What is Mohs surgery?
Picture of Mohs surgery scar
Mohs surgery, also referred to as Mohs micrographic surgery (MMS), is a special technique that utilizes both pathology and surgery to remove skin cancers in a physician's office. Tissue containing the tumor is frozen on a cryostat, and very thin horizontal slices of tissue are cut from the bottom-most layers of the skin tumor, positioned on a glass slide, and stained by a technician. The physician then examines these thin layers under a microscope. A dermatologist removes and examines repeated saucer-shaped layers of tissue until no more skin cancer is visible, only healthy tissue.
The procedure includes special dyes that allow the surgeon to remove the entire malignancy using a tumor map showing the sites of any residual cancer cells. During MMS there is an examination of the entire tumor's margins simultaneously while the patient is waiting. If more cancer cells are seen under the microscope, tissue is removed at those sites of involvement. Each skin layer that is removed is called a "level." If no more cancer cells are seen at a particular level, then it is deemed "clear" (no more tumor) and no additional levels are removed.
By removing only tissue where cancer is known to be present, the technique combines a very high cure rate with excellent preservation of normal skin. Once the cancer has been fully removed, the surgeon will determine the type of repair for the best cosmetic result. The surgeon may refer patients to another physician for wound closure, may close the wound immediately, or may let the wound heal on its own.
MMS is special because the entire edge and under-surface of each skin cancer layer is microscopically examined for the presence of cancer cells. Traditional surgical pathology techniques use vertical sections (bread loafing) and are only helpful to evaluate 1%-3% of the tumor margins, thereby increasing the chances that a small tumor cluster may be missed and left behind. MMS enables the examination of 100% of the entire tumor's margin, thereby reducing the chance of recurrence of the skin cancer.
MMS surgery is significantly more labor intensive than conventional methods of skin cancer treatment, and as a result, it is more expensive. This type of surgery is typically reserved only for certain situations, which include recurrent cancers that have not been cured by conventional techniques, large tumors with indistinct margins, tumors that occur in cosmetically sensitive areas like the mid-facial tissues, and scarring basal cell cancers, which are known to have a high recurrence rate.
MMS is scheduled with the understanding that the patient may be spending the entire day in the office depending on the number of levels necessary to clear the tumor. Most of these procedures are performed with the patient in the waiting room awaiting the verdict from the Mohs surgeon.