DFSP — SCE Dermatology MCQ

Correct answer: B — Dermatofibrosarcoma protuberans

The correct answer is B, Dermatofibrosarcoma protuberans. This diagnosis is confirmed by the combination of a firm, slowly enlarging trunk nodule with the classic histology of monotonous spindle cells in a storiform (cartwheel) pattern that infiltrates subcutaneous fat in a honeycomb pattern, together with strong CD34 positivity, which is the recognised immunohistochemical hallmark distinguishing it from its benign mimics. DFSP is a low grade, locally infiltrative fibrohistiocytic tumour driven by the COL1A1-PDGFB fusion, and current UK practice favours Mohs micrographic surgery over wide local excision because of lower recurrence rates given the tumour's deceptively wide subclinical spread. The trunk is the most common site, fitting the stem exactly. Why the other options are wrong: D. Leiomyosarcoma: arises from smooth muscle, showing fascicles of eosinophilic spindle cells with cigar-shaped nuclei that stain for desmin and smooth muscle actin, not CD34, and lacks the storiform pattern. A. Keloid: is a reactive dermal scar composed of thick, hyalinised collagen bundles without a spindle cell proliferation, storiform architecture, or CD34 positivity, and follows trauma or surgery. E. Atypical fibroxanthoma: occurs on sun damaged skin of elderly patients, shows pleomorphic epithelioid and spindle cells with marked cytological atypia and mitoses, and is CD34 negative, typically positive for CD10. C. Dermatofibroma: is a benign dermal nodule with a grenz zone, peripheral collagen trapping, and factor XIIIa positivity, but is characteristically CD34 negative, the opposite pattern to DFSP. Key point: Storiform spindle cell architecture with honeycomb fat infiltration and strong CD34 positivity is diagnostic of dermatofibrosarcoma protuberans and distinguishes it from CD34 negative dermatofibroma.

Reference: UK Health Security Agency / Clinical and Experimental Dermatology, 'Review of dermatofibrosarcoma protuberans', Clin Exp Dermatol 2023;48(4):297 - https://academic.oup.com/ced/article-abstract/48/4/297/6887192