Atypical fibroxanthoma/pleomorphic dermal sarcoma of the scalp with aberrant expression of HMB-45: a pitfall in dermatopathology.

Atypical fibroxanthoma (AFX) has been considered as the non-infiltrating precursor lesion of pleomorphic dermal sarcoma (PDS), which shows an aggressive clinical behavior, because of its extensive invasion of the deeper skin layers. Although these two tumors may represent two stages of the same disease, it can be difficult to differentiate between them, because of their similar clinical and histological features 1. Furthermore, they must be distinguished from a spindled variant of squamous carcinoma, melanoma and leiomyosarcoma 2. AFX/PDS still remains a diagnosis of exclusion, that needs to combine immunohistochemical markers for a definitive diagnosis. Usually AFX/PDS shows positivity for CD10, CD99, CD68, vimentin and lysozyme, while S100, HMB45, MART-1, cytokeratins, CD34, CD31, desmin and h-caldesmon are absent. We report a case of 89-year-old male, with a history of squamous cell carcinoma removed from the right ear, presented to our department with a recently growing, ulcerated and bleeding 2 cm nodule on the scalp. After surgery the tumor recurred with infiltration to the cranial theca. The final histological diagnosis was "pleomorphic dermal sarcoma" (PDS), which showed an unexpected positivity for HMB45. We present, to the best of our knowledge, the first case of AFX/PDS with an aberrant diffuse expression of HMB45 and an aggressive biological behavior, that leads us to a difficult exclusion diagnosis.