Derm Cases · Dermatology / Surgical dermatology / Skin cancer
OSCE — counsel for Mohs micrographic surgery: indications, margin control, and outcomes
An 8-minute OSCE station on Mohs micrographic surgery indications (H-zone, aggressive histology, recurrence), 100% en face margin control vs bread-loafing, cure rates for BCC/SCC, and counselling on same-day staged excision and reconstruction.
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Study tools
Target exams
NEET-PGINICETUSMLEPLABMRCP
Prompt
An 8-minute OSCE station on Mohs micrographic surgery indications (H-zone, aggressive histology, recurrence), 100% en face margin control vs bread-loafing, cure rates for BCC/SCC, and counselling on same-day staged excision and reconstruction.
Brief (to candidate)
A 67-year-old has a biopsy-proven infiltrative BCC on the nasal ala (H-zone). She asks why she cannot “just have it cut out with a big margin” like her previous arm lesion. You have 8 minutes to explain Mohs, when it is indicated, how margin control differs from standard excision, and expected cure rates.
Candidate instructions
- Define Mohs micrographic surgery in plain language.
- List high-yield indications (site, histology, recurrence, immunosuppression).
- Contrast 100% en face frozen-section margins with bread-loaf vertical sectioning.
- Quote approximate cure rates for primary/recurrent BCC and primary SCC.
- Outline the same-day stages → clear margins → reconstruction pathway.
Examiner checklist (mark each domain / 10)
| Domain | Key actions expected |
|---|---|
| Definition | Staged excision where surgeon maps tumour, processes horizontal (en face) frozen sections of the entire peripheral and deep margin, and re-excises only positive areas until clear (tissue-sparing, complete circumferential peripheral and deep margin assessment)[1][3] |
| Indications | H-zone face (periorbital, perinasal, periauricular, perioral, ala, nasolabial, helix); recurrent NMSC; aggressive subtypes (infiltrative/morpheaform/micronodular BCC, poorly differentiated SCC, PNI); poorly defined borders; immunosuppression; selected DFSP and other infiltrative tumours where available[1][4][6] |
| Why not standard WLE alone | Bread-loafing samples a small fraction of the margin; subclinical extension at high-risk sites risks incomplete removal and larger reconstruction later[1][3] |
| Outcomes | Among highest for NMSC: roughly 98–99% primary BCC, 94–96% recurrent BCC, 96–97% primary cutaneous SCC (context-dependent literature figures acceptable if in this range)[1] |
| Process counselling | Local anaesthesia; multiple stages same day; waiting between stages; reconstruction only after clear margins; scar/functional risk on face; rare need for further reconstruction |
| Limitations | Not universal first-line for all low-risk trunk/limb NMSC; melanoma and some tumours need specialised protocols/referral; access may be limited in some systems |
| Shared decision | Address cosmesis vs completeness; alternatives (standard excision, radiation, topical for highly selected low-risk) when appropriate |
Model key actions
- Recommend Mohs for H-zone infiltrative BCC because of complete margin control and tissue sparing.[1][4]
- Explain en face 100% margin vs incomplete bread-loaf sampling.[3]
- Set realistic cure-rate expectations and same-day reconstruction plan.[1]
Common errors
- Offering only “wide local excision with 5 mm margin” for H-zone aggressive BCC without discussing Mohs.
- Claiming Mohs “removes all cancer cells under a microscope before surgery” without describing staged margin mapping.
- Quoting Mohs as mandatory for every low-risk truncal superficial BCC.
- Ignoring reconstruction and functional risk on the nose/eyelid.
- Confusing Mohs with simple frozen section of a random margin slice.
References4ShowHide
- [1]Bittner GC, Cerci FB, Kubo EM, et al. Mohs micrographic surgery: a review of indications, technique, outcomes, and considerations. Anais Brasileiros de Dermatologia, 2021.PMID 33849752
- [3]Golda N, Hruza G. Mohs Micrographic Surgery. Dermatologic Clinics, 2023.PMID 36410982
- [4]Marzuka AG, Book SE. Basal cell carcinoma: pathogenesis, epidemiology, clinical features, diagnosis, histopathology, and management. The Yale journal of biology and medicine, 2015.PMID 26029015
- [6]Mullen JT Dermatofibrosarcoma Protuberans: Wide Local Excision Versus Mohs Micrographic Surgery. Surgical oncology clinics of North America, 2016.PMID 27591501