Variant EWS-WT1 chimeric product in the desmoplastic small round cell tumor

EWSR1::WT1 Fusions in Neoplasms Other Than Conventional Desmoplastic Small Round Cell Tumor: Three Tumors Occurring Outside the Female Genital Tract

Desmoplastic small round cell tumor (DSRCT) is a high-grade, primitive round cell sarcoma classically associated with prominent desmoplastic stroma, coexpression of keratin and desmin, and a characteristic EWSR1::WT1 gene fusion. DSRCT typically arises in the abdominopelvic cavity of young males with diffuse peritoneal spread and poor overall survival. Although originally considered to be pathognomonic for DSRCT, EWSR1::WT1 gene fusions have recently been detected in rare tumors lacking the characteristic morphologic and immunohistochemical features of DSRCT. Here, we report 3 additional cases of neoplasms other than conventional DSCRCT with EWSR1::WT1 gene fusions that occurred outside the female genital tract. Two occurred in the abdominopelvic cavities of a 27-year-old man and a 12-year-old girl, whereas the third arose in the axillary soft tissue of an 85-year-old man. All cases lacked prominent desmoplastic stroma and were instead solid and cystic with peripheral fibrous pseudocapsules and occasional intervening fibrous septa. Necrosis was either absent (1/3) or rare (2/3), and mitotic activity was low (<1 to 3 per 10 hpf). In immunohistochemical studies, there was expression of smooth muscle actin (3/3) and desmin (3/3), rare to focal reactivity for EMA (2/3), and variable expression of CK AE1/AE3 (1/3). Myogenin and MyoD1 were negative, and C-terminus-specific WT1 was positive in both cases tested (2/2). All 3 tumors followed a more indolent clinical course with 2 cases demonstrating no evidence of disease at 20 and 44 months after resection. The patient from case 3 died of other causes at 14 months with no evidence of recurrence. DNA methylation profiling showed that the 3 cases clustered with DSRCT; however, they demonstrated fewer copy number variations with 2 cases having a flat profile (0% copy number variation). Differential methylation analysis with hierarchical clustering further showed variation between the 3 cases and conventional DSRCT. Although further study is needed, our results, in addition to previous reports, suggest that EWSR1::WT1 gene fusions occur in rare and seemingly distinctive tumors other than conventional DSRCT with indolent behavior. Proper classification of these unusual soft tissue tumors with EWSR1::WT1 gene fusions requires direct correlation with tumor morphology and clinical behavior, which is essential to avoid overtreatment with aggressive chemotherapy.

Desmoplastic Small Round Cell Tumors With EWS-WT1 Transcript Expression: Should We Consider Children and Adult Patients Differently?

Desmoplastic small round cell tumor (DSRCT) is an aggressive sarcoma occurring in the young, teenager, and adult populations. The aim of this study is to compare initial tumor presentation, therapeutic management and scalability between pediatric and adult DSRCT patients and investigate the possibility of specific therapeutic approaches. A multicenter retrospective study of 81 Franco-Belgian medical files with DSRCT harboring Ewing sarcoma-Wilm tumor transcript was made. Median age was 17 years (3 to 58) with 42 children (13.5 y [3;17]) and 39 adults (28 y [18;58]). No significant differences were found between the 2 groups regarding initial symptoms and metastasis at diagnosis. The therapeutic approaches were similar for both groups: use of neoadjuvant chemotherapy (78.6% vs. 79.5%, P=1), primary surgery (71.4% vs. 69.2%, P=0.73), adjuvant chemotherapy (54.8% vs. 61.5%, P=0.99), radiotherapy (23.8% and 10.3%, P=0.11) and intraperitoneal chemotherapy (14.3% vs. 2.6%; P=0.11). Median time to recurrence was 12 versus 18 months (P=0.13). Overall survival at 2 years and recurrence free were 46.4% versus 60.1% (P=0.83) and 14.3% versus 16%, respectively (P=0.16). Clinical presentation, initial therapeutics and outcome of DSRCT are equivalent suggesting that similar management should be considered for children and adults with DSRCT.

Desmoplastic Small Round Cell Tumor with "Pure" Spindle Cell Morphology and Novel EWS-WT1 Fusion Transcript: Expanding the Morphological and Molecular Spectrum of This Rare Entity

Background: Desmoplastic small round cell tumor (DSRCT) is a rare pediatric soft tissue neoplasm composed of small round tumor cells with prominent stromal desmoplasia, polyphenotypic differentiation and EWSR1-WT1 gene fusion. We, herein, present a unique case of DSRCT, exhibiting a pure spindle cell morphology, absence of desmoplastic stroma and showing a novel EWS-WT1 fusion transcript. Methods: A 12-year-old boy presented multiple intra-abdominal, confluent and mass-forming nodules that affected the entire abdominal and pelvic cavities. Results: Histologically, the nodules were composed of spindle cells with scant cytoplasm and oval nuclei arranged into short, intersecting fascicles and set in a scant, non-desmoplastic, stroma. Immunohistochemically, neoplastic cells were stained with vimentin, desmin, WT-1 (C-terminus antibodies) and EMA. Reverse-transcriptase polymerase chain reaction (RT-PCR) analysis showed the presence of an unusual chimeric transcript, composed of an in-frame junction of exon 9 of EWS to exon 7 of WT1, confirming the histological diagnosis of DSRCT. Conclusions: The present case contributes to widen the morphological spectrum of this entity; notably, the additional presence of a novel chimeric fusion transcript contributes to making the present case even more unique. Whether the detection of the above-mentioned fusion transcripts could explain the unusual morphology of the tumor remains to be established.

Essentials for Pathological Evaluation of Peritoneal Surface Malignancies and Synoptic Reporting of Cytoreductive Surgery Specimens-A review and evidence-based guide

Peritoneal surface oncology has emerged as a subspecialty of surgical oncology, with the growing popularity of surgical treatment of peritoneal metastases comprising of cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC). Pathological evaluation plays a key role in multidisciplinary management but there are still many areas where there are no guidelines or consensus on reporting. Some tumors presenting to a peritoneal surface oncology unit are rare and pathologists my not be familiar with diagnosing and classifying those. In this manuscript, we have reviewed the evidence regarding various aspects of histopathological evaluation of peritoneal tumors. It includes establishing a diagnosis, appropriate classification and staging of common and rare tumors and evaluation of pathological response to chemotherapy. In many instances, the information captured is of prognostic value alone with no direct therapeutic implications. But proper capturing of such information is vital for generating evidence that will guide future treatment trends and research. There are no guidelines/data set for reporting cytoreductive surgery specimens. Based on the authors' experience, a format for handling/grossing and synoptic reporting of these specimens is provided.

Desmoplastic small round-cell tumour of the peritoneal cavity: case report and literature review

Desmoplastic small round-cell tumour is a very rare neoplasm, which usually arises from the abdominal or pelvic peritoneum of adolescents and young adults. Early diagnosis is difficult, because most tumours present with non-specific gastrointestinal symptoms after a long asymptomatic period. It is generally a very aggressive tumour, which grows rapidly with poor prognosis and an overall five-year survival rate of 15% despite multimodal treatment. Despite multiple treatment strategies, the management of desmoplastic small round-cell tumour still remains a clinical challenge and no consensus about a therapeutic protocol has been established. A 35-year-old man presented with mild abdominal pain, constipation and weight gain, and was eventually diagnosed with desmoplastic small round-cell tumour, which was shown to be limited to the abdomen. After incomplete debulking surgery, radiotherapy and chemotherapy, he developed multiple metastatic nodular foci in chest and the pleura and, unfortunately, he died due to disease progression.

Desmoplastic small round cell tumor: evaluation of reverse transcription-polymerase chain reaction and fluorescence in situ hybridization as ancillary molecular diagnostic techniques

Desmoplastic small round cell tumor (DSRCT) is a rare, biologically aggressive soft tissue neoplasm of uncertain differentiation, most often arising in the abdominal and pelvic cavities of adolescents and young adults with a striking male predominance. Histologically, it is characterized by islands of uniform small round cells in prominent desmoplastic stroma, and it has a polyimmunophenotypic profile, typically expressing WT1 and cytokeratin, desmin, and neural/neuroendocrine differentiation markers to varying degrees. Tumors at other sites and with variant morphology are more rarely described. DSRCT is associated with a recurrent t(11;22)(p13;q12) translocation, leading to the characteristic EWSR1-WT1 gene fusion. Fluorescence in situ hybridization (FISH), to detect EWSR1 rearrangement, and reverse transcription-polymerase chain reaction (RT-PCR) to assess for EWSR1-WT1 fusion transcripts are routine diagnostic ancillary tools. We present a large institutional comparative series of FISH and RT-PCR for DSRCT diagnosis. Twenty-six specimens (from 25 patients) histologically diagnosed as DSRCT were assessed for EWSR1 rearrangement and EWSR1-WT1 fusion transcripts. Of these 26 specimens, 24 yielded positive results with either FISH or RT-PCR or both. FISH was performed in 23 samples, with EWSR1 rearrangement seen in 21 (91.3%). RT-PCR was performed in 18 samples, of which 13 (72.2%) harbored EWSR1-WT1 fusion transcripts. The sensitivity of FISH in detecting DSRCT was 91.3%, and that of RT-PCR was 92.8% following omission of four technical failures. Therefore, both methods are comparable in terms of sensitivity. FISH is more sensitive if technical failures for RT-PCR are taken into account, and RT-PCR is more specific in confirming DSRCT. Both methods complement each other by confirming cases that the other method may not. In isolation, FISH is a relatively non-specific diagnostic adjunct due to the number of different neoplasms that can harbor EWSR1 rearrangement, such as Ewing sarcoma. However, in cases with appropriate morphology and a typical pattern of immunostaining, FISH is confirmatory of the diagnosis.

Abdominal desmoplastic small round cell tumor without extraperitoneal metastases: Is there a benefit for HIPEC after macroscopically complete cytoreductive surgery?

Background

Desmoplastic Small Round Cell Tumor (DSRCT) is a rare disease affecting predominantly children and young adults and for which the benefit of hyperthermic intraperitoneal chemotherapy (HIPEC) after complete cytoreductive surgery (CCRS) remains unknown.

Methods

To identify patients with DSRCT without extraperitoneal metastases (EPM) who underwent CCRS between 1991 and 2015, a retrospective nation-wide survey was conducted by crossing the prospective and retrospective databases of the French Network for Rare Peritoneal Malignancies, French Reference Network in Sarcoma Pathology, French Sarcoma Clinical Network and French Pediatric Cancer Society.

Results

Among the 107 patients with DSRCT, 48 had no EPM and underwent CCRS. The median peritoneal cancer index (PCI) was 9 (range: 2–27). Among these 48 patients, 38 (79%) had pre- and/or postoperative chemotherapy and 23 (48%) postoperative whole abdominopelvic radiotherapy (WAP-RT). Intraperitoneal chemotherapy was administered to 11 patients (23%): two received early postoperative intraperitoneal chemotherapy (EPIC) and nine HIPEC. After a median follow-up of 30 months, the median overall survival (OS) of the entire cohort was 42 months. The 2-y and 5-y OS were 72% and 19%. The 2-y and 5-y disease-free survival (DFS) were 30% and 12%. WAP-RT was the only variable associated with longer peritoneal recurrence-free survival and DFS after CCRS. The influence of HIPEC/EPIC on OS and DFS was not statistically conclusive.

Conclusion

The benefit of HIPEC is still unknown and should be evaluated in a prospective trial. The value of postoperative WAP-RT seems to be confirmed.

Desmoplastic small round cell tumor: current management and recent findings

Desmoplastic small round cell tumor (DSRCT) is a rare and highly aggressive mesenchymal tumor that develops in the abdominal cavity of young men adults. Patients typically present with symptoms of abdominal sarcomatosis. Diagnosis is based on histological analysis of biopsies which typically show small round blue cells in nests separated by an abundant desmoplastic stroma. DSRCT is associated with a unique chromosomal translocation t(11:22) (p 13; q 12) that involves the EWSR1 and WT1 genes. The prognosis is particularly poor; median survival ranges from 17 to 25 months, largely due to the presentation of the majority of patients with metastatic disease. Management of DSRCT remains challenging and current schemes lack a significant cure rate despite the use of aggressive treatments such as polychemotherapy, debulking surgery and whole abdominal radiation. Several methods are being evaluated to improve survival: addition of chemotherapy and targeted therapies to standard neoadjuvant protocol, completion of surgical resection with HIPEC, postoperative IMRT, treatment of hepatic metastases with [(90)Y]Yttrium microsphere liver embolization.

Ewing sarcoma/peripheral primitive neuroectodermal tumor and related tumors

Ewing sarcoma/peripheral primitive neuroectodermal tumor (EWS/pPNET) and other tumors with EWS gene rearrangements encompass a malignant and intermediate neoplasm with a broad anatomic distribution and a wide age range but a predilection for soft tissue in children, adolescents, and young adults. The overlapping histologic, immunohistochemical and cytogenetic and molecular genetic features create diagnostic challenges despite significant clinical and prognostic differences. Ewing sarcoma is the 3rd most common sarcoma in children and adolescents, and desmoplastic small round cell tumor is a rare neoplasm that occurs more often in older children, adolescents, and young adults. Pathologic examination is complemented by immunohistochemistry, cytogenetics, and molecular genetics. This article reviews the clinicopathologic features of EWS/pPNET and desmoplastic small round cell tumor in the spectrum of tumors with EWS gene rearrangements. Other tumors with different histopathologic features and an EWS gene rearrangement are discussed elsewhere in this volume.

Desmoplastic small round cell tumor of the pancreas: An unusual primary site for an uncommon tumor

Pancreas is a rare location for desmoplastic small round cell tumor. The present case highlights the dilemma in diagnosis and ascertaining the site of tumor origin. Morphologic and immunohistochemical features were complemented with the molecular markers and tumor origin which was initially nebulous was subsequently confirmed on exploratory laparotomy.

A novel EWS-WT1 gene fusion product in desmoplastic small round cell tumor is a potent transactivator of the insulin-like growth factor-I receptor (IGF-IR) gene

Desmoplastic small round cell tumor (DSRCT) is a primitive sarcoma characterized by a recurrent chromosomal translocation, t(11;22)(p13;q12), which fuses the 5' exons of the EWS gene to the 3' exons of the WT1 gene. EWS-WT1 chimeras are heterogeneous as a result of fusions of different regions of the EWS gene to the WT1 gene. We report here a rare and novel EWS-WT1 variant, EWS-WT1 5/10, in a 6-year-old boy diagnosed with DSRCT and analyze the potential transactivation effect of the fusion oncoprotein. The predicted product is comprised of the N-terminal transactivation domain of EWS and lacks any sequence derived from the WT1 gene product. Nonetheless, the truncated protein was able to stimulate expression of the insulin-like growth factor-I receptor gene, a potent antiapoptotic receptor tyrosine kinase with potentially important roles in DSRCT etiology. These findings raise the possibility that the oncogenic potential of EWS-WT1 fusions is not necessarily a consequence of the fusion protein product per se.

PDGF-A, PDGF-Rbeta, TGFbeta3 and bone morphogenic protein-4 in desmoplastic small round cell tumors with EWS-WT1 gene fusion product and their role in stromal desmoplasia: an immunohistochemical study

Histologically, desmoplastic small round cell tumor is composed of the characteristic neoplastic small round cells with divergent differentiation, and distinct desmoplastic stroma. Genetically, the tumor shows a characteristic 11;22 translocation, involving the EWS gene on chromosome 22 and the WT1gene on chromosome 11 to produce an EWS-WT1 fusion gene which generates a chimeric protein functioning as a novel transcription factor that activates expression of target genes such as PDGF-A. Expression of PDGF-A, a potent growth factor for fibroblasts, has been detected in desmoplastic small round cell tumors and has been linked to the characteristic desmoplasia in these tumors. Bone morphogenic proteins, which are members of the TGFbeta superfamily play a complex role in regulating cell growth and differentiation and bone formation but have not been evaluated in desmoplastic small round cell tumors. In all, 24 desmoplastic small round cell tumors with EWS-WT1 fusion product confirmed by RT-PCR analysis were evaluated for expression of PDGF-A, PDGF-Rbeta, TGFbeta3 and bone morphogenic protein-4 by standard immunohistochemical methods with antigen retrieval on paraffin sections. Immunoreactivity was evaluated semiquantitively. Tumor-associated desmoplasia was quantified using a three-tier scale on hematoxylin- and eosin-stained sections. Desmoplastic small round cell tumors showed variable immunoreactivity with TGFbeta3 (21/24), BMP4 (14/21), PDGF-A (19/24) and PDGF-Rbeta (16/22). Less frequently, the stromal cells showed reactivity with TGFbeta3, PDGF-Rbeta and PDGF-A. Tumor-associated desmoplasia was prominent in eight, intermediate in seven and weak in nine cases. There was no correlation between tumor-associated desmoplasia and the markers tested except PDGF-A. In contrast to a previous study, our study showed that the level of PDGF-A expression inversely correlated with tumor-associated desmoplasia. Other targets of the EWS-WT1 transcription factor other than PDGF-A may be directly responsible for the prominent tumor-associated desmoplasia seen in desmoplastic small round cell tumor.

The IGFI receptor gene: a molecular target for disrupted transcription factors

The biological actions of the insulin-like growth factors, IGFI and IGFII, are mediated by their activation of the IGFI receptor (IGFIR), a transmembrane heterotetramer linked to the RAS-RAF-MAPK and PI3K-PKB/AKT signal transduction cascades. The IGFIR displays potent mitogenic, antiapoptotic, and transforming activities, and is a prerequisite for oncogenic transformation. A number of transcription factors have been identified that control the expression of this gene and therefore determine, to a significant extent, the proliferative status of the cell. The purpose of this review is to summarize data showing that, under normal physiological conditions, expression of the IGFIR is under inhibitory control by a family of negative growth regulators or tumor suppressors. Cells with a reduced number of cell-surface receptors are unable to progress through the cell cycle and remain in a postmitotic state. Loss-of-function mutation of tumor suppressors in certain cancers results in transcriptional derepression of the IGFIR gene, with ensuing increases in the levels of IGFIR and increased proliferative capacity. Understanding the molecular mechanisms responsible for transcriptional regulation of the IGFIR gene will prove important in designing novel therapies aimed at targeting the IGF axis.

Whole abdominopelvic radiotherapy for desmoplastic small round-cell tumor

PURPOSE

Desmoplastic small round-cell tumor (DSRCT) is a rare, recently described intraperitoneal malignancy occurring predominantly in adolescent boys. Our objective was to evaluate the feasibility and outcome of whole abdominopelvic irradiation (WAPI) as part of a combined modality protocol for patients with DSRCT.

METHODS AND MATERIALS

The records of all 21 patients treated with WAPI for DSRCT at our institution from 1992 to 2001 were retrospectively reviewed. Patients were treated on an institutional protocol with 7 cycles of an alkylator-based chemotherapy. After maximal surgical debulking, patients were treated with external beam radiotherapy to the whole abdomen and pelvis to a dose of 30 Gy.

RESULTS

All 21 patients completed the prescribed treatment. The median follow-up was 28 months. The overall survival and relapse-free survival rate at 3 years was 48% and 19%, respectively. The median survival was 32 months, and the median time to relapse was 19 months. Most relapses were intraperitoneal and/or hepatic. Acute toxicities included Radiation Therapy Oncology Group Grade 2 upper and lower gastrointestinal toxicity in 81% and 71% of patients, respectively. All patients experienced acute hematologic toxicity, with Grade 4 thrombocytopenia, leukopenia, and anemia in 76%, 29%, and 33%, respectively. The major long-term toxicity was small bowel obstruction, which occurred in 7 patients (33%) after surgery and WAPI.

CONCLUSION

DSRCT is a rare and highly lethal disease, requiring aggressive multimodality therapy. WAPI is feasible in conjunction with intensive chemotherapy and surgery. Hematologic and gastrointestinal toxicities are expected but manageable with diligent supportive care. The long-term efficacy of this therapy remains disappointing, thus novel approaches are being investigated.

Establishment and characterization of a novel human desmoplastic small round cell tumor cell line, JN-DSRCT-1

The exact nature of the desmoplastic small round cell tumor (DSRCT) remains controversial. More detailed analyses might be facilitated by the establishment of permanent DSRCT cell lines. To date, however, no human DSRCT cell line has been reported. In this study, we report the establishment of a new human cell line, JN-DSRCT-1, from the pleural effusion of a 7-year-old boy with pulmonary metastasis from a typical intra-abdominal DSRCT. JN-DSRCT-1 cells were small round or spindle shaped with oval nuclei and have been maintained continuously in vitro for over 190 passages during more than 40 months. Histologic features of the heterotransplanted tumors in severe combined immunodeficiency mouse were essentially the same as those of the original DSRCT, revealing nests or clusters of small round cells embedded in an abundant desmoplastic stroma. Both in vitro and in vivo, the cells exhibited immunopositive reactions for vimentin, desmin, cytokeratins (AE1/AE3 and CAM 5.2), epithelial membrane antigen, neuron-specific antigen, and CD57 (Leu-7). JN-DSRCT-1 cells exhibited a pathognomonic t(11;22)(p13;q12) translocation by cytogenetic analysis. In addition, RT-PCR and sequencing analysis revealed a chimeric transcriptional message of the Ewing's sarcoma gene exon 10 fused to the Wilms' tumor gene exon 8. To our knowledge, this is the first permanent human DSRCT cell line. The JN-DSRCT-1 cell line, which exhibits the unique morphologic and genetic characteristics of DSRCT, will be extremely useful for a variety of important studies such as the pathogenic mechanism, biologic behavior, and therapeutic model of human DSRCT.

Practical application of molecular genetic testing as an aid to the surgical pathologic diagnosis of sarcomas: a prospective study

The strong correlation of specific reciprocal translocations with individual tumor types and the demonstration that polymerase chain reaction (PCR)-based methods can detect translocations in tissue samples have stimulated interest in the role of molecular genetic testing in diagnostic surgical pathology. To evaluate the clinical utility of PCR-based molecular analysis of soft tissue neoplasms in routine surgical pathology, 131 consecutive soft tissue tumors submitted for molecular genetic testing at a tertiary care teaching hospital were prospectively analyzed over a 36-month period. RT-PCR was used to test tumor RNA for fusion transcripts characteristic of malignant round cell tumors (including Ewing sarcoma/primitive neuroectodermal tumor, desmoplastic small round cell tumor, and alveolar rhabdomyosarcoma), spindle cell tumors (including synovial sarcoma and congenital fibrosarcoma), and fatty tumors (myxoid liposarcoma). DNA sequence analysis was used to confirm the identity of all PCR products, and the PCR results were compared with the histopathologic diagnosis. We found that sufficient RNA for RT-PCR-based testing was recovered from 96% of the 131 cases and the percentage of tumors that tested positive for the associated characteristic fusion transcript was in general agreement with those reported in the literature. DNA sequence analysis of PCR products identified both variant transcripts and spurious PCR products, underscoring the value of product confirmation steps when testing formalin-fixed, paraffin-embedded tissue. Only in rare cases did testing yield a genetic result that was discordant with the histopathologic diagnosis. We conclude that PCR-based testing is a useful adjunct for the diagnosis of malignant small round cell tumors, spindle cell tumors, and other miscellaneous neoplasms in routine surgical pathology practice.

Transcriptional regulation of IGF-I receptor gene expression by novel isoforms of the EWS-WT1 fusion protein

The EWS family of genes is involved in numerous chromosomal translocations that are characteristic of a variety of sarcomas. A recently described member of this group is desmoplastic small round cell tumor (DSRCT), which is characterized by a recurrent t(11;22)(p13;q12) translocation that fuses the 5' exons of the EWS gene to the 3' exons of the WT1 gene. The originally described chimera comprises exons 1-7 of EWS and exons 8-10 of WT1. We have previously reported that the WT1 protein represses the expression of the IGF-I receptor gene, whereas the EWS(1-7)-WT1(8-10) fusion protein activates IGF-I receptor gene expression. It has recently become apparent that EWS-WT1 chimeras produced in DSCRT are heterogeneous as a result of fusions of different regions of the EWS gene to the WT1 gene. We have recently characterized additional EWS-WT1 translocations that involve the juxtaposition of EWS exons 7 or 8 to WT1 exon 8, and an EWS-WT1 chimera that lacks EWS exon 6. The chimeric transcription factors encoded by these various translocations differ in their DNA-binding characteristics and their ability to transactivate the IGF-I receptor promoter. These data suggest that the molecular pathology of DSRCT is more complex than previously appreciated, and that this diversity may provide the foundation for predictive genotype-phenotype correlations in the future.

Pathologic, cytologic and immunohistochemical findings of an intra-abdominal desmoplastic small round cell tumor in a 15-year-old male

The intra-abdominal desmoplastic small round cell tumor is a rare neoplasm. It usually occurs in young males and diffusely involves the peritoneum and pursues an aggressive clinical course. The present patient was a 15-year-old male who experienced abdominal pain and abdominal swelling. The patient was diagnosed with an intestinal myogenic sarcoma, and surgery for tumor resection was performed in June 1999. The tumor was a 20 x 15 x 15 cm well-defined mass in the peritoneum involving the transverse colon and stomach with peritoneal disseminations and splenic metastasis. Microscopic findings were well-defined nests composed of small round cells and separated by abundant desmoplastic stroma. Cytologically, the tumor cells consisted of small, round to oval cells with a scant amount of light blue cytoplasm. Immunohistochemically, the tumor cells were positive for anti-epithelial membrane antigen, vimentin, desmin, neuron-specific enolase and WT1 protein antibodies. Similar pathologic features with other small round cell tumors may lead to differential diagnostic difficulties that require the application of ancillary diagnostic methods, such as immunohistochemistry and cytogenetic techniques.