Mohs Micrographic Surgery
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Direct answer
Mohs micrographic surgery is a staged excision in which the tumour is removed layer by layer and every margin is examined under the microscope before the operation ends. Frederic Mohs devised the technique to give complete margin control, which is why it is reserved for high-risk skin cancers. The main indications include large or ill-defined basal cell carcinomas, recurrent tumours and squamous cell carcinomas in cosmetically or functionally sensitive sites.
What you must remember
- Principle: complete circumferential and peripheral deep margin assessment, meaning the whole margin is examined at each stage rather than a few sampled sections.
- Horizontal sections: the excised tissue is cut horizontally, so the entire margin can be seen, unlike conventional vertical sampling.
- Tissue sparing: only the tumour and a thin rim of normal tissue are removed at each stage, preserving healthy skin.
- High-risk basal cell carcinoma: large, recurrent, morpheaform and perineurally invading tumours are typical indications.
- H-zone: the central face, including the eyelids, nose, lips, ears and chin, where tumours are often treated with Mohs.
- Processing: frozen sections give same-day results, while paraffin processing gives better tissue quality for detailed review.
- Consent: patients are told that the procedure may need several stages and that the wound is repaired only after clearance.
- Reconstruction: after clearance, the defect may be closed directly, repaired with a flap or graft, or left to heal by secondary intention.
The Mohs cycle in six steps
Mohs surgery is a repeatable cycle, and each stage moves the surgeon closer to a tumour-free margin. The steps below summarise the standard sequence in practice.
- Mapping: the visible tumour border is drawn on a diagram, with ink and orientation marks that match the tissue.
- Excision: a thin layer of tissue, including the tumour and a small rim of normal skin, is removed.
- Processing: the specimen is divided into sections, orientated and processed, so every margin is cut horizontally.
- Examination: the surgeon reviews the margins under the microscope and maps any residual tumour onto the diagram.
- Repeat or clear: tumour is removed again from the positive area only, and stages continue until the margins are clear.
- Reconstruction: once clear, the defect is repaired, using closure, a flap, a graft or healing by secondary intention.
The diagram forms part of the medical record, which allows the surgeon to track exactly where residual tumour was found at each stage.
Where the exam tries to catch you
Stems on Mohs surgery test whether you know when it is indicated and why repair waits until after clearance.
- Trap: using Mohs for every skin cancer. Correct: low-risk, well-defined superficial tumours are usually treated by standard excision, curettage or topical therapy, and Mohs is reserved for high-risk tumours.
- Trap: assuming a single excision with a few sampled margins clears the tumour. Correct: conventional vertical sampling covers only part of the margin, while Mohs examines the entire margin at each stage.
- Trap: repairing the defect before the margins are clear. Correct: reconstruction waits for clearance, because a repair would hide residual tumour and make the margins impossible to examine.
Frequently asked questions
Which skin cancers are the main indications for Mohs micrographic surgery?
High-risk basal cell carcinomas and squamous cell carcinomas are the main indications for Mohs micrographic surgery, especially large, recurrent, morpheaform or perineurally invasive tumours. Lesions in the H-zone of the face and tumours with ill-defined clinical margins also benefit from the technique.
How does Mohs surgery differ from standard excision with margins?
Standard excision removes a fixed margin and examines a few sections vertically, so residual tumour may be missed. Mohs surgery removes tumour stage by stage and examines the whole margin horizontally, giving a more complete clearance of the tumour.
What does CCPDMA mean in Mohs micrographic surgery?
CCPDMA stands for complete circumferential and peripheral deep margin assessment, meaning the whole peripheral and deep margin is examined at each stage. This is the key feature that gives Mohs surgery its high cure rates, and it separates the technique from conventional excision.
Why is reconstruction delayed until Mohs clearance?
Reconstruction is delayed because a repair covering the defect would hide any residual tumour and prevent examination of the margins. Once tumour-free margins are confirmed, the surgeon chooses a closure, flap or graft suited to the site.