Ossifying dermatofibroma with osteoclast-like giant cells: report of a case and literature review.

Dermatofibromas are fibrohistiocytic lesions with numerous histologic variants. Ossifying dermatofibroma with osteoclast-like giant cells is an uncommon variant that has only rarely been reported. We report another case of ossifying dermatofibroma with osteoclast-like giant cells and describe the immunohistochemical expression pattern of these rare lesions. A 72-year-old male presented with a 3.5-cm subcutaneous nodule on the posterior right shoulder of several years duration. The excision specimen showed a large, dermal-based, well-circumscribed, nonencapsulated heterogenous spindle cell proliferation. Large islands of spindled cells arranged in a storiform pattern were separated by broad fibrous bands. Collections of multinucleated giant cells were present predominantly at the periphery of the spindle cell islands. In addition, small islands of bone with osteoblastic rimming were present multifocally, concentrated in the central portion of the lesion. The spindle cells express factor XIIIA, smooth muscle actin, and CD68 on immunohistochemical stains, confirming a fibrohistiocytic origin. There was no immunohistochemical expression for S100 protein, panmel, CD57, cytokeratin, neuron-specific enolase, or CD34. A broad differential diagnosis, including variants of melanoma and osteosarcoma, should be considered when analyzing cutaneous lesions with a fibrohistiocytic component admixed with giant cells and metaplastic bone.