Clinicopathologic features of undifferentiated round cell sarcomas of bone & soft tissues: An attempt to unravel the

Background & objectives: Certain genetically defined undifferentiated round cell sarcomas, namely BCOR-CCNB3 and CIC-DUX4 positive, have been described. Here we present detailed clinicopathologic features and molecular results in such cases. Methods: Fifty one cases of undifferentiated round cell sarcomas, including 32 cases, tested for BCOR-CCNB3 and CIC-DUX4 fusions, by reverse transcription polymerase chain reaction technique and 44 tumours, for CCNB3 immunostaining, were analyzed. Results: Twenty seven (52.9%) tumours occurred in males and 24 (47%) in females; in soft tissues (38; 74.5%), commonly, trunk and extremities and bones (13; 25.4%), frequently, femur and tibia. Five of 32 (15.6%) tested cases were positive for BCOR-CCNB3 fusion and seven (21.8%) for CIC-DUX4 fusions. Histopathologically, CIC-DUX4-positive sarcomas comprised nodular aggregates of round to polygonal cells, containing hyperchromatic nuclei, prominent nucleoli and moderate cytoplasm, with focal myxoid stroma and variable necrosis, in certain cases. BCOR-CCNB3- positive sarcomas mostly comprised diffusely arranged, round to oval to short spindly cells with angulated nuclei, vesicular chromatin, inconspicuous nucleoli and interspersed vessels. Immunohistochemically, tumour cells were positive for MIC2 in 24 of 49 (48.9%) and CCNB3 in 12 of 44 (27.2%) cases. Four of five BCOR-CCNB3-positive sarcomas showed CCNB3 immunostaining and 6 of 7 CIC-DUX4-positive sarcomas displayed WT1 immunostaining. Most patients (27/37) (72.9%) underwent surgical resection and chemotherapy. Median overall survival was 12 months, and disease-free survival was seven months. Interpretation & conclusions: Undifferentiated round cell sarcomas are rare; mostly occur in soft tissues of extremities, with CIC-DUX4 positive, as these are relatively more frequent than BCOR-CCNB3 positive sarcomas. CCNB3 and WT1 are useful immunostains for triaging such cases for BCOR-CCNB3 and CIC-DUX4 fusion testing, respectively. Overall, these are relatively aggressive tumours, especially CIC-DUX4-positive sarcomas.