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by Chrysalyne D. Schmults, M.D, Harvard & Director of Mohs Center. Mohs Appropriate for early stage skin cancers. Updated 5/2022

Michele Kelsey isn’t sure how long the “irregular spot” had been there before she started to pay attention to it. “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.

A few months later 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.’”

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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.

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.

What Is Mohs Surgery?

To understand Mohs surgery, it helps to first understand what happens in conventional surgery for a skin cancer. 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.

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.”

Mohs Surgery: What to Expect

Mohs surgery is typically performed in the doctors 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.

The vast majority of true skin cancers are basal cell carcinomas and squamous cell carcinomas. When diagnosed early, these tend to be easily treatable. Melanoma is a more serious type of skin cancer. Though when caught early some melanomas are treated by simple excision, more than half the melanomas diagnosed are invasive and require wide and deep excision of the lesion as well as lymph node removal. Recurrent melanomas may require additional surgery, radiation, or chemotherapy.

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.”

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.

Survival Moderately Improved After MMS vs Traditional Excision for Melanoma

Compared with traditional excision, Mohs micrographic surgery (MMS) improved rates of overall survival in patients with early-stage invasive melanoma.

Mohs micrographic surgery has is increasingly used for the treatment of melanoma however current treatment guidelines for invasive melanoma without nodal metastases recommend traditional surgery obtaining a wide margin excision. There has been little published information about how MMS compared to wide margin surgical removal of the cancer.

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Dr Cheraghlou and colleagues from Yale have conducted a review of 70,319 patients with stage I melanoma who underwent MMS or WME and published their results in the Journal of Dermatology. The patient data was collected from the National Cancer Database, which represents approximately 50% of newly diagnosed melanoma cases in the United States

Patients were considered for evaluation if they had stage I melanoma diagnosed in accordance with the American Joint Committee on Cancer Staging Manual, 8th edition between January 1, 2004, and December 31, 2014. They identified 67,085 individuals treated with wide medical incision and 3234 who underwent MMS and when compared determine they MMS has a modest survival benefit relative to wide medical incision.

Although this study was retrospective in nature and academic facilities were more likely than nonacademic facilities to use MMS the findings suggest that Mohs surgery may provide an alternative approach to traditional wide medical excision for appropriately selected individuals with stage I melanoma.4

Surgery and the Need for Reconstruction

Surgical treatment of skin cancer typically involves removal of the cancerous lesion along with a margin of surrounding skin. The entire affected area is excised, leaving a defect that extends into the deeper layers of skin. In some areas—such as around the eyes, nose, and mouth—there may be limits to the amount of skin that can be removed. In such cases, there are additional functional as well as aesthetic considerations, so Mohs micrographic surgery may be used.

Mohs surgery allows for serial excisions of sensitive areas and immediate examination of the tissue to ensure complete tumor removal while sparing as much normal tissue as possible. This is also a valuable technique with skin cancers that have recurred and with cancers with indistinct borders that have spread to the deeper layers of the skin. Mohs surgery can produce a higher chance of cure than standard resections in these types of cases.

Timing of Repair

Whenever possible, open surgical wounds are closed immediately. This reduces pain and discomfort, eliminates the need to care for an open wound, and speeds healing. There are some situations, however, when immediate closure may not be in a patient’s best interest.

At the time of the resection, surgeons use their clinical judgment or frozen section evaluation to ensure that the tumor has been completely removed. Frozen section examination of tissue gives immediate information but is not 100 percent accurate. Large tumors may make it more difficult to get a wide margin, and some tumors have indistinct borders, with fingers of tissue spreading under the skin that are not apparent on clinical examination. In these cases, it is in the patient’s best interest to have confirmation of clear margins before proceeding with reconstruction.

If immediate closure is performed when cancer has been left behind, previously cancer-free tissue may become contaminated. To achieve complete removal of the cancer, a significant amount of additional tissue along with the known positive margin must then be removed. If tissue rearrangement is done for closure, the exact location of the positive margin may not be clear, complicating effective treatment. To avoid this situation, closure of a skin cancer defect may be delayed until confirmation of clear margins is assured by the final pathology from permanent sections, which may take several days. Patients are generally treated with a dressing over the open wound.

Reconstructive Options

The size and the location of the defect, the type of skin cancer, and the surgeon’s confidence of a clean resection determine which type of reconstruction is used. Options for treatment include simply allowing a wound to heal on its own (dressings only), direct closure, skin grafting, or flap closure (moving tissue around to allow for closure of complex wounds).

Allowing a wound to heal without surgical closure is called healing by secondary intention. This can be a good choice when a small wound is in an area where it will heal on its own with less scarring. The middle corner of the eye is one such area. Generally, however, surgical reconstruction will be completed after tumor resection. In many cases, the dermatologist or plastic surgeon who performed the tumor resection will also perform the reconstruction. In other cases, tumor resection is done by one physician, who then works with a plastic surgeon for the reconstruction.

Reconstruction with Direct Closure

The majority of skin cancers are small and well defined. In these cases, treatment may be simple. In areas of natural laxity, skin edges may be easily pulled together and closed, primarily after excision of a small skin cancer. Even in cases where the closure is not so easy, because of either a larger wound or less inherent laxity in the surrounding skin, the skin edges may be undermined and stretched to achieve a simple primary closure. This type of reconstruction is generally done with local anesthesia at the time of the cancer removal.

Reconstruction with Skin Grafts

The skin is the body’s largest organ. There is a lot of it, and it’s stretchy. It is also a multilayered organ that is able to heal itself when wounded, growing a new layer of outer skin to cover superficial defects. All these factors allow skin to be used as a graft (moved from one location to another, relying on the new location for survival and blood supply) to heal open wounds. When skin cancer treatment leaves a more complicated defect, skin grafting may also be used for closure.

There are two types of skin grafts: split-thickness skin grafts, which use only a partial layer of the skin, and full-thickness skin grafts, which use all layers. Split-thickness grafts are harvested by taking a thin shaving of the outer layer of skin. This leaves a raw spot (like a skinned knee) where the graft was harvested. In one to two weeks, a new outer layer of skin reforms over the deeper layer of skin. Any site on the body with a broad, flat expanse of skin can be used. Donor sites remain discolored for several months after healing and may have permanent alterations of texture or slightly raised scars. Full-thickness grafts are harvested by taking a complete piece of skin down to the underlying tissue layer. The resulting defect at the donor site is closed by simply pulling the skin edges together, leaving a linear scar. Full-thickness grafts are limited primarily by size—by what can be closed—and they are best taken from areas where there is natural laxity and skin excess and where the resulting scar can be best hidden.

When used for reconstruction of skin cancer defects, both split- and full-thickness grafts are useful. Small defects on the face are best treated with full-thickness grafts. For the best cosmetic result, the skin is taken from a nearby location that has similar color and thickness. Large defects in locations less prominent than the face are more easily addressed with split-thickness grafts, which provide simple, functional solutions when skin coverage is needed.

Reconstruction with Tissue Rearrangement

When a simple closure of a skin cancer defect cannot be done, reconstructive surgeons bring in tissue from elsewhere on the body. This follows the Robin Hood principle—taking from areas rich in tissue to give to areas that are deficient. Tissue moved from one location to another is called a flap. Flaps differ from grafts in that they have an inherent blood supply and therefore are not reliant on the new site for survival. Flaps can be designed from skin adjacent to the open wound (a local flap) or from distant areas (a regional flap). When no other good options are available for large complex wounds, tissue flaps may also be removed from a remote part of the body, moved to the site of the problem wound, and plugged in to a new blood supply. This is called a free flap.

Local Flaps

Local flaps are frequently used on the face. Open wounds near the eyelid, nose, or mouth may cause distortion of these features if the skin is simply pulled together to close the wound. To prevent distortion, skin flaps can be designed to take advantage of areas with skin to spare as well as natural facial creases and contours. Tissue can be rearranged, spreading the tension over a large area, where it goes unnoticed. Whenever possible, flaps are designed to place the resulting scars into natural creases and shadowed areas of the face so that they are less noticeable. Local flaps are designed to take maximum advantage of areas of relative tissue excess. They may be designed as semicircular rotation flaps, rhomboid-shaped flaps, pennant-shaped flaps, and bilobed flaps.

Regional Flaps

Regional flaps recruit larger blocks of tissue from farther away to close more-complex wounds. For example, when a large portion of the nose needs resurfacing, the forehead skin can provide ideal coverage. The flap is elevated from the forehead and attached to a long leash of tissue that contains the blood vessels that feed it. These come from the area just above the central part of the eyebrow. This leash of tissue either bridges the intact skin to connect the flap to the open wound or is buried under the intact skin. If it is left external, the leash is divided after three weeks, once the flap has established a sufficient blood supply at its new location. The forehead has a large, broad surface and ample laxity when pulled in from the sides to the midline. This allows the majority of the donor site to be closed without significant deformity.

When a portion of the donor site cannot be closed, it is left to heal by itself. Because wounds contract as they heal, the resulting scar is much smaller than the initial wound; and when positioned at the top of the forehead adjacent to the hairline, the scar is minimally perceptible. If the surface of the nose were treated in the same way, it would severely distort the architecture of the nose and obliterate its natural contours. Wound contraction and scarring may also cause functional problems by narrowing the airway.

Free Flaps

A free flap is a block of tissue that is removed from the body and attached to a length of artery and vein that supply it. This allows the tissue to be moved to a distant location, where tissue is needed. To ensure survival, an artery and a vein at the wound location must be recruited to plug in to the flap vessels to restore blood supply to the tissue. This type of solution for skin cancer defects is used when extensive coverage over a complex area is needed. It is also a valuable tool when there is no good-quality tissue nearby to close a skin cancer defect, such as when cancer arises in a bed of skin that has undergone radiation therapy or in an area of burn scars.

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Multilayered Reconstructions

When skin cancer defects are reconstructed over broad, flat surfaces, skin grafts or single-layer flaps suffice. Areas of the body with three-dimensional architecture and critical functions, such as the eyelids and the nose, may require multilayered reconstructions. Layers for internal lining, structural stability, and external skin coverage—all may be required. Combinations of flaps and skin, cartilage, and bone grafts may be used. Composite grafts, which contain more than one tissue type, such as skin and cartilage, may also be helpful. A common example of this uses a portion of the ear rim, containing skin and cartilage, to reconstruct the border of the nasal aperture. The cartilage prevents the graft from collapsing and maintains the desired curvature.

Recovery After Reconstruction from Skin Cancer

Healing times after skin cancer reconstructive surgery vary depending on the site and the type of closure used. Procedures on the lower legs often involve grafts, and several weeks of immobilization and elevation of the extremity may follow. Lymph node dissections also significantly prolong the healing period. Small procedures on the face, including local flaps, may have little if any impact on daily activities and cause only minor discomfort.

One thing all procedures have in common is scarring. Scars take several months—even up to a year—to mature. During the various phases of healing, scars can become raised, thickened, and discolored. Over time scars fade, flatten, and soften. Small flaps on the face may appear puffy or pillowed for prolonged periods. This swelling is due to fluid buildup in the rearranged tissues, and it resolves gradually as new drainage channels are established. Surgeons may use surgical tapes, silicone gel sheeting, or compressive wraps to treat scars prophylactically, but keloid scars—red, raised formations of fibrous scar tissue—if they develop, likely require additional treatment. Given enough time, most people heal their surgical sites with acceptable scars without the need for any topical creams or scar treatments.


  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.
  2. American College of Mohs Surgery (Patient Education)
  4. JAMA Dermatol. 2019 Sep 25. Epub ahead of print.