A case of myopericytoma on the lower leg.

Myopericytoma (MP) is a benign tumor composed of cells that show apparent differentiation towards putative perivascular myoid cells called myopericytes. It arises most commonly in the dermis or subcutaneous tissue of the extremities in adults. The most common presentation is a well-circumscribed, slow-growing painless firm mass. A 45-year-old woman presented with a 2-year history of a painless, slowly growing 0.9×0.7 cm sized firm mass in the subcutaneous tissue of the posterior side of the right lower leg. We presumed this lesion to be an epidermal cyst, pilomatricoma or calcinosis cutis and performed an excisional biopsy. The histologic examination showed that it was composed of spindle-shaped myoid-appearing cells in a concentric arrangement, intimately associated with thin-walled vascular channels. Lesional spindle cells were diffusely positive for smooth muscle actin and were negative for CD34, desmin and S100 protein. From these findings, we diagnosed this lesion as a myopericytoma.