Unusually differentiating immature nerve sheath myxoma in association with dermal melanocytosis

A 44-year-old woman presented with a slightly elevated, erythematous lesion, with partially blue-black areas. The nonpigmented area histologically showed a "dissecting" fascicular growth pattern, similar to one of the patterns seen in the cellular type of nerve sheath myxoma. The clinically pigmented part of the lesion consisted of diffusely infiltrating, broad and poorly delineated fascicles often showing nerve sheath differentiation, embedded in a highly myxomatous stroma. No part of the lesion showed the plexiform pattern typical of the classic type of nerve sheath myxoma; rather, the lesion had some common features of neurofibroma, and also was characteristically associated with a considerable number of scattered dermal melanocytes. However, based on the fascicular histologic pattern showing nerve sheath differentiation within mucinous matrix, S-100 protein-negative immunophenotype, and electron microscopic features, we considered the whole lesion in the present case to be an entity within the spectrum of nerve sheath myxoma, either mixed-type nerve sheath myxoma or unusually differentiating immature nerve sheath myxoma, except for the associated dermal melanocytosis. Because of the intimate association of the dermal melanocytes with this nerve sheath myxoma with divergent differentiation, this lesion can also be considered as a distinctive type of benign neoplasm derived from pluripotent neural crest cells.