Lentigo maligna melanoma, also known as Hutchinson freckle, occurs in elderly patients who have a history of a premalignant lesion called lentigo maligna. The lesion often appears on the face or on other areas exposed to the sun. Histologic features include cellular pleomorphism with the juxtaposition of normal and bizarre melanocytes, principally in the basal epidermis. There is diffuse intraepidermal distribution of relatively monomorphous, large malignant melanocytes in noninvasive areas. Lesions are more common in patients older than 60 years.
Vertical invasion is evident as a nodular lesion on a background of uneven pigmentation.
Histologically, early in situ lesions show hyperplasia of melanocytes, often with prominent dendrites in the basal layer, followed by extension of aggregates of melanocytes into the upper epidermis. These findings help distinguish this melanoma from benign nevi, which may show melanin pigment but not extension of melanocytes in the upper layers. In 34 of 35 cases of nonmucosal acral lentiginous melanoma reviewed in one study, the lesions were located on the soles. Twenty-five percent of the patients gave a history of antecedent trauma to the involved site in the form of puncture wounds, friction blisters, or contact dermatitis. The poor outcome of acral lentiginous melanoma is a result of the frequent long delay in diagnosis rather than inherently aggressive biologic behavior.