Lentiginous melanoma: a histologic pattern of melanoma to be distinguished from lentiginous nevus

Atypical lentiginous melanocytic proliferations in elderly patients continue to pose a diagnostic dilemma with lesions variably categorized as dysplastic nevus, atypical junctional nevus, melanoma in situ (early or evolving) and premalignant melanosis. We present pigmented lesions from 16 patients (seven male and nine female) and with the exception of one case, all were older than 50 years of age. The anatomical sites included trunk (7), head and neck (6) and upper extremity (3). The clinical diagnosis was variable and included lentigo maligna, atypical nevus, pigmented basal cell carcinoma, seborrheic keratosis and lentigo. The initial biopsies mimicked lentiginous nevus or dysplastic nevus and were characterized by a lentiginous proliferation of melanocytes at the dermoepidermal junction both as single cells and as small nests with areas of confluent growth, extending to the edges of the biopsy. The retiform epidermis was maintained and pagetoid spread of melanocytes was not prominent in hematoxylin- and eosin- stained sections. Dermal fibrosis was variably present and the melanocytic proliferation demonstrated cytological atypia. The subsequent re-excisions demonstrated similar atypical melanocytic proliferation occurring over a broad area flanking the prior biopsy sites. The diagnosis of melanoma was more easily recognized in the complete excision specimens. Immunohistochemical stains for Mitf and Mart-1 highlighted the extent of the basalar melanocytic proliferation as well as foci of pagetoid spread by melanocytes. Familiarity with this pattern of early melanoma should facilitate proper classification of lentiginous melanocytic proliferations in biopsies from older adults.