BCOR-Altered Sarcoma in a 6-Month-Old: Diagnostic Challenges and Morphological Insights.

BCOR-altered sarcomas (BAS) are a subset of undifferentiated small round cell sarcomas characterized by alterations involving the BCOR gene. These tumors often pose diagnostic challenges due to their morphological overlap with other pediatric neoplasms. We report a case of BAS in an infant presenting with bilateral lower limb weakness. Imaging revealed a large multilobulated retroperitoneal mass. Histopathology showed prominent rhabdoid cells, alongside small round cells and spindle cells in a fascicular pattern. The tumor was positive for BCOR, Cyclin D1, TLE-1, BCL-6, and SATB2, while INI1 and BRG expression were retained. Fluoresence in situ hybridization, ruled out BCOR::CCNB3 fusion and NTRK 1/2/3 rearrangements. Based on the histopathological, immunophenotypic, and molecular profile, a diagnosis of BAS was established. The patient was started on a Ewing sarcoma chemotherapy regimen, showing a favourable response. This case highlights that BAS may rarely show a prominent rhabdoid feature and utility of ancillary diagnostic modalities.