Low-grade fibromyxoid sarcoma with prominent giant rosettes and heterotopic ossification

Low-grade fibromyxoid sarcoma is a rare soft tissue sarcoma of fibroblastic differentiation characterized by a deceptively benign morphologic appearance with almost consistent MUC4 expression and recurrent chromosomal translocations, t(7;16)(q34;p11) and t(11;16)(p11;p11), resulting in the FUS-CREB3L2 and FUS-CREB3L1 fusion genes, respectively. A subset of the tumors show peculiar histologic features, designated as giant rosettes, and were formerly referred to as hyalinizing spindle cell tumor with giant rosettes. We herein report a case of low-grade fibromyxoid sarcoma showing the presence of numerous giant rosettes, with and without collagenous centers, distributed throughout the lesion and unusual rim-like heterotopic ossification. Such a case might present a diagnostic challenge. The diagnosis of the tumor was confirmed by positive immunoreactivity to MUC4 and the FUS-CREB3L2 fusion detected by molecular testing using formalin-fixed, paraffin-embedded tissue. This case, which has such unusual clinicopathologic features, would help to further expand our knowledge regarding the morphologic diversities of low-grade fibromyxoid sarcoma.