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Mohs micrographic surgery

Mohs surgery; Mohs MMS

Mohs micrographic surgery is a tissue-sparing surgical technique with on-table frozen-section margin assessment of horizontally cut sections, allowing complete histological evaluation of 100% of the surgical margin. It offers the lowest recurrence rates for high-risk facial non-melanoma skin cancer and is the UK gold standard for selected BCC and cSCC.

5-yr cure (primary BCC)~99%vs 90% standard excision
5-yr cure (recurrent BCC)~94%vs ~80% standard re-excision
Margin assessed100%vs <1% with breadloafing
Tissue spared~50%vs predetermined margin
CurrentLast reviewed 22 March 2026
Mohs technique — horizontal sectioning
Schematic: horizontal frozen sections allow visualisation of 100% of the deep and peripheral surgical margin in one slide.
Mohs map and tissue specimen
Mohs mapping: orientation of the specimen with skin marker correspondence to the Mohs map.

Indications

BCC

  • H-zone of the face (central face, periocular, nasolabial folds, ears).
  • Recurrent BCC.
  • Diameter >2 cm.
  • High-risk histology — morphoeic, infiltrative, micronodular, basosquamous.
  • Poorly defined clinical borders.
  • Where maximal tissue sparing matters (eyelid, nasal ala, helix).

cSCC

  • H-zone facial cSCC.
  • Recurrent cSCC.
  • BWH T2b. (BWH T3 — bone invasion, ≥ 4 risk factors or otherwise advanced disease — generally requires staging imaging plus composite resection and MDT discussion (± adjuvant radiotherapy) rather than Mohs.)
  • Immunosuppressed patients (OTR, CLL).
  • Perineural invasion present (in addition to wide margin).

Other

  • Lentigo maligna — frozen-section Mohs with MART-1 / SOX10 immunostaining (specialist centres).
  • DFSP — Mohs with CD34 IHC reduces recurrence vs wide local excision.
  • Sebaceous carcinoma (periocular).
  • Microcystic adnexal carcinoma.
  • Atypical fibroxanthoma.

Technique

  1. Mark the visible tumour with a 1–2 mm clinical margin and orientation marks (often 12 o'clock).
  2. Excise stage 1 — disc of skin removed at a 45° bevel, en bloc with the underlying soft tissue. Specimen marked with tissue dye for orientation, divided into segments, mapped on a Mohs map.
  3. Frozen section — segments are flattened onto glass for horizontal cryosectioning. The surgeon (or pathologist) reads the entire deep and peripheral margin in one slide per segment.
  4. Map positive margins back to the patient using the Mohs map and excise only the area with residual tumour (stage 2). Repeat.
  5. Reconstruct when clear margins confirmed — usually same-day.

Evidence

The strongest evidence comes from the Dutch RCT of Mohs vs standard surgical excision for facial BCC (van Loo 2014). For primary BCC there is a modest absolute benefit; for recurrent BCC the benefit is larger (~10 percentage points difference in recurrence at 10 years).

  • 10-year recurrence: primary BCC ~4% (Mohs) vs ~12% (standard).
  • 10-year recurrence: recurrent BCC ~4% (Mohs) vs ~14% (standard).
  • cSCC: Mohs reduces 5-year recurrence in high-risk subtypes vs standard excision (observational evidence).

UK availability and pathway

Mohs services in the UK are provided by accredited Mohs centres (most major teaching hospitals; some private). Referral pathways vary by Cancer Alliance — typically through the local skin cancer MDT. Demand often exceeds capacity; lead times of 6–12 weeks are common.

British Society for Dermatological Surgery (BSDS) maintains accreditation criteria for Mohs surgeons in the UK.

When Mohs is not available

  • Wide local excision with appropriate margin and paraffin-embedded margin assessment ('staged excision' if delayed reconstruction).
  • 2-stage 'square procedure' with paraffin permanent sections of the entire peripheral margin (alternative to Mohs for lentigo maligna).
  • Definitive radiotherapy where surgery not feasible.

Complications

  • Bleeding, infection, scar.
  • Nerve injury (specific to anatomical site).
  • Aesthetic morbidity from tissue loss.
  • Long operative time for multi-stage cases.
  • Anxiety / fatigue for patient through multiple stages.

References

  1. van Loo E et al. Surgical excision versus Mohs micrographic surgery for basal cell carcinoma of the face: 10-year follow-up. Eur J Cancer; 2014.
  2. British Society for Dermatological Surgery. UK Mohs surgery accreditation standards.
  3. Connolly SM et al. AAD/ACMS/ASDSA/ASMS 2012 appropriate use criteria for Mohs micrographic surgery. J Am Acad Dermatol; 2012;67:531–50.

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