Slow-growing firm plaque on the shoulder

Dermatofibrosarcoma protuberans (DFSP) of the right shoulder — COL1A1-PDGFB rearranged — treated by slow Mohs excision and adjacent-tissue rearrangement reconstruction

Learning points

• DFSP is an indolent dermal sarcoma with characteristic tentacle-like sub-clinical extension well beyond the apparent clinical lesion. Local recurrence after inadequate excision is the principal management problem.
• Most cases harbour the COL1A1-PDGFB t(17;22) translocation — the molecular basis for imatinib activity in advanced disease.
• Diagnostic immunohistochemistry: CD34 strongly positive, factor XIIIa negative (the reverse of dermatofibroma). Negative for S100, SMA, desmin.
• Adequate biopsy: a deep punch or incisional biopsy that includes subcutaneous fat — the spindle-cell infiltrate extends in a 'honeycomb' pattern into fat. Shave biopsies routinely miss the diagnosis.
• Definitive surgery: wide local excision with 2–3 cm peripheral margins to investing fascia / underlying muscle, or slow Mohs / staged excision with permanent-section margins. Slow Mohs is increasingly favoured because of high local control rates while preserving tissue.
• Imaging: MRI of the affected area for pre-operative assessment of extent / fascial involvement; CT chest for baseline staging (metastasis is uncommon, ~5%, predominantly to lung).
• Adjuvant imatinib 400–800 mg daily for unresectable, recurrent or metastatic DFSP; pre-operative imatinib can downstage borderline cases.
• Long-term recurrence surveillance is essential — recurrences can occur > 10 years after primary treatment.