What is Mohs surgery?
In the early 1940’s, Dr Frederick Mohs, Professor of Surgery at the University of Wisconsin, developed this advanced surgical treatment for skin cancer. Mohs surgery is highly specialised treatment for the total removal of skin cancers. Mohs surgery is a subspeciality of Dermatology and your dermatologist has done further training registered with the Australian College of Dermatologists.
Mohs surgery is most commonly used to treat basal cell carcinomas and intraepithelial carcinomas or squamous cell carcinomas. It may also be used to treat a variety of rare tumours.
When a BCC or SCC is excised with other surgery, it is removed with a margin of 3-4mm around the tumour. This removes the tumour completely in 95% of cases; however, in 5% of cases, pathology analysis may confirm the tumour to be present at the edge of the specimen. This means that the cancer has not been fully removed, and necessitates further surgery, with re-excision of the scar at a later date.
Mohs surgery, by contrast, is all done on the day, including the pathology. Your basal cell carcinoma is marked out, and excised with a margin of approximately 1mm. This is performed under local anaesthetic. A long-acting anaesthetic is used, to ensure you are comfortable throughout the day. After the excision, a dressing applied to the open wound, and you return to the waiting area. This initial stage takes 10-15 minutes.
This tissue is then divided into several portions, with each section marked and inked. A surgical map is drawn, with each section numbered, to maintain orientation. The tissue is then frozen, and a technician makes microscope slides of the tumour’s margins. The horizontal sectioning technique allows visualisation of more than 99% of the tumour’s margins, compared to less than 1% seen in normal pathology sections.
These microscope slides are then reviewed by your Mohs surgeon, and you are brought back in to the operating theatre. If the slides are clear, then your surgical defect can be repaired (see surgical repair below). If tumour is present at the edge, then your Mohs surgeon will take a further tissue layer, with the site dictated by the surgical map. It is common to need more than one surgical stage before the tumour is cleared, and some patients require several.
There are thus two main advantages of Mohs surgery. Firstly, the technique spares normal tissue that doesn’t need to be excised, and may be lost ‘unnecessarily’ with a conventional surgical technique. This may well result in a smaller surgical defect, that can be reconstructed with a smaller repair, and achieve a superior cosmetic outcome. Secondly the technique carries a higher cure rate than conventional surgery.
Very rarely, the skin cancer cannot be totally removed, and further surgery or radiotherapy may be needed at a later date. In less than 1% of cases, perineural invasion is identified. This means the tumour has invaded the nerves in the area. In this setting, radiotherapy may be required post-operatively.
Why have Mohs surgery?
For a number of reasons, the benefits of Mohs surgery are most applicable to tumours on the head and neck, or digits.
Mohs surgery is most commonly used for basal cell carcinomas, particularly those of an aggressive subtype such as micronodular, infiltrative and sclerosing types. These types are more likely to have tentacle-like extensions of cancer cells, which track beyond what is clinically apparent when the skin is examined with the eye. The microscope allows visualisation of these extensions. Mohs surgery is also particularly useful in tumours which are ill-defined, previously incompletely excised, or recurrent.