Answer: Desmoplastic melanoma

Histology: The skin shows marked dermal elastosis, indicative of prior sun damage. There are areas where the vessels are perpendicular to the surface and associated with collagen, consistent with a prior biopsy. However, in the deep dermis and subcutaneous tissue there is a proliferation of spindle cells with hyperchromatic nuclei, associated with balls of lymphocytes. These atypical spindle cells are immunoreactive for S100 protein. These are the typical features of desmoplastic melanoma.

Discussion: A scar would lack atypical cells of the current lesion, would typically not have balls of lymphocytes within it, and would demonstrate less extensive S100 protein immunoreactivity. Fibromatosis would lack the pleomorphism of the current case, and would feature bland spindle cells with more uniform abundant collagen deposition. Neurofibroma would label for S100 protein, but would do so less profusely and would lack the atypia of the current case.

Desmoplastic malignant melanoma can closely simulate scar. When an intraepidermal melanoma component is present, it is often of the lentigo maligna type that classically arises in the chronically sun damaged skin of the head and neck. Perineural invasion and rounded clusters of lymphocytes are helpful findings.