MYOFIBROMATOSIS-LIKE HEMANGIOPERICYTOMA METASTASIZING AS DIFFERENTIATED VASCULAR SMOOTH-MUSCLE AND MYOSARCOMA - MYOPERICYTES AS A SUBSET OF MYOFIBROBLASTS

A thyroid hemangiopericytoma that was resected in a 5-year-old boy recurred insidiously in the larynx 8 years later. Marked cicatricial mucosal inflammation prevented a definitive pathologic diagnosis of recurrence until a nodule grew to obstruct the airway 15 years after initial surgery. After excision of the nodule, a larger sarcomatous metastasis was discovered in the upper esophagus and resected, but the patient eventually succumbed to widespread disease at the age of 20 years. The original tumor contained atypical pericytes and bundles of hyalinizing smooth muscle abutting on "staghorn vessels," a pattern similar to infantile myofibromatosis. Desmin immunostaining was negative in the pericytes but positive in smooth-muscle cells dispersed singly as well as in bundles. Both elements reacted strongly for vimentin and the alpha-isoform of actin (alpha-SMA) found in normal smooth muscle and pericytes. A third cell type showing dendritic processes and immunoreactivity for all three antigens was interpreted as a myopericyte. Spindled cells in multiple subsequent mucosal biopsy specimens stained retrospectively also positive for these antigens. Large bundles of vascular smooth muscle surrounded by radiating myocytes characterized the occluding laryngeal nodule. In the esophageal metastasis, which showed no histologic features typical of hemangiopericytoma, numerous mitotically active, small, vimentin+, desmin+, alpha-SMA+ cells often maintained shortened processes and tended to form nodular aggregates about capillaries. Single rows of pericytes accreted to endothelial tubes. Ultrastructurally, some cells contained myofilaments and irregular dense material or showed rare cell junctions and variable investment by a basal lamina. This case vividly illustrates the potential for tumors composed predominantly of pericytes to differentiate toward smooth muscle and also identifies myopericytes as vimentin+, desmin+, alpha-actin+ dendritic cells. It suggests further that some tumors with a hemangiopericytomatous pattern and a presumed myofibroblastic component, such as infantile myofibromatosis, may reflect a benign disturbance in the differentiation of smooth muscle and pericytes, giving rise to myopericytes rather than true myofibroblasts.