Melanotic schwannoma of soft tissues. Electron-microscopic observations and review of literature.

This is a report of an unusual case of melanotic schwannoma located in the soft tissue of the shoulder. By light microscopy, the partially pigmented, encapsulated mass showed tumor lobules displaying an organoid pattern and benign cytologic features. By electron microscopy, the tumor cells disclosed numerous interdigitating blunt cytoplasmic processes invested by a thin, continuous basal lamina with formation of mesaxons and moderate amount of glycogen as well as immature and mature melanosomes. The patient was reported to be alive and well 7 years after local excision of the mass. We also reviewed 21 additional cases from the literature. The most frequent sites of involvement were: the spinal nerve roots and adjacent structures (12 cases), soft tissues (five cases), heart (one case), esophageal wall (one case), acoustic nerve (one case), and mandible (one case). Twelve patients were alive and well (follow-up periods ranging from 9 months to 13 years after surgical resection), and one died of surgical complications. Eight patients were lost to follow-up. Three patients had local recurrences due to incomplete excision of the mass. Ten tumors were studied electron microscopically and displayed characteristic features of both Schwann cells and melanocytes. This unique dual differentiation probably reflects their common ancestry from neural crest cells.