Ossifying fibromyxoid tumor with atypical histological features: a case report.

Ossifying fibromyxoid tumor of soft tissues (OFMT) is considered a rare mesenchymal neoplasm. Its main histological features are sheets and ill-defined lobules of rounded bland cells within a fibromyxoid background and a thick collagenous capsule with an incomplete rim of lamellar bone. This lesion occurs mostly in the soft tissues of the lower extremities and limb girdles. In this paper, we describe a mesenchymal tumor removed from the right thigh of a 41 year-old-woman. The neoplasm differed histologically from typical forms of OFMT for areas of moderate cellularity and atypia, nuclear enlargement and small nucleoli. Focally, stromal tongues of osteoid were centrally and irregularly located within the lesion with evident spindling of tumor cells around them. The mitotic activity was low (up to 19 per 50 HPF) and atypical figures were rarely seen. The tumor was positive to S-100 protein, vimentin, CD10, CD56, CD99, ASMA, calponin and collagen IV. Rare elements were positive for cytokeratin AE1/AE3. To the best of our knowledge, this is the first case of atypical OFMT reported to be positive for calponin. The patient is currently alive and well with no evidence of disease at 96 months following surgery. In spite of low-grade histology, OFMT has high local recurrence rate and low metastatic potential, primarily in the lungs, even several years after surgical removal. The recognition of this entity is important. In this report the authors address differential diagnosis and enigmatic histogenesis of this neoplasm.