Desmoplastic melanoma: a diagnostic trap for the unwary.

Desmoplastic melanoma (DM) is an uncommon type of spindle cell melanoma in which the malignant cells are separated by collagen fibres or fibrous stroma. DM is prone to misdiagnosis because of unfamiliarity with its often bland appearance and dissimilarity to other conventional types of melanoma. It displays variable cytological atypia, cellularity and stromal fibrosis and more often than not has an accompanying atypical junctional component. Neurotropism is a common associated feature (in at least 30% of cases) and when it occurs such tumours are termed 'desmoplastic neurotropic melanomas' (DNM). The neurotropism may be perineural or intraneural and often extends beyond the desmoplastic component. In addition, some DMs show evidence of neural differentiation ('neural transformation'). DM may also present as a recurrence or occasionally as a metastasis from other types of melanoma. A high index of suspicion and knowledge of its often subtle morphological appearance are necessary to avoid misdiagnosis.