OSCE — indurated trunk plaque: DFSP diagnosis, Mohs preference and imatinib pathway

Brief (to candidate)

A 42-year-old man has a slowly enlarging, firm blue-red-brown indurated plaque on the shoulder/trunk for 3 years, now becoming multinodular and “protuberant.” A prior clinic called it a scar/keloid. You have 8 minutes to suspect DFSP, plan biopsy/IHC, choose surgery (Mohs vs wide margins), and outline imatinib use and fibrosarcomatous risk.

Candidate instructions

1. Recognise classic DFSP morphology and demographics.
2. Plan biopsy and key IHC (CD34 vs Factor XIIIa).
3. Explain COL1A1–PDGFB fusion / PDGFR pathway.
4. Prefer Mohs or wide excision with generous margins; discuss recurrence rates conceptually.
5. Identify fibrosarcomatous transformation as metastasis risk escalator.
6. Indicate imatinib for unresectable/recurrent/metastatic disease.

Examiner checklist (mark each domain / 10)

Model key actions

• Suspect DFSP in chronic indurated trunk plaque mislabelled keloid/scar; biopsy with CD34.^[3][6]
• Plan Mohs (or wide margins) as definitive local therapy.^[1]
• Use imatinib when unresectable/metastatic; escalate if fibrosarcomatous change.^[4][1]

Common errors

• Treating DFSP as keloid with steroids alone.
• Narrow excision like a dermatofibroma.
• Missing CD34/Factor XIIIa distinction from DF.
• Ignoring fibrosarcomatous histology as a game-changer for metastasis risk.
• Failing to re-excise positive margins.