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The information on this page was reviewed and approved by
Maurie Markman, MD, President, Medicine & Science at CTCA.

This page was updated on June 7, 2021.

Mohs surgery

Mohs micrographic surgery (also known as MMS or Mohs) is a type of outpatient surgical procedure used to treat multiple types of skin cancer.

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.

There are several reasons Mohs surgery may be recommended instead of another treatment option, including:

  • Tumor has regrown after previous treatments
  • Tumor is located in an area of the skin that is scarred or had previously been exposed to radiation treatment 
  • Tumor is large, penetrates deep into the skin, or doctors can’t tell the extent of the cancer
  • Edges of the tumor aren’t obvious
  • Tumor has a high risk of recurring if removed
  • Tumor is located near the eyelids, nose, ears, lips or hands
  • Patient has a compromised immune system

Mohs is sometimes used for the more deadly melanoma skin cancers when they’re small (stage 0 melanoma or melanoma in situ).

What are the risks?

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

  • Infection
  • Nerve damage
  • Scars
  • Bleeding

How to prepare for surgery

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

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

Your doctor may also recommend you stop 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 your care team about other preparations or suggestions for making your surgery day more comfortable.

What to expect during surgery

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

  • First, the doctor examines the tumor and positions you so he or she is able to access it during surgery.
  • Your doctor lays a surgical drape over your body and numbs the skin.
  • You’ll 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 your doctor better visualize the different cell types. Your doctor reviews the stained slices of skin tissue under a microscope in search of cancerous cells.

If cancer is found, your 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 tumors need two to three layers of excision to be fully removed. More layers may be needed for bigger tumors or those that penetrate deeper into the skin.

When your 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.

What to expect after surgery

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 your doctor about recommended pain medicines to take. Acetaminophen should be OK, but aspirin or ibuprofen may cause bleeding.

Call your doctor if you notice any of these symptoms:

  • 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
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