Atypical melanocytic lesions of the thigh with spitzoid and dysplastic features: a clinicopathologic study of 29 cases.

Melanocytic nevi in certain anatomic locations can display unusual histopathologic features potentially creating diagnostic uncertainty. Benign melanocytic nevi in sites such as acral, genital, and flexural areas may show unusual architecture and cytologic atypia, which can mimic dysplastic nevi and, sometimes, melanoma. Twenty-nine benign melanocytic skin lesions were identified in the thigh of 26 women and 3 men who showed atypical histologic features, including both dysplastic and spitzoid features. All lesions measured less than 1 cm in diameter. Eighteen cases showed features of compound nevi, and 11 were junctional. In all cases, the lesions displayed spitzoid features including large epithelioid and/or spindle cells, some melanocytes with ganglion-like cytomorphology, and focal suprabasilar upward migration. All cases showed dysplastic features. Clinical follow-up was available in 26 patients; no recurrences or metastases were observed. The demographic features and the anatomical sites of these melanocytic nevi appear to be reproducible, and in the assessment of histologically difficult cases, these data are helpful. It is our opinion that the malignant potential of these lesions with hybrid patterns (spitzoid/dysplastic) has yet to be determined, and although we favor these lesions to be benign, longer follow-up may yield a more complete understanding of their biologic potential.