Plexiform spindle cell naevus: a distinctive variant of plexiform melanocytic naevus.

Twelve cases of a unique plexiform melanocytic naevus that we have termed plexiform spindle cell naevus are reported. The lesions affected young individuals (mean age 22.5 years) of both sexes and were most frequently located on the shoulders and back. The lesions clinically were slightly raised and blue or darkly pigmented, suggesting blue naevus. Histologically these tumours had a symmetrical wedge-shaped configuration, as seen in typical Spitz naevus, with the apex directed toward the deep reticular dermis or subcutis. The pigmented spindle cells were disposed in fascicles in association with neurovascular bundles and adnexal structures, imparting a plexiform architecture to the lesion. The predominant cell type consisted of spindle cells containing a granular melanin and elongated nuclei. Low-grade cellular atypia was commonly noted. Varying numbers of epithelioid cells were observed in most of the cases. In two cases studied, the naevus cells showed S-100 protein and HMB-45 immunoreactivity. The differential diagnosis of plexiform spindle cell naevus includes malignant melanoma, and spindle and epithelioid cell (Spitz) naevus, blue naevus and combined naevus. Plexiform spindle cell naevus is a distinctive type of pigmented spindle naevus distinguished from the above entities by its striking plexiform architecture, predominance of melanin-containing spindle cells and lack of significant cellular atypia.