Desmoplastic Small Round Cell Tumor: Study of Cytomorphologic and Immunophenotypical Features in Seven Cases, One With Unusual Rhabdoid Morphology.

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Desmoplastic small round cell tumor (DSRCT) is a rare malignant mesenchymal neoplasm of polyphenotypic differentiation, characterized by EWSR1-WT1 gene fusion. Despite multimodality therapy, the tumor carries poor prognosis. We report cytomorphological, immunohistochemical (IHC) and molecular features of seven cases of DSRCT from four patients. A retrospective review of cytology smears, biopsies and molecular features of cases diagnosed as DSRCT at Department of Pathology, Henry Ford Health System, and Indiana University, USA between 2016 and 2022 were performed. A total of seven cases of DSRCT from four patients were included in this study. The cytological findings were correlated with concurrent or subsequent biopsy findings. All patients were male, with age ranging from 21 to 29 years. Two patients had primary tumor in the abdomen, one in the retroperitoneum, and one in the inguinal region. The tumors measured from 7.5 to 17 cm in size. Six out of seven cases showed typical hypercellular smears of small round cells with inconspicuous nucleoli, scant cytoplasm, and rare nuclear molding. The cytomorphology in Case no. 1 was unusual, showing discohesive pleomorphic tumor cells with large irregular nuclei, prominent nucleoli, occasional binucleation, and frequent mitotic activity. A subset of tumor cells demonstrated rhabdoid features. The tumor cells were positive for keratins (dot-like), desmin, WT-1, and CD15. In Case no. 4, the immunohistochemistry for desmin and WT-1 was unusually negative reflecting the variability of the staining pattern. Three cases were positive for EWSR1 rearrangement by fluorescence insitu hybridization (FISH) and one positive for EWSR1-WTI gene fusion by reverse transcriptase-polymerase chain reaction (RT-PCR). All seven cases had concurrent/subsequent surgical pathology diagnosis consistent with DSRCT. Cytologic diagnosis for DSRCT is challenging but can be achieved with appropriate cytomorphology, IHC profiles, molecular studies, and demographic information. Rare rhabdoid differentiation can occur in DSRCT and can be present in effusion cytology.