Mohs surgery

This page was reviewed under our medical and editorial policy by

Frederick L. Durden, Jr, MD, Plastic and Reconstructive Surgeon & Microsurgical Reconstructive Surgeon

This page was updated on October 20, 2022.

Mohs surgery (also known as Mohs micrographic surgery, MMS or Mohs) is a type of outpatient surgical procedure used to treat multiple types of skin cancer. During the procedure, the surgeon removes a layer of tissue from the cancerous region and examines it under a microscope to look for remaining cancer cells that were not removed during the procedure. If cells are detected, the surgeon will remove another layer of tissue, and so on, until no more cancer cells have been detected.

Dr. Frederic Mohs developed the surgery as a medical student at the University of Wisconsin-Madison in the 1930s, and it’s been considered an effective treatment option for keratoses (precancerous lesions), early-stage melanomas and carcinomas.

Specifically, Mohs helps with the two most common cancers in the world: basal cell carcinomas and squamous cell carcinomas.

What are the risks of Mohs surgery?

The risks of Mohs surgery are generally low. Possible complications during the procedure include:

  • Infection
  • Nerve damage
  • Scars
  • Bleeding

How to prepare for Mohs surgery

Mohs surgery may take several hours or even a second day of surgery. Patients should make sure to reserve enough time away from work or family obligations for the procedure, travel and recovery.

Ahead of the procedure, doctors may ask the patient to stop taking drugs such as aspirin and blood thinners, but patients should check with the care team before stopping any medications.

The doctor may also recommend that the patient stops smoking because it’s linked to increased risk and complications after Mohs surgery, especially if a flap of skin or skin graft is being used to reconstruct the area.

Ask the care team about other preparations or suggestions for making surgery day more comfortable.

What to expect during Mohs surgery

Mohs micrographic surgery usually happens over the course of a day. Two visits may be necessary if the cancerous area is larger or if reconstruction is needed after the doctor has removed the cancer cells.

  • First, the doctor examines the cancerous area and positions the patient so he or she is able to access the cancer during surgery.
  • The doctor lays a surgical drape over the patient's body and numbs the skin.
  • The patient will stay awake while the doctor cuts the tumor out and examines it.

The surgery continues in several steps, each one removing another layer of the potentially cancerous tissue. The first layer typically includes all of the visible tumor and a thin slice of the skin and tissue under it. The tissue is then flash-frozen and sliced into very thin layers. These thin sections of the tumor may be treated with special stains to help the doctor better visualize the different cell types. The doctor reviews the stained slices of skin tissue under a microscope in search of cancerous cells.

If cancer is found, the doctor may numb the area again if needed and cut another thin layer. Those cells are then examined, and the cycle starts again.

The process of cutting and testing each layer takes about an hour in total, and most cancers need two to three layers of excision to be fully removed. More layers may be needed for bigger cancerous areas or those that penetrate deeper into the skin.

When the doctor has removed all of the cancerous tissue, the wound is sealed with a pressure dressing, stitches, or an electric probe that heats the skin and cauterizes the blood vessels. If the wound is large, the doctor may use a skin flap (skin from one side of the wound) or graft (skin from another part of the body) to close it up and reconstruct the area.

Mohs surgery recovery

Proper care after surgery minimizes the risk of infection or excessive scarring.

Post-op instructions may include:

  • Avoid touching the wound or dressing for several days.
  • Avoid strenuous movements or activities that may stretch or rip stitches or reopen the wound.
  • Gently clean the wound with cool water and soap once or twice a day, and pat it dry before replacing the bandage.
  • Apply petroleum jelly or an antibiotic ointment to keep the area moist while it heals.
  • Check with the doctor about recommended pain medicines to take. Acetaminophen should be OK, but aspirin or ibuprofen may cause bleeding.

Call the doctor if any of these symptoms occur:

  • Nerve damage, in the form of numbness or burning sensations
  • Redness, pain or pus
  • Bleeding that doesn’t stop after 10 minutes of pressure
  • Excessive pain
  • Fever over 100 degrees Fahrenheit

When will patients get Mohs surgery results?

Most patients get Mohs surgery results immediately since the doctor reviews the tissue samples during the actual procedure. The patient typically sees the physician again a week or two after the procedure to confirm that the wound is healing properly.

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