Ossifying spindled and epithelioid tumour: Expanding the clinical and morphologic spectrum of a recently described entity.

BACKGROUND Ossifying spindled and epithelioid tumour (OSET) is a recently defined soft tissue neoplasm with characteristic features, including a peripheral shell of bone or pseudocapsule, keratin expression, and indolent behaviour. Here, we present five unique cases of OSET with novel clinical and morphologic features. MATERIALS AND METHODS Cases of OSET were collected. Clinicopathologic and molecular features were documented. RESULTS Cases of OSET were collected from lesions arising in the extremities of five patients, ranging in age from 8 to 58 years. Histologically, all tumours were well circumscribed and keratin-positive. Three showed a mixed spindle and epithelioid cell morphology, whereas two were composed predominantly of epithelioid cells. Two cases harboured mitotic rates of ≥5 mitotic figures per 10 high-power fields and contained necrosis. While peripheral pseudoencapsulation was consistent, two cases contained minimal peripheral ossification (approximately 5%), and two were entirely non-ossifying. One OSET case showed multifocal disease with synchronous tumours involving the elbow and wrist of the upper extremity; another represented a local recurrence 7 years after resection of the primary tumour; the remaining three patients are disease-free. Next-generation sequencing of tumour RNA revealed an SRSF7::NFATC3 fusion in the three cases evaluated. CONCLUSION Our cohort expands the clinical and morphologic spectrum of this entity to include tumours with increased mitotic activity and necrosis, as well as highlights the first two examples of non-ossifying OSET. Moreover, while confirming the overall indolent behaviour of OSET, we described cases that demonstrate primary multifocal disease and local recurrence.