Desmoplastic melanoma.

Desmoplastic melanoma (DM) is a variant of spindle cell melanoma characterized by the presence of abundant fibrous matrix. It is typically found in the head and neck region on chronically sun-damaged skin of older individuals. Early detection is uncommon, because its clinical features are not distinctive. DM is prone to misdiagnosis not only clinically but also histologically. It may simulate a sclerosing melanocytic nevus and various benign and malignant nonmelanocytic lesions. Among melanomas said to be desmoplastic by various pathologists there is significant variation with regard to the extent of intratumoral fibrosis. It may be prominent throughout the entire tumor (pure DM) or represent a portion of an otherwise nondesmoplastic melanoma (combined DM). Immunophenotypically, DM are usually strongly and homogeneously positive for S-100 protein, but are often negative or only focally positive for melanocyte differentiation antigens. DM differs from conventional melanoma in its clinical course. It is associated with a higher tendency for local recurrence, but metastases to regional lymph nodes are less common.