Mohs micrographic surgery — Viva

Q1: Definition and principle (2 min)

What is Mohs micrographic surgery, and what single feature distinguishes it from standard excision?

Mohs micrographic surgery is the microscopically controlled, tissue-sparing excision of skin cancer in which the operating surgeon examines the entire surgical margin on horizontal (en face) frozen sections and re-excises only the tumour-positive areas until clear, then reconstructs the same day. The single distinguishing feature is complete circumferential and deep margin assessment versus standard excision, which bread-loafs the specimen and samples only a small fraction of the margin. The surgeon also reads their own slides, which no other skin-cancer technique requires.^[1]

Examiner probe: What does "en face" mean and why is the bevel important? — En face means the deep margin is sectioned horizontally so the entire edge is on one slide; the bevelled edge lets the saucerised specimen flatten on the cryostat so the peripheral and deep margins lie in one plane.^[1]

Q2: Indications (3 min)

Give the absolute indications for Mohs, and tell me why DFSP is a standout indication.

Absolute indications: BCC or SCC on the H-zone of the face (periorbital, perinasal, periauricular, perioral, nasal ala and tip, nasolabial fold, auricular helix); recurrent tumours; aggressive histology (infiltrative, morpheaform/sclerosing, micronodular BCC; poorly differentiated SCC; perineural invasion); poorly defined clinical margins; and the infiltrative non-melanoma tumours — DFSP, atypical fibroxanthoma, Merkel cell carcinoma, extramammary Paget disease, microcystic adnexal carcinoma. DFSP is a standout because its slender CD34-positive cells infiltrate subcutaneous fat with extensive subclinical extension, so standard wide excision carries a high local recurrence; Mohs (often slow-Mohs with permanent paraffin sections) gives the lowest recurrence of any treatment.^[1][3]

Examiner probe: Is Mohs indicated for invasive melanoma? — Not routinely; standard excision with Breslow-based margins. The exception is lentigo maligna and melanoma in situ on sun-damaged skin, where subclinical extension defeats narrow margins and Mohs with immunostaining is increasingly used.^[1]

Q3: Technique (2 min)

Take me through one Mohs stage.

Mark the lesion, debulk with a curette to define the true border, infiltrate local anaesthetic. Excise a saucerised layer with a narrow rim at a bevelled edge. Cut reference notches and colour-code the edges with dyes, and draw an anatomical map linking each colour to a clock position on the patient. Orient the specimen deep-side up (en face), freeze on the cryostat, cut thin sections, and stain. The Mohs surgeon reads the slides while the patient waits. If tumour touches the margin, use the map to return to that exact site and re-excise only that area. Repeat until every margin is clear, then reconstruct the same day.^[1]

Examiner probe: How is this different from a generic frozen section? — A generic frozen section is a single representative vertical cut sent to a separate pathologist, sampling only a fraction of the margin; Mohs sections are horizontal en face preparations of the entire true margin, read by the operating surgeon using a mapped specimen.^[1]

Q4: Outcomes and comparison with standard excision (3 min)

What cure rates would you quote, and how do they compare with standard excision?

Mohs achieves higher cure rates than standard excision, with the gap widest where Mohs is indicated — face, recurrent, aggressive histology. The margin difference explains the cure difference: bread-loafing samples only a small fraction of the margin, so subclinical extension is missed; Mohs examines the entire margin. Cosmetic outcomes are also superior because Mohs is tissue-sparing and the surgeon reconstructs the exact defect the same day with confirmed negative margins.^[1][3]

Q5: Complications, training and cost (2 min)

What are the main complications, who is trained to perform Mohs, and is it cost-effective?

Complications include bleeding, infection, dehiscence, flap or graft failure, nerve injury, ectropion, scarring and recurrence. Mohs is performed by surgeons with dedicated Mohs training across dermatological surgery, dermatopathology and laboratory management. Cost: Mohs is more expensive per procedure but cost-effective for high-risk tumours because it avoids the downstream costs of recurrence and re-operation.^[1][3]

References3ShowHide

1. [1]Connolly SM, Baker DR, Coldiron BM, et al. AAD/ACMS/ASDSA/ASMS 2012 appropriate use criteria for Mohs micrographic surgery: a report of the American Academy of Dermatology, American College of Mohs Surgery, American Society for Dermatologic Surgery Association, and the American Society for Mohs Surgery. J Am Acad Dermatol, 2012.PMID 22959232
2. [2]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
3. [3]Mullen JT, Feng L, Xing Y, et al. Dermatofibrosarcoma Protuberans: Wide Local Excision Versus Mohs Micrographic Surgery. Surgical oncology clinics of North America, 2016.PMID 27591501