Low-grade fibromyxoid sarcoma was first described more than 20 years ago. Subsequently, it was discovered to carry the recurrent chromosomal translocation t(7;16)(q33;p11) encoding a FUS-CREB3L2 fusion oncoprotein. Molecular tests for this pathognomonic gene fusion can confirm the identity of histologic variants (such as hyalinizing spindle cell tumor with giant rosettes) and suggest that some cases of sclerosing epithelioid fibrosarcoma may represent a high-grade version of this entity. We present a case of an ossifying tumor of the perineum that required an open biopsy and fluorescent in situ hybridization testing for FUS and CREB3L2 for diagnosis as a variant of low-grade fibromyxoid sarcoma. Subsequent excision revealed characteristic areas with collagen rosettes as well as foci of heterotopic ossification. Significant ossification, which is well documented in entities such as synovial sarcoma, ossifying fibromyxoid tumor, and extraskeletal osteosarcoma, has not been reported previously in low-grade fibromyxoid sarcoma. This case demonstrates the value of having a distinctive confirmatory molecular pathology test for diagnosis and expands our knowledge of the histologic variants possible in low-grade fibromyxoid sarcoma.
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