uncommon [26]. All of the B-cell lymphomas would be expected to express LCA, in contrast to DSRCT, although this may be lacking in anaplastic large cell lymphoma (CD30-positive) and precursor T-lymphoblastic lymphoma (TDT-positive). Leukemias (chloroma, granulocytic sarcoma or extramedullary myeloid tumor) infrequently involve the ovary in a clinically significant way, although they are commonly found at autopsy in patients who died of leukemia [27]. They express CD43, myeloperoxidase and muramidase.

Granulosa cell tumor vs DSRCT
Both adult and juvenile granulosa cell tumors [28] can resemble DSRCT, although they are infrequently disseminated at presentation. Adult granulosa cell tumors (AGCTs) only uncommonly present in adolescence, in contrast to DSRCTs. These tumors can show nested growth of small cells with scant cytoplasm, but microfollicles, pseudopapillary patterns and prominent nuclear grooves are all much more common in AGCTs. Juvenile granulosa cell tumor (JGCT), particularly when high stage at presentation and mitotically active, can also mimic DSRCT, but the cells of JGCTs generally contain more cytoplasm. Follicle-like spaces are also a common feature of JGCTs. Like DSRCT, these tumors frequently express WT1, and JGCT, in particular, can express EMA at low levels. Diffuse expression of EMA, desmin and keratin is not expected. Granulosa cell tumors also express inhibin, calretinin and, frequently, CD99. I am not aware of reports of DSRCTs with hormonal manifestations, which are commonly encountered in granulosa cell tumors.

Dysgerminoma vs DSRCT
In most instances, the morphologic appearance of dysgerminoma is sufficiently unique and reproducible to enable differentiation between it and DSRCT. Instead of growing in nests, dysgerminoma grows in sheets separated by lymphocyte-bearing fibrous trabeculae. The cells contain prominent clear or pink cytoplasm and a large nucleus with a distinctive and large nucleolus. Dysgerminoma only very rarely expresses keratins diffusely and does not express desmin. Expression of PLAP, CD117 and OCT 3/4 is typical.

Rhabdomyosarcoma vs DSRCT
Primary ovarian rhabdomyosarcomas are extraordinary [29]. A more common, but still very rare occurrence is rhabdomyosarcoma metastatic to the ovary [30, 31]. Rhabdomyosarcomatous components of more frequently occurring gynecologic tumors may predominate, giving the impression of rhabdomyosarcoma. Such tumors include adenosarcoma and carcinosarcoma, and rarely, Sertoli-Leydig cell tumors and immature teratomas, among others. Like DSRCT, rhabdomyosarcomas almost always show some desmin staining; unlike DSRCT, rhabdomyosarcomas are essentially always at least focally myogenin and Myo-D1 positive.

Neuroectodermal tumors vs DSRCT
Both primary and metastatic neuroectodermal tumors have been described to involve ovary [31, 32, 33]. Primary tumors that might resemble DSRCT include neuroblastoma, medulloblastoma, primitive neuroectodermal tumor of central type [32, 33] and primitive neuroectodermal of peripheral type (Ewing-like) [34]. Neuroblastoma, in particular, can metastasize to the ovary [31]. None of these tumors demonstrates the characteristic immunophenotype of DSRCT. Strikingly, some peripheral-type PNETs demonstrate architectural growth patterns that mimic DSRCT. These are immunophenotypically and genotypically distinct from DSRCT, however.

Tumors with rhabdoid features
Many tumors, including carcinomas (particularly the large cell variant of small cell hypercalcemic carcinoma), sarcomas, melanomas and even lymphomas, can demonstrate rhabdoid features. Two specific rhabdoid entities are renal/extrarenal rhabdoid tumor and epithelioid sarcoma of the proximal type.

In theory, it is possible that a renal or extrarenal rhabdoid tumor could be mistaken for DSRCT, but I have not heard of such a case. Extrarenal rhabdoid tumors, related to CNS tumors bearing the same name, occur in young and middle-aged patients. Like DSRCT, they bear a specific (but different) genotype. Familial examples of central nervous system cases show mutation of INI-1 (with consequent loss of heterozygosity) on chromosome 22q and sporadic extrarenal rhabdoid tumors demonstrate homozygous INI-1 deletions [35]. Most express EMA and vimentin. They would be expected to lack expression of INI-1 using the BAF47 monoclonal antibody [36].

Another theoretical consideration, though not a practical one, is the proximal type of epithelioid sarcoma [37], a tumor that occurs in the buttocks and vulva of young to middle-aged patients. Most express keratins, EMA and vimentin and many express desmin and CD34. Some authors have suggested that, although they have been described as epithelioid sarcoma variants, they are instead extrarenal rhabdoid tumor variants. One example we studied showed homozygous deletion of INI-1, identical to extrarenal rhabdoid tumor. Loss of INI-1/BAF47 expression has also been reported anecdotally [38].

Although such tumors have a rhabdoid appearance, they can also be mistaken for squamous carcinomas, both because of the tumor's location and because of the tumor cells' abundant eosinophilic cytoplasm. The absence of intercytoplasmic bridges and epithelial involvement along with expression of CD34 and lack of BAF47 all favor extrarenal rhabdoid tumor/proximal epithelioid sarcoma over squamous carcinoma.

Other miscellaneous tumors (epithelioid smooth muscle tumors, endometrioid stromal sarcoma, mesothelioma and melanoma)
It is also theoretically possible that epithelioid smooth muscle tumors, endometrioid stromal sarcoma, mesothelioma and melanomas could resemble DSRCT. The keratin, desmin and WT1 positive immunophenotype of DSRCT contrasts with endometrioid stromal sarcoma, mesothelioma and melanoma, although mesothelioma can on occasion express desmin. In contrast to mesothelioma, DSRCT frequently expresses Ber-EP4, Leu-M1 and lacks CK5/6. Using immunohistochemistry alone to distinguish epithelioid smooth muscle tumors might be problematic, however. Close clinical correlation, morphologic study and gene rearrangement studies could be contributory.

A managerial approach
As discussed, DSRCT can be separated from its histologic mimics because of characteristic and rather specific genotypic and immunophenotypic features. Any diagnostically difficult lesion with some features of DSRCT should therefore be studied for t(11;22)(p13;q12) or the EWS/WT1 chimeric transcript. It is probably not necessary to study the genotype of lesions demonstrating classic morphology and immunophenotype, but the threshold should be low when there are any doubts about the diagnosis. DSRCT is usually, unfortunately a lethal disease, and one that triggers a diagnosis-specific treatment regimen.

Table 1. DSRCT Differential Diagnosis

Desmin EMA WT1 t(11;22)(p13;q12)
DSRCT + + + +
Small cell hyperCA++ - + + -
Lymphoma/leukemia - - R -
Granulosa1 -/+ - +/- -
Dysgerminoma - - ? -
Rhabdomyosarcoma2 + R R -
Neuroectodermal3 - R R -
Endometrioid stromal -/+ - + -
Mesothelioma4 R + + -

Abbreviations: DSRCT—desmoplastic small round cell tumor; hyperCA++--hypercalcemic; + —positive; - —negative; -/+ —usually negative; +/- —usually positive; R—rare

1 Also expresses inhibin. JGCT can express EMA focally.

2 Also expresses myogenin and Myo D1

3 PNETs of peripheral type also express CD99 strongly and diffusely

4 Also express CK5/6, but not Ber-EP4 or Leu-M1

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