Mohs Surgery

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Medical Author:

Nili N. Alai, MD, FAAD

Nili N. Alai, MD, FAAD

Dr. Alai is an actively practicing medical and surgical dermatologist in south Orange County, California. She has been a professor of dermatology and family medicine at the University of California, Irvine since 2000. She is U.S. board-certified in dermatology, a 10-year-certified fellow of the American Academy of Dermatology, and Fellow of the American Society of Mohs Surgery.

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Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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What is Mohs micrographic surgery (MMS)? Why is the procedure called Mohs?

Mohs micrographic surgery is a minor surgical procedure and special method of removing skin cancers using local anesthesia (numbing). The majority of cases are performed in the physician's office. Mohs is a very precise, highly detailed technique whereby small layers of skin are sequentially removed and immediately examined under the microscope until the samples indicate that the skin cancer is completely removed.

The procedure uses frozen sections of skin that are then stained with special dyes. The dyed frozen pieces of skin are further examined under the microscope and a tumor map is drawn by the Mohs surgeon. The freezing process allows an immediate examination of the entire tumor margin and tissue histology (microscopic examination of cells).

If more cancer cells or "roots" are seen under the microscope, then another skin layer is removed and again examined. Each skin layer that is removed is called a "level." If no more cancer roots are seen, then it is called "clear" (no more tumor) and no additional levels are needed.

By removing only tissue where cancer is known to be present, the technique combines a very high cure rate with good preservation of normal skin. Once the cancer has been fully removed, the Mohs surgeon looks at the wound to determine the method to obtain the best wound repair and cosmetic result for you.

Mohs is special because the entire edge and undersurface of each skin cancer layer is carefully examined under the microscope for the presence of very small cancer cells. With regular or traditional surgery, only about 1%-3% of the tumor margins are actually examined, thereby increasing the chances that a small tumor root would be missed and left behind. Mohs allows for examination of 100% of the tumor margins thereby reducing the chance that tumor cells will be left behind.

Mohs is usually scheduled only on certain days in the doctor's office because of the required equipment, tissue stains (dye), Mohs technologists, and microscopes. Most of these procedures are generally performed with the patient waiting in the office for the tissue to be "read" or interpreted by the Mohs surgeon.

Mohs is named after its inventor, Dr. Frederic Mohs, who first described the technique in 1941.

From top: (top) Basal cell carcinoma on the neck; (middle) site after the tumor roots were cleared; (bottom) post-op closure of the wound with sutures