CPC15 Case series of (not so) benign lesions

Abstract

Case 1. A 35-year-old man developed a recurrent 20-mm cellular dermatofibroma on the left shoulder after initial incomplete excision. Despite Mohs surgery, deep margin involvement persisted. Histology showed hypercellular spindle cells with increased mitoses and storiform patterns (factor XIIIa+, CD34−). Case 2. A 35-year-old Bangladeshi woman presented with a rapidly growing 25-mm nodular lesion in the right axilla with surrounding induration. Molecular testing excluded dermatofibrosarcoma protuberans (absent COL1A1–PDGFB fusion). Magnetic resonance imaging (MRI) revealed involvement of the deltoid and pectoralis major muscles. The final diagnosis was cellular fibrohistiocytoma with metastatic potential. Surgery was deferred due to pregnancy. Case 3. A 55-year-old man had a thigh lesion that haemorrhaged severely, causing symptomatic anaemia requiring transfusions. Giant aneurysmal dermatofibroma metastasized to the right-groin lymph nodes. Lymph-node dissection confirmed three tumour conglomerates (up to 125 mm) with obturator node involvement. Follow-up revealed progressive external iliac lymph-node disease requiring repeat excision. Case 4. A 21-year-old woman underwent Mohs surgery for dermatofibroma extending into muscle. Despite positive deep margins, MRI showed no residual disease. She opted for observation. All cases lacked evidence of gene fusion on sarcoma targeted RNA panel testing. These cases demonstrate that histologically ‘benign’ dermatofibromas can exhibit aggressive behaviour. Cellular and aneurysmal variants have 5–20% recurrence rates and rare metastatic potential – challenging traditional benign classification. Critical distinctions from DFSP include dermatofibroma having a factor XIIIa+ CD34− profile vs. DFSP having a CD34+ factor XIIIa− immunoprofile with pathognomonic COL1A1–PDGFB fusion. Complete excision with clear margins is mandatory for cellular or aneurysmal variants, with long-term surveillance essential. These lesions occupy a diagnostic grey zone – histologically benign yet behaviourally aggressive. Their recurrence and rare metastatic potential demand vigilant follow-up. Benign histology does not guarantee benign behaviour.