This pathology case study focuses on the diagnosis of Desmoplastic Small Round Cell Tumor (DSRCT) in a young male patient presenting with a tumor in the peritoneal and retroperitoneal cavities. Microscopically, the tumor is characterized by malignant small round blue cells arranged in distinct islands or nests separated by prominent desmoplastic, fibrous stroma—a classic pattern for DSRCT. The video details the critical immunohistochemical profile required for diagnosis, particularly the co-expression of Keratin/EMA and Desmin, alongside nuclear staining for the C-terminus of WT1. Furthermore, the molecular hallmark, the EWS-R1/WT1 gene fusion, is discussed, highlighting the crucial need for specific molecular testing (dual FISH probes or RT-PCR) as a generic EWS-R1 break-apart probe is insufficient for differentiation from other round cell sarcomas, such as Ewing sarcoma. DSRCT is noted as a highly aggressive tumor with a strong male predominance.
Desmoplastic Small Round Cell Tumor (DSRCT): Diagnosis and Molecular Characteristics
0:03 Case Presentation: The case involves a small round blue cell tumor in the peritoneal cavity (abdomen) of a young boy, extending into the retroperitoneum near the adrenal gland.
0:48 Key Microscopic Pattern: The most important diagnostic clue is the arrangement of the small round blue cells into distinct nests or islands separated by abundant desmoplastic fibrous and vascularized stroma.
1:13 Diagnosis and Prognosis: This pattern, particularly in a young male in the abdominal/pelvic area, is classic for Desmoplastic Small Round Cell Tumor (DSRCT). Unfortunately, DSRCTs are highly aggressive and generally have a poor prognosis.
1:37 Microscopic Differential Diagnosis: The solid variant of alveolar rhabdomyosarcoma must be considered, as it can mimic DSRCT's nodular and fibrously divided appearance.
2:03 Critical Immunohistochemistry (IHC): The hallmark IHC finding for DSRCT is the co-expression of Keratin or EMA plus Desmin. This combination is rare in other tumor types (though rhabdomyosarcoma is a caveat).
2:33 WT1 Staining: DSRCT shows nuclear staining for WT1, specifically using an antibody that targets the C-terminus of the WT1 protein. Cytoplasmic WT1 staining is non-specific.
3:21 Non-Specific Marker Warning:CD99 is often positive but is non-specific ("a marker of small round blueness"). It is sensitive for Ewing sarcoma but cannot be trusted alone to differentiate DSRCT from Ewing sarcoma.
4:05 Molecular Basis: DSRCT is driven by a characteristic gene translocation: the fusion of the EWS-R1 gene and the WT1 gene (EWS-R1/WT1 fusion).
4:41 Molecular Testing Pitfall: Because EWS-R1 is commonly rearranged in many tumors (including Ewing sarcoma), a standard EWS-R1 break-apart FISH probe will be positive in both DSRCT and Ewing sarcoma and will not provide differentiation.
5:18 Molecular Confirmation Takeaway: To definitively prove DSRCT, specific molecular testing is required, such as a dual FISH probe (for EWS-R1 and WT1) or RT-PCR to detect the EWS-R1/WT1 fusion transcript.