Myopericytoma of the kidney.

Myopericytoma is a rare, histologically distinctive tumor that shows evidence of differentiation toward perivascular myoid cells. Myopericytoma is largely considered a neoplasm of skin and soft tissues, with examples of this lesion involving visceral sites being extremely limited. We present the clinical and pathologic details of an unusual case of myopericytoma occurring in the kidney. Histologically, the tumor was richly vascularized and composed of a perivascular proliferation of oval to spindle-shaped cells with bland cytologic features. The neoplastic cells were arranged in a concentric fashion around vascular lumina and also surrounded dilated, branching vessels, with a glomangiopericytomatous appearance. Mitotic figures were inconspicuous, and necrosis was absent. Perivascular myoid differentiation was supported by positive immunoreactivity for muscle-specific and smooth muscle actins, and absence of reactivity for desmin. The present case serves to expand the anatomical distribution of myopericytoma and also broadens the spectrum of primary mesenchymal neoplasms that may be encountered in the kidney.