Reduced S100 Protein Expression in Malignant Ossifying Fibromyxoid Tumors: A Case Report.

Ossifying fibromyxoid tumor of soft parts (OFMT) is a rare mesenchymal neoplasm of uncertain lineage. OMFT normally has a benign clinical course, and malignant variants are considered unusual. Criteria defining malignancy have not yet been clearly identified and universally accepted, and there is diagnostic uncertainty between pathologists as to how best to recognize a malignant variant. We present the case of a 68-year-old male patient who, following initial diagnosis of typical OFMT in the left scapular region, presented to the sarcoma service 9 years later with a short history of a solid lesion in the right calf. Biopsy confirmed metastatic OFMT and further imaging identified three other radiologically similar but distant lesions, which were later resected. The histology of the initial biopsy was reviewed, and the original observations were found to be accurate and due to current diagnostic criteria, the specimen was reported as typical. We propose that this case report contributes to a growing body of literature suggesting that negative S100 expression may be a useful feature in identifying and characterizing malignant OFMT.