Mohs micrographic surgery involves a specific sequence of surgery and pathological investigation. An outline of the process is shown below:

1. The visible tumour is outlined with a marker pen.
2. A local anaesthetic is injected into the area.
3. Once the anaesthetic has taken effect (several minutes) the visible portion of the tumour including a small safety margin is surgically removed.
4. The scalpel is initially angled at 45º then travels horizontally to produce a disc of tissue. The tissue is divided into sections, which are colour-coded with tissue dyes. Reference marks are made on the skin to show the site of these sections.
5. A map of the surgical site and the sections of removed tissue is drawn.
6. The tissue is processed by a specialist histotechnologist, who applies certain chemicals, freezes the specimen, and sections it into tiny slices with a microtome.
7. The undersurface and edges of each section are microscopically examined by the dermatologist for evidence of cancer cells.
8. If cancer cells are found their location is marked on the map and another layer of cancerous tissue can be precisely removed from the patient according to the map. By using this technique only areas with cancer cells are sequentially removed and normal healthy tissue is preserved.
9. The removal process continues layer by layer until microscopically there is no longer any evidence of cancer remaining in the surgical site.
10. Depending on the size of the resulting wound it may be left to heal on its own, closed with stitches, or reconstructed with a skin graft or flap. The procedure usually takes about 3 hours to complete but in some cases where the tumour is large may take a whole day or longer.

Mohs micrographic surgery is very effective at completely removing skin cancers while sparing normal healthy tissue. Cure rates have been found to exceed 99% for new (primary) cancers and 95% for recurrent cancers. Five-year recurrence rates for the different treatments of primary and recurrent basal cell carcinoma are shown in the table below.

Skin cancers may form with undefined edges and lengthy rootlike extensions that can grow deeply or laterally from the clinically visible lesion. The MMS technique allows almost 100% of the tumour margins to be microscopically examined, very much moree than traditional histological methods. Hence MMS is particularly suitable for the treatment of difficult skin cancers because it is able to identify and remove all cancer tissue including that found in finger-like extensions. This allows higher cure rates and less scarring.

MMS is primarily used to treat basal and squamous cell carcinomas, but can be used to treat less common skin cancers including melanoma. It is particularly useful in the following circumstances.

• Recurrent or incompletely excised BCC and SCC
• Primary BCC and SCC where the edges of the cancer cannot be clearly defined
• BCC and SCC in an area where it is important to preserve healthy tissue for maximum functional and cosmetic result, such as eyelids, nose, ears, lips, genitals, hands, feet
• BCC and SCC that is large (> 2cm in diameter) or growing rapidly.
• Cancer arising in scars or in sites of previous radiation therapy
• High risk or aggressive types of SCC (e.g. infiltrative histology, poorly differentiated)

MMS is also recommended where a patient may have multiple, small, less aggressive tumours situated in the same surgical area, and in young patients who may expect to have further skin cancers on the face in future.

However, most BCC and SCC are clearly defined tumours in low risk sites, and can be dealt with by simple excisional surgery or other methods.

As with all surgical procedures there is a risk of complications. For MMS possible complications include:

• Bleeding – bleeding risks should be identified preoperative medical history. Patients should stop medications that increase the risk of bleeding, e.g. warfarin, aspirin, unless this is medically risky for them.
• Nerve damage resulting in skin numbness – small sensory nerve fibres in the skin may be severed during removal of tissue layers. Usually the problem is temporary as new nerve fibres grow back. The surgeon will work cautiously around nerves controlling movement to avoid damaging them, unless absolutely necessary in order to completely remove the tumour.
• Infection – rare if the area being treated is kept clean. Oral antibiotics may be required for patients with large wounds or with co-existing medical conditions, e.g. diabetes.
• Postoperative problems, e.g. bleeding, clots under the tissue, death of the skin used to repair the wounds, graft failure.

A specially trained dermatologist or dermatologic surgeon performs Mohs micrographic surgery. The MMS surgeon needs to have skills in dermatology, oncology, pathology and reconstructive surgery. Occasionally, other specialist practitioners may be called in to assist, e.g. plastic surgeon to reconstruct the wound, head/neck surgeon to treat the deep component of the tumour.