A 92-year-old man had a 2-month history of head nodule with bleeding. A partial biopsy of the lesion was performed, and histologically it showed proliferation of fibroblast-like spindle cells with marked cellular pleomorphism and atypia. Immunohistochemically, the tumor cells were focally and weakly positive for α-SMA, h-caldesmon, and calponin, but were negatively stained for desmin. Thus, leiomyosarcoma(LMS)was suspected. The nodule was widely excised and histological examination revealed that it was a dermal well-circumscribed nodule composed of haphazardly arranged spindle cells and pleomorphic epithelioid cells admixed with multi-nuclear giant cells and showing atypical mitotic activity. Immunohistochemically, the tumor cells were focally positive for α-SMA and h-caldesmon, but were negatively stained for desmin, calponin, CK5/6, p40, p63, AE1/AE3, S-100p, CD31, and CD34. Thus, we diagnosed the nodule as atypical fibroxanthoma(AFX). AFX should be distinguished from several spindle cell tumors or epithelioid cell tumors such as undifferentiated pleomorphic sarcoma. In our case, LMS was listed as one of the differential diagnoses, but the tumor was finally diagnosed as AFX due to its clear demarcation in the dermis, with only focally positive immunostaining for α-SMA and h-caldesmon. Partial biopsy was inadequate for the differential diagnosis between AFX and LMS, and evaluation of the entire tumor was necessary to achieve a definitive diagnosis.
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