CD34+ pigmented fibrous proliferations: the morphologic overlap between pigmented dermatofibromas and Bednar tumors.

Pigmented dermatofibrosarcoma protuberans (DFSP; Bednar tumor) constitutes 5%-10% of all cases of DFSP and shows morphologic features that overlap with melanocytic and fibrous proliferations. We report 2 unusual cases of pigmented fibrous proliferations that demonstrate features of dermatofibromas and DFSP. The first case is that of a 19-year-old man with a 3-year history of a slowly growing pigmented lesion on the right arm. On clinical exam, the lesion was a 7-mm firm pigmented papulonodular lesion. The second case is that of a 31-year-old woman with a 4- to 5-year history of a slowly enlarging, asymptomatic "dark area" on the right buttock. On clinical exam, the lesion was a 2-cm darkly pigmented flat nodule. Morphologically, both lesions are primarily dermal proliferations of spindled cells admixed with pigmented dendritic melanocytes. The lesional cells trap collagen fibers at the periphery and there is basal cell hyperpigmentation. Adnexal structures are effaced, but significant trapping of subcutaneous fat is not present. By immunohistochemistry, both lesions show focal CD34 positivity but are negative for Factor XIIIa and melanocytic markers. Although overlap between standard dermatofibromas and DFSP is well documented in the literature, pigmented fibrous lesions with features of both entities are not well described.