Aneurysmal dermatofibroma mimicking both clinical and dermoscopic malignant melanoma and Kaposi's sarcoma

Aneurysmal dermatofibroma (AD) or aneurysmal fibrous histiocytoma (AFH) is a relatively rare form of histiocytoma representing less than 2% of total cases. It shares many clinical and dermoscopic similarities with skin tumors, especially malignant melanoma and Kaposi's sarcoma, which can make differentiation problematic. We report the case of a 53-year-old man, who presents with a black nodular tumor with increased consistency, edges infiltrated from the surface to depth, spontaneous and sensitive to touch that shows rapid growth in the last three months. Dermoscopically, the central region consists of intricate areas colored in red, violet, blue-white and black. On the periphery stand two rings, centrally white and peripherally pigmented, with an abundance of polymorphic capillaries. To clarify the diagnosis, the lesion was widely excised and histopathological examination was performed, which revealed immunophenotypical tumor cells negative for HMB-45 and S100, and numerous CD68 macrophages between tumor cells. This aneurysmal fibrous histiocytoma shows both clinical and dermoscopically discrete differential criteria, which are not specific and that make it difficult to distinguish from malignant melanoma and Kaposi's sarcoma, and required performing histopathology and immunohistochemistry.