Mohs micrographic surgery

Author: Vanessa Ngan, Staff Writer, 2005.

What is Mohs micrographic surgery?

Mohs micrographic surgery (MMS) is also called Mohs surgery, Mohs microscopically controlled surgery, and margin controlled surgery. It is a surgical technique for the removal of certain skin cancers by taking special horizontal sections of skin tissue and examining them under a microscope. It differs from other surgical excision techniques which normally involve examination of vertical sections. The technique was first developed by Dr Frederic E Mohs in the 1930s and has been refined and perfected over the last 50 years, although the unique process of examining horizontal tissue sections, colour coding excised specimens and creating a map to identify location of remaining cancer cells to be excised remains the cornerstone of the procedure.

The cure rate for MMS is very high (up to 99%) compared with other treatments for skin cancer, thus MMS has become the treatment of choice for basal cell carcinomas (BCC) and squamous cell carcinomas (SCC) with high risk for recurrence.

What does the procedure involve?

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.

How effective is Mohs micrographic surgery?

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.

TreatmentPrimary BCC, % recurrence rateRecurrent BCC, % recurrence rate
Mohs migrographic surgery 1.0 5.6
Surgical excision 10.1 17.4
Radiotherapy 9.7 9.8
Cryotherapy 7.5 13.0
Curettage and cautery 7.7 40.0
Five-year recurrence rates for the different treatments of primary and recurrent basal cell carcinoma*

*Medscape Reference at http://emedicine.medscape.com/article/876110-overview. Accessed 18 November 2008.

Which tumours are suitable for Mohs micrographic surgery?

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.

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.

What are the complications of Mohs micrographic surgery?

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

Who performs Mohs micrographic surgery?

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.

Consult your doctor or dermatologist to find a suitable MMS surgeon.

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