Mohs Surgery Principle — SCE Dermatology MCQ

Correct answer: C — Mohs provides 100% peripheral and deep margin assessment through systematic horizontal frozen section mapping, compared to standard 'bread-loaf' histology which samples only approximately 1-2% of the surgical margin

The correct answer is C: Mohs provides 100% peripheral and deep margin assessment through systematic horizontal frozen section mapping, compared to standard 'bread-loaf' histology which samples only approximately 1-2% of the surgical margin. In Mohs surgery the excised disc of tissue is mapped, colour-coded and processed as horizontal (en-face) frozen sections, allowing the surgeon to examine the entire peripheral and deep margin in a single pass. Standard excision specimens are instead bread-loafed into vertical sections at intervals, which only samples a small fraction of the true margin and can miss finger-like tumour extensions typical of recurrent or morphoeic BCC. This complete margin control is why BAD guidance recommends Mohs as first-line for recurrent BCC with an additional high-risk factor, particularly at high-risk sites such as the nose, where tissue conservation and cure rate both matter. Why the other options are wrong: E. Mohs is faster: Mohs is a multi-stage, iterative process (excision, mapping, processing, re-excision if needed) that is typically slower overall than a single standard excision, not faster. B. Mohs removes more tissue: Mohs is designed to spare normal tissue by removing margins in thin, precisely mapped layers only where tumour is confirmed histologically, so it generally removes less tissue than standard excision with fixed margins. D. Mohs uses a different anaesthetic: Mohs is almost always performed under local anaesthetic, the same as standard excision; anaesthetic technique is not the distinguishing feature. A. Mohs does not require suturing: Reconstruction and suturing (or flap/graft closure) are still required after Mohs once clear margins are confirmed; the defect still needs definitive closure. Key point: Mohs is superior for recurrent BCC because it achieves complete circumferential and deep margin examination via mapped frozen sections, unlike bread-loafing which only samples a tiny percentage of the margin and can miss residual tumour.

Reference: British Association of Dermatologists (BAD) Guidelines for the management of adults with basal cell carcinoma 2021 (Nasr et al., British Journal of Dermatology 2021), recommendation R10: 'Offer Mohs micrographic surgery as a first-line treatment option to adults with recurrent BCC with at least one other high-risk factor'. https://onlinelibrary.wiley.com/doi/10.1111/bjd.20524