Malignant hemangiopericytoma of the forearm.

Hemangiopericytoma is a tumor that arises from primitive perivascular cells in many sites of the body. A painless tumor may be present for years. Malignant hemangiopericytoma of the upper extremity is rare, with only 14 cases previously reported in the English literature. Because of the often unpredictable behavior of this tumor, extended follow-ups are strongly recommended.