Neuromuscular choristoma. A clinicopathologic study of two cases.

Neuromuscular choristoma, also known as benign "triton" tumor, is an uncommon peripheral nerve tumor usually occurring in early childhood. We report two cases, a 6-year-old girl with a solitary tumor of the supraclavicular fossa, and a 14-year-old girl with multiple small neuromuscular choristomas surrounding a recurrent massive musculoaponeurotic fibromatosis of the right axilla. These choristomas were bound firmly to the involved nerves, and consisted histologically of bundles of mature striated muscle fibers intermingled with myelinated nerve fibers. Our review of the literature indicates that these tumors are benign, regress with time, do not recur, and are usually not accompanied by functional impairment of the affected nerves before biopsy, so that wide or complete excision should not be attempted. Histogenetically, the striated muscle fibers in the nerves probably derive from nondifferentiated ectomesenchymal cells.