Mohs skin cancer surgery has a very high ‘cure’ rate

In Mohs surgery, the tumour is removed in a series of stages, and the excised tissue is histologically analysed in the laboratory for cancer cells at each step of the procedure.

“The patient has local anaesthetic infiltrated and the tumour is then de-bulked using a curette or a scalpel, and that will provide us lesional tissue to correlate with the margins,” Dr Bui explains.

“The tumour is excised conservatively in the first stage, with a 45-degree bevel yielding a dome-like tissue specimen. This enables us to flatten out the tissue for mounting onto slides to examine the peripheral and deep margins.

“That gets prepared, inked and colour-correlated to a map of the area on the patient’s skin. This mapping process is critical to enable any residual tumour identified microscopically to be traced back to an exact anatomic location on the patient.”

The tissue is sectioned into smaller blocks, rapidly frozen, and then cut using a cryostat in a horizontal plane into very thin sections, five to seven microns. The sections are then stained using haematoxylin and eosin before being mounted onto microscope slides, and the dermatologist examines the tissue margins for cancer cells.

“Once those slides are processed,” Dr Bui explains, “and, let’s say, the majority of the slides are clear but a small nest of tumour cells remain at the nine o’clock mark, then I’d take another separate stage, just focusing on that nine o’clock margin, and continue until I am able to confirm the tissue is tumour-free.”

In wide excision for a basal cell carcinoma (BCC), the doctor would apply a conventional margin of 3mm to 4mm all the way around the cancer, excise the entire area, close the defect, and refer the tissue for examination by a pathologist.

“An estimation is made as to where the tumour starts and finishes based on macroscopic and dermoscopic inspection,” Dr Bui explains, “and extra surrounding tissue is removed, sacrificing potentially healthy skin in the process of empirically removing tumour and subclinical extension. Definitive pathology results are generally available several days to weeks following the procedure.

“Whereas with Mohs surgery, pathologic examination of the tissue occurs on the day. That’s why a fairly conservative first stage can often be utilised and then allow further tissue removal to be directed by the results seen under the microscope.