Low-grade fibromyxoid sarcoma is a rare, benign-appearing soft tissue neoplasm with an aggressive clinical course characterized by multiple local recurrences over several years, with ultimate spread to lung and occasionally to bone. Thus far, a total of 24 cases of low-grade fibromyxoid sarcoma have been reported in the literature. The authors present an additional case that grossly and microscopically emphasizes a pronounced lobular pattern of contrasting areas of cellularity showing high proliferative activity, as demonstrated by a proliferation marker, Ki 67 with MIB-1, and hypocellular areas with prominent myxoid component and abundant collagen fibrils. There was predominance of delicate capillary-sized stromal vessels with collagenized walls in both cellular and myxoid areas. The unusual features in this case were osseous metaplasia, prominent intranuclear pseudoinclusions, DNA tetraploidy, and membrane-bound intracytoplasmic fat vacuoles. The immunoprofile and cytologic and ultrastructural features are described. After the excision of the tumor, the patient was treated with radiotherapy without chemotherapy. The patient has been observed for 26 months and is alive without the evidence of disease. The postoperative follow-up with axial computed tomography at 24 months showed no evidence of disease, except postsurgical fibrotic changes.

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