Plexiform spitz nevus: an intradermal spitz nevus with plexiform growth pattern.

Two cases of a distinctive variant of Spitz (spindle and epithelioid cell) nevus are described. One lesion developed on the lower leg of a 17-year-old boy and the other lesion on the back of a 52-year-old man. The microscopic appearance was characterized by a plexiform arrangement of bundles and lobules of enlarged spindle to epithelioid melanocytes throughout the superficial and deep dermis. Intraepidermal melanocytic proliferation was unappreciated. Some lobules were circumscribed by a thin rim of compressed fibrous tissue. In both cases a myxoid stroma was present. The cells had abundant eosinophilic cytoplasm with well-defined borders. The nuclei were enlarged, consistently ovoid and vesicular, with small nucleoli. Both cases contained scattered multinucleate giant cells similar to those observed in classical form of Spitz nevi. No melanin pigment was detectable by light microscopy. No mitoses were observed in one case and a rare mitosis was present in the other. Tumor cells were strongly immunoreactive for S-100, but not for HMB-45, desmin, and actin. The differential diagnosis of this distinctive tumor includes desmoplastic/neurotropic melanoma, plexiform spindle cell nevus, cellular blue nevus, plexiform neurofibroma, and cellular neurothekeoma. The designation of "plexiform Spitz nevus" is chosen to emphasize its distinctive plexiform growth pattern.