Fat necrosis with an associated lymphocytic infiltrate represents a histopathologic clue that distinguishes cellular dermatofibroma from dermatofibrosarcoma protuberans.

BACKGROUND: Cellular dermatofibromas (CDFs) and dermatofibrosarcoma protuberans (DFSP) can be challenging to differentiate from one another. Morphologically, both entities commonly extend into the subcutis, exhibit high cellularity with limited cytologic atypia and have a mixed fascicular-to-storiform growth pattern. We sought to evaluate the significance of fat necrosis with an associated lymphocytic infiltrate as a histopathologic clue for distinguishing CDFs from DFSP. METHODS: We identified cases in our pathology database with a primary diagnosis of CDF or DFSP. Punch or excisional biopsy specimens with extension into the subcutis were selected. Previously biopsied lesions and specimens that did not interact with the subcutis were excluded. Histopathologic features were evaluated in hematoxylin and eosin stained sections. RESULTS: Fat necrosis with lymphocytic infiltrate was present in 20/20 cases of CDF. None of the 20 DFSP cases had fat necrosis with lymphocytic infiltrate although 4/20 had fat necrosis alone. CONCLUSIONS: Fat necrosis with associated lymphocytic response can aid in the distinction between CDF and DFSP.