Mohs Micrographic Surgery Technique and Indications
The defining principle
Mohs achieves complete margin examination by mapping tissue in a contiguous plane. The critical technical steps: scoring and orienting the specimen, taking saucerized stages with a bevel, inking the true margin, relaxing incisions to flatten the tissue, and correlating the map with histology. Candidates must understand why the peripheral and deep margins are examined at 100% - this is the fundamental advantage over vertical-section bread-loafing, which samples a small fraction of the margin.
Indications and staging
Know appropriate indications (recurrent tumors, anatomic high-risk sites, ill-defined borders, positive margins after prior excision, genetic syndromes such as Gorlin) and relative contraindications. Understand the role of preoperative biopsy and the decision to convert to a staged approach when tumors are larger than initially mapped.
Traps
A frequent trap is confusing the frozen-section processing with the mapping principle - Mohs requires both. Another: assuming Mohs is indicated for every lesion on the nose; small, well-defined primary BCCs may be treated by other means. Also know that relaxing incisions are made in the dermis/subcutis, not through the margin, and that ink must not contaminate the margin.
Worked example
A stage shows tumor at the 3 o'clock peripheral margin on the map. The next stage is taken only from that sector, with the map reoriented so the new specimen's 3 o'clock corresponds to the involved edge. Failure to maintain orientation is the classic source of error in questions.