Answer: Melanoma in situ

Histology: Within the epidermis there is a proliferation of atypical melanocytes at all levels. The cells have a pagetoid appearance with some cells and nests present at the dermal-epidermal junction. Within the dermis are heavily pigmented macrophages (melanophages). The lesion is not circumscribed with scattered atypical melanocytes extending as individual units beyond the main confines of the lesion.

Discussion: Paget’s disease may show a similar spread of atypical cells throughout the epidermis. However, Paget’s disease occurs exclusively within the nipple, which is an unusual site for melanoma. Also, Paget’s disease does not show cells at the dermal-epidermal junction yet rather there is a preserved basal cell layer with atypical Paget’s cells present higher up in the epidermis. Paget’s disease may show some melanin deposition yet not to the extent seen in this case. In instances where the distinction between Paget’s disease and melanoma in situ is difficult, immunohistochemistry for S100, HMB45, keratin, and CEA can readily distinguish between the two. In an active junctional nevus the cells are arranged in well-formed nests at the dermal-epidermal junction without spread of atypical individual melanocytes through all levels of the epidermis. Whereas with melanoma in situ, one may see heavily pigmented melanophages in the dermis, in invasive melanoma there are atypical melanocytes present in the dermis. In cases where the heavy pigmentation makes it difficult to assess morphology, bleaching the slides may demonstrate the nature of the dermal cells more clearly. Alternatively, one can immunohistochemically identify the nature of the dermal cells as histiocytic in origin.