BCOR-CCNB3-Positive Soft Tissue Sarcoma with Round and Spindle Histology: A Series of 4 Cases Highlighting the Pitfall of Mimicking Poorly Differentiated Synovial Sarcoma.

Aims: BCOR-CCNB3 sarcoma is a genetically defined undifferentiated malignancy with Ewing sarcoma (ES)-like round cells, and preferentially affects the bones of male adolescents. Sarcomas harbouring BCOR-CCNB3 rarely arise from soft tissues; therefore, we aimed to report four cases to expand the clinicopathological spectrum. Methods and results: By reverse transcription polymerase chain reaction and confirmatory sequencing, we detected a BCOR-CCNB3 transcript in primary undifferentiated sarcomas of the deep musculature of four male patients, comprising two teenagers (aged 14 and 17 years) and two adults (aged 34 and 44 years). The tumours originated in the back (n = 2), pelvis (n = 1), and thigh (n = 1), and were 70-140 mm in size (mean, 107 mm). All tumours showed sheets of primitive round or ovoid cells with vesicular nuclei, active mitosis (28-41/10 high-power fields), variably prominent nucleoli, and geographical necrosis. This major component transformed into fascicles of elongated spindle cells with staghorn vessels and a myxoid reticular stroma, accounting for 10-50% of areas. All cases were positive for CD99, three were positive for TLE1, and one was positive for EMA, indicating poorly differentiated synovial sarcomas (PDSSs). Nuclear cyclin B3 reactivity was present in all cases, but not in molecularly confirmed atypical ESs and PDSSs. At the last follow-up (median, 13.5 months), one patient had died of lung metastasis, two were alive with tumours, and one was tumour-free. Conclusions: BCOR-CCNB3-positive sarcomas may primarily occur in soft tissues of adults and show PDSS-mimicking round-cell and spindle-cell histology with aggressive behaviour. Cyclin B3 is useful for selecting candidates for BCOR-CCNB3 molecular testing.