Mohs Micrographic Surgery: Technique, Indications, and Margin Logic
Why Mohs works
Mohs micrographic surgery achieves complete margin control by excising tissue, mapping it, and examining 100% of the peripheral and deep margins in a single horizontal plane. This contrasts with conventional vertical "bread-loaf" sections, which sample only a fraction of the margin. The result is the highest cure rate with maximal tissue conservation, which matters in cosmetically or functionally sensitive areas.
Indications and traps
Classic indications include basal cell carcinoma and squamous cell carcinoma in high-risk locations (nose, ear, eyelid, lip), recurrent tumors, tumors with aggressive histology (morpheaform, perineural invasion), and large or ill-defined lesions. Mohs is also used for dermatofibrosarcoma protuberans and other selected tumors. Traps: Mohs is not the default for every skin cancer; low-risk small tumors on the trunk may be treated with excision or electrodesiccation. Also, Mohs is not appropriate for melanoma in most settings, though specialized immunostaining techniques exist.
The operative sequence
Know the stages: preoperative mapping and marking, excision with a beveled edge, tissue orientation with nicks and a map, frozen-section processing, and margin interpretation. If margins are positive, a second stage is taken from the mapped area. Reconstruction follows clearance.
Practical rules
Anticoagulation is generally not stopped for Mohs. Frozen sections are read the same day. The surgeon acts as both surgeon and pathologist, so documentation of mapping is critical.
Worked example: A recurrent basal cell carcinoma on the nasal ala has ill-defined clinical borders. Mohs is preferred over wide local excision because recurrence and incomplete excision risk are high and tissue conservation is critical. The trap answer, re-excision with margins, may fail to clear subclinical extension.
Rule of thumb: High-risk location, recurrent tumor, aggressive histology, or ill-defined borders = think Mohs.