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