The Rarity in the Rarity: A Rare Case of Bednar Tumor (Pigmented Dermatofibrosarcoma Protuberans) Localized on Acral Skin With Negative COL1A1-PDGFB Fusion on Fluorescence In Situ Hybridization.

Abstract: Dermatofibrosarcoma protuberans (DFSP) is a rare, low-grade fibroblastic tumor that typically exhibits CD34 positivity and a storiform growth pattern. Although commonly found on the trunk and proximal extremities, DFSP can also present in less frequent locations, such as the head, neck, and acral sites. This report discusses a rare case of pigmented DFSP, also known as Bednar tumor, located on the back of the left hand in a 48-year-old woman. Histological examination revealed a mesenchymal proliferation with a storiform pattern and occasional dendritic melanocytes. Immunohistochemical analysis confirmed strong CD34 positivity while excluding other markers. Despite the absence of the typical COL1A1-PDGFB gene fusion, a diagnosis of pigmented DFSP was rendered. This case underscores the importance of considering DFSP in differential diagnoses of acral tumors and suggests the possibility of noncanonical genetic rearrangements in DFSP occurring in atypical locations. The patient remains under close clinical and ultrasound follow-up because of the potential for local recurrence.