Mohs Surgery - Mohs Procedure

Mohs Procedure

The Mohs procedure is essentially a pathology sectioning method that allows for the complete examination of the surgical margin. It is different from the standard bread loafing technique of sectioning, where random samples of the surgical margin are examined.

Mohs surgery is performed in four steps:

• Surgical removal of tissue (Surgical Oncology)
• Mapping the piece of tissue, freezing and cutting the tissue between 5 and 10 micrometers using a cryostat, and staining with hematoxylin and eosin (H&E) or other stains (including Toluidine Blue)
• Interpretation of microscope slides (Pathology)
• Possible reconstruction of the surgical defect (Reconstructive Surgery)

The procedure is usually performed in a physician's office under local anesthetic. A small scalpel is utilized to cut around the visible tumor. A very small surgical margin is utilized, usually with 1 to 1.5 mm of "free margin" or uninvolved skin. The amount of free margin removed is much less than the usual 4 to 6 mm required for the standard excision of skin cancers. After each surgical removal of tissue, the specimen is processed, cut on the cryostat and placed on slides, stained with H&E and then read by the Mohs surgeon/pathologist who examines the sections for cancerous cells. If cancer is found, its location is marked on the map (drawing of the tissue) and the surgeon removes the indicated cancerous tissue from the patient. This procedure is repeated until no further cancer is found. For best results a reconstructive plastic surgeon may be necessary.

The method is well described in current references. The mapping combined with the "smashing the pie pan" method of processing is the essence of CCPDMA surgery. If one imagines an aluminum pie pan as the blood covered surgical margin, and the top of the pie is the crust covered surface of the skin – the goal is to flatten the aluminum pie pan into one flat sheet, mark it, stain it, and examine it under the microscope. Another author uses the example of peeling the skin off an orange. Imagine an orange cut in half as the CCPDMA layer. The peel is the surgical margin. One can remove this peel and flatten it out on a glass slide to examine the roots of the invasive cancer. The mapping is simply how one stains and labels the sections for a microscopic examination. The sections can be processed in one piece (using relaxing incisions at multiple points, or hemisectioned like a "Pac-Man" figure), cut in halves, cut in quarters, or cut in multiple pieces. Single piece processing is acceptable for small cancers, and multiple piece sectioning facilitates processing and prevent artifacts. Single piece sectioning prevents errors introduced by soft, hard-to-handle tissue; or from accidental dropping or mislabeling of specimen. Multiple sectioning prevents compression artifacts, separation of tissue, and other logistical problems with handling large thin sheets of frozen skin.

Some physicians believe that frozen section histology is the same as Mohs micrographic surgery, but it is not. Mohs surgery is performed using fresh tissue while frozen section histology may or may not be CCPDMA. Non-CCPDMA histology usually utilizes a random tissue sampling technique called "bread loafing". Bread loafing is a statistical sampling method which examines less than 5% of the total surgical margin (imagine pulling 5 slices of bread out of a whole loaf of sliced bread and examining only those 5 slices to visualize the whole loaf). In CCPDMA processing, the entire surgical margin is examined (imagine one who examined the entire outside crust of the same loaf of bread). In statistical terms, the more slices of bread one examines, the lower the "false negative" rate will become. False negatives occur when a pathologist reads cancer excision as "free of residual carcinoma", even though cancer might be present in the wound and missed because of the random sampling. In reality, most pathology labs examine only 3 to 8 sections of the "loaf" in their margin determination. The alternatives to Mohs surgery are CCPDMA based surgical excision and non-CCPDMA surgical excision. Dr. Mohs and CCPDMA pathologists have perfected methods of examining the entire surgical margin.

Mohs surgery often leaves an open wound, which a plastic surgeon is ideally suited to close.