In short, because 100% of the surgical margin is examined for remaining tumor in your skin after the tumor is excised.
Mohs surgery is unique in that the surgeon is also the pathologist. This allows us to map a piece of tissue that is excised like a clock face so that if there is remaining tumor, we only need to go back to exactly where the tumor can be seen under the microscope. In this way, we can achieve a very high cure rate (>99%) while preserving the maximum amount of normal tissue possible.
The Mohs technique is commonly used to treat nonmelanoma skin cancers. It was developed by Dr. Frederic Mohs in the 1930s. It is only appropriate for some types of skin cancers: those in specific areas such as the face or hands, those with aggressive histopathology, and those in patients who are immunosuppressed or otherwise at high risk of aggressive cancers.
First, the biopsy site is marked and confirmed with the patient. The area is then number with injection lidocaine. The tumor extent then is defined by scraping the surface, and because tumor cells are not as well connected to each other as normal skin is, the tumor will fall apart while normal skin stays intact. Then, the tumor is excised and the tissue is mapped like a clockface. Then comes the time-consuming part: the tissue is then taken to a lab on the premises and frozen, embedded, cut into thin sections, then stained. After the slides are ready, the surgeon then examines them under microscopy. Using the map, we can identify exactly where the skin cancer remains on the patient, if any. Coming back to the patient, the surgeon then uses the map to remove any additional tumor until tumor is completely gone at the edges of the pieces of tissue taken.
Once the tumor has been completely removed, reconstruction of the surgical wound is performed. The reconstruction depends enormously on patient goals, the location of the wound, and the size and depth of the wound. It is difficult to predict what repair will be performed at the outset of the surgery, but after the tumor has been removed, the surgeon will discuss repair options with the patient. Special considerations include wounds that are close to the eyes, nose and lips, because the repair should not pull on any of those important structures. An excellent cosmetic and functional outcome, in addition to removing the cancer, are always our main goals!