Desmoplastic Malignant Melanoma on the Buttock of an 18-Year-Old Girl

Melanocytic proliferation in young people may sometimes pose a diagnostic dilemma. This is particularly so when a desmoplastic component is present. Because the two main differential diagnoses, desmoplastic malignant melanoma and desmoplastic Spitz nevus, share some morphologic features, the diagnosis of desmoplastic malignant melanoma may be overlooked. Distinction between the two is important because they show completely different biological behavior. The age of patient, site of lesion, histologic findings of melanocytic atypia, neurotropism, mitosis, and maturation help to distinguish the two entities. We report a case of desmoplastic malignant melanoma occurring in the buttock of an 18-year-old Chinese girl. Histologically, it had typical features of desmoplastic malignant melanoma with junctional melanocytic atypia and prominent neurotropism. Clinical and histologic differences between desmoplastic malignant melanoma and desmoplastic Spitz nevus are reviewed. We conclude that although desmoplastic Spitz nevus occurs much more commonly in adolescents, desmoplastic malignant melanoma can occur in this age group and even in non-sunexposed skin. Microscopic findings remain the mainstay that guides the final diagnosis.