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“It was a tiny patch of dry, flaky, red skin—smaller than a number 2 pencil eraser—right below my nose and above my lip,” she recalls.

That was in the summer of 2009. At Thanksgiving the spot was still there. “My brother, who’s a doctor, looked at it and said, ‘That could be a basal cell cancer.’”

Michele, 45, a real estate broker on Nantucket, an island off Cape Cod in Massachusetts, quickly made an appointment to see her dermatologist in Boston, which meant a harrowing two-and-a-half-hour trip by air and road in a snowstorm. Her doctor biopsied the mole and confirmed that it was indeed a basal cell carcinoma. Because the mole was on Michele’s face, the dermatologist recommended Mohs surgery to remove it and referred her to a surgeon with special training in the procedure.

What Is Mohs Surgery?

To understand Mohs surgery, it helps to first understand what happens in conventional surgery for a skin cancer like Michele had. Typically, the surgeon removes the visible tumor plus a margin of several millimeters of tissue around it. The tissue is sent to a pathology lab, where it is cut into thin slices for examination under a microscope.

“The pathologist examines a representative sample of the surgical margin—around 1 percent of the total margin” to look for evidence of remaining cancer cells, explains Chrysalyne D. Schmults, MD, MSCE, an assistant professor of dermatology at Harvard Medical School, director of the Mohs and Dermatologic Surgery Center at Brigham and Women’s Hospital in Boston, and the surgeon who performed the procedure on Michelle.

Doctor and patient then wait several days for the results of the pathology examination. If any cancer cells are found in the margin, the patient must undergo a second surgery to remove the remaining cancer.

In Mohs micrographic surgery (MMS), by contrast, a specially trained surgeon removes the tumor one thin layer at a time and immediately—while the patient waits—examines each layer under a microscope for remaining cancer cells in the margin.

“The same doctor is removing the cancer and looking at it under the microscope to determine whether it’s all out,” says Dr. Schmults. “We look at nearly 100 percent of the margin, so if there is any residual tumor, we can see exactly where it is. If we need to remove more tissue, we can do it very precisely.

“Most tumors are removed in a single stage, but in some cases we need to remove a second layer,” she continues. “In rare cases a tumor is quite a bit bigger than it looks, and we need to remove several layers of tissue to get all of it.”

When Might Mohs Surgery Be the Best Choice?

Most basal cell and squamous cell cancers (the two most common types of skin cancer) are highly curable with either conventional or Mohs surgery, says Dr. Schmults. Mohs surgery offers some advantages, particularly for recurrent tumors; “infiltrating” tumors that grow in long, thin strands rather than as a clump of tissue; and tumors on the face. In a randomized clinical trial published in 2008, patients with recurrent facial basal cell cancers treated with Mohs surgery were significantly less likely to have another recurrence than patients treated with conventional skin cancer surgery.1

Mohs surgery offers the most tissue-sparing approach to skin cancer surgery, says Simon Yoo, MD, associate professor of dermatology at Northwestern University’s Feinberg School of Medicine: “We can take out a smaller margin around the tumor because we’re examining all of it. When it’s important to minimize scarring, Mohs surgery is the preferred approach.”

Another advantage of MMS, says Dr. Schmults, is that “patients leave knowing their tumor is clear—they don’t have to wait several days for a pathology report.”

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Depending on the circumstances, she adds, Mohs surgery might not be the most appropriate treatment option such as when skin tumors have spread to other areas in nearby skin or to the lymph nodes.

An “Infiltrating” Tumor

The tumor on Michele Kelsey’s face turned out to be an infiltrating one. Although it looked small to the naked eye, “below the surface it was deeper and wider than it appeared,” she says. Five layers of tissue needed to be removed before the margins were clear.

For about a year, Michelle had a “red, angry” scar, but after several cosmetic procedures the scar is now barely visible. “Most people tell me they don’t even notice it,” she says. Most patients do not need any scar treatments after Mohs surgery but Mohs surgeons can perform these treatments if needed.

“The advantage of Mohs in a case like Michele’s is that we could remove the tumor very precisely; and because of our training in reconstruction, we could also give a good cosmetic result, even though her tumor was large and in a tricky area over her lip.” says Dr. Schmults.  _


  1. Mosterd K, Krekels GA, Nieman FH, et al. Surgical excision versus Mohs’ micrographic surgery for primary and recurrent basal-cell carcinoma of the face: a prospective randomised controlled trial with 5-years’ follow-up. Lancet Oncology. 2008;9(12):1149-56. doi: 10.1016/S1470-2045(08)70260-2.

What’s in a Name?

Mohs surgery is named for Frederick Mohs, MD, who began to develop the technique as a medical student in the 1930s. Over a long career, he refined the procedure and taught it to other surgeons. In 1967 he founded and became the first president of the American College of Mohs Surgery,

The Accreditation Council for Graduate Medical Education oversees a one- to two-year post-residency fellowship training program in Mohs surgery, skin tumor pathology,

Mohs surgery is typically performed in a dermatologist’s office under local anesthesia. Patients should expect to be there for several hours, says Dr. Schmults. Much of that time is spent waiting while the excised tissue is processed and examined.

“After the tumor has been completely removed and we know the margins are clear microscopically, in most cases the Mohs surgeon will reconstruct the wound the same day,” says Dr. Schmults.


American College of Mohs Surgery (Patient Education)