Mohs Micrographic Surgery

Cutaneous Oncology (Skin Cancer)

UCSF is a nationally recognized center of excellence for the treatment of skin cancer. All of our doctors have completed an advanced fellowship in Mohs Micrographic Surgery, are some of the leading experts in the field, and have trained numerous physicians from around the world. We are able to provide the best treatment options and cure rates for skin cancer while giving outstanding cosmetic results with expertise in reconstruction. Our skin cancer patients have access to all the best resources that are available at UCSF, allowing treatment options that are not available in other practice environments. In addition, UCSF Dermatologic Surgery Center has a multi-disciplinary tumor board for challenging and unusual skin cancer tumors and conditions, and we are referred patients from community dermatologists and surgeons from around the region.

Mohs micrographic surgery is a specialized technique for treating skin cancers. Mohs surgeons are uniquely trained as cancer surgeons, pathologists, and reconstructive surgeons. Initially developed by Frederic Mohs, MD in the 1930s, and then modified by UCSF physicians in the 1970s, Mohs surgery involves removing precise layers of the skin and evaluating the entire surgical margin with a microscope while the patient waits. Repeated layers are taken until the margins are clear and the cancer is removed. Following the removal, immediate surgical reconstruction can be performed with the security of knowing the cancer has been removed. The tissue is evaluated in a special way to look at 100% of the surgical margin, giving the patient the confidence of knowing with the highest probability that the cancer has been removed. In addition, because the cancer is removed one layer at a time, Mohs surgery results in the smallest possible surgical defect because no extra skin is removed. While most commonly used to treat basal cell carcinoma and squamous cell carcinoma, Mohs surgery can be used for a wide range of skin cancers. Since the technique is very precise, patients can get the highest cure rate, while minimizing the size of the surgical defect and resulting scar.  

The Mohs Surgery Procedure

The Mohs surgical process involves a repeated series of surgical excisions followed by a microscopic examination of the tissue to assess if any tumor cells remain. Some tumors that appear small on the clinical exam may have extensive invasion underneath normal-appearing skin, resulting in a larger surgical defect than would be expected. It is therefore impossible to predict a final size until all surgery is complete. As Mohs surgery is used to treat complex skin cancers, approximately half of all treated tumors require 2 or more stages for complete excision.

Step 1: Anesthesia

The tumor site is locally infused with anesthesia to completely numb the tissue. General anesthesia is not required for Mohs micrographic surgery.

Step 2: Stage I - Removal of visible tumor

Once the skin has been completely numbed, the tumor is gently scraped with a curette, a semi-sharp, scoop-shaped instrument. This helps define the clinical margin between tumor cells and healthy tissue. The first thin, saucer-shaped "layer" of tissue is then surgically removed by the Mohs surgeon. An electric needle may be used to stop the bleeding. This process takes approximately 10-20 minutes.

Step 3: Mapping the tumor

Once a "layer" of tissue has been removed, a "map" or drawing of the tissue and its orientation to local landmarks (e.g. nose, cheek, etc) is made to serve as a guide to the precise location of the tumor. The tissue is labeled and color-coded to correlate with its position on the map. The tissue sections are processed and then examined by the surgeon to thoroughly evaluate for evidence of remaining cancer cells. It takes approximately 60 minutes to process, stain and examine a tissue section. During this processing period, your wound will be bandaged and you may leave the operative suite.

Step 4: Additional stages - Ensuring all cancer cells are removed

If any section of the tissue demonstrates cancer cells at the margin, the surgeon returns to that specific area of the tumor, as indicated by the map, and removes another thin layer of tissue only from the precise area where cancer cells were detected. The newly excised tissue is again mapped, color-coded, processed, and examined for additional cancer cells. If microscopic analysis still shows evidence of disease, the process continues layer-by-layer until the cancer is completely removed. By beginning early in the morning, Mohs surgery is generally finished in one day. Sometimes, however, a tumor may be extensive enough to necessitate continuing surgery a second day.

This selective removal of tumors allows for the preservation of as much of the surrounding normal tissue as possible. Because this systematic microscopic search reveals the roots of the skin cancer, Mohs surgery offers the highest chance for complete removal of cancer while sparing the normal tissue. Cure rates typically exceed 99% for new cancers, and 95% for recurrent cancers.

There are a number of special circumstances when the Mohs surgery technique is modified to accommodate issues that go beyond traditional “tissue sparing”. Tumors such as melanoma, Merkel cell carcinoma, malignant fibrous histiocytoma, dermatofibrosarcoma protuberans, and some others can be aggressive and life-threatening. In this event, our emphasis is based on complete tumor removal with an appropriate wide local margin, followed immediately by the first Mohs layer. This sequence is performed to achieve the highest possible confidence level that the resultant skin defect site can be repaired immediately in the confident knowledge that the entire tumor has been resected.

Step 5: Reconstruction

Fellowship-trained Mohs surgeons are experts in the reconstruction of skin defects. Reconstruction is individualized to preserve normal function and maximize aesthetic outcomes. The best method of repairing the wound following surgery is determined only after the cancer is completely removed, as the final defect cannot be predicted prior to surgery. Stitches may be used to close the wound side-to-side, or a skin graft or a flap may be designed. Sometimes, a wound may be allowed to heal naturally.

Healing by spontaneous granulation involves letting the wound heal by itself naturally. This offers a good chance to observe the wound as it heals after the removal of a difficult tumor. Experience has taught us that there are certain areas of the body where nature will heal a wound as nicely as any further surgical procedures. There are also times when a wound will be left to heal knowing that if the resultant scar is unacceptable, some form of cosmetic surgery can be performed at a later date.

Closing the wound with stitches is often performed on a small lesion. This involves some adjustment of the wound and sewing the skin edges together. This procedure speeds healing and can offer a good cosmetic result. For example, the scar can be hidden in a wrinkle line.

Skin grafts involve covering a surgical site with skin from another area of the body. There are three types of skin grafts. The first is called a split-thickness graft. This is a thin shave of skin, usually taken from the thigh, which is used to cover a surgical wound. This can be either permanent coverage or temporary coverage before another cosmetic procedure is done at a later date. The second graft type is the full-thickness graft. This graft provides a thicker layer of skin to achieve desired results. In this instance, skin is usually removed from behind the ear or around the collarbone (the donor site), and stitched to cover a wound. The donor site is then sutured together to provide a good cosmetic result. A third type of graft uses skin and cartilage. This usually comes from the ear and is used to repair defects in the nose.

Skin flaps involve the movement of adjacent, healthy tissue to cover a surgical site. Where practical, they are chosen because of the excellent cosmetic match of nearby skin.

In rare cases, when Mohs surgery is extremely extensive or when removal of the tumor results in functional impairment, we may recommend that you visit one of several consultant surgeons for reconstruction.   

Frequently Asked Questions

Are all Mohs surgeons the same? Why choose a fellowship trained surgeon?

When choosing a doctor to treat your skin cancer, you want to select someone with the advanced training, expertise, and experience to give you the best results ( All the physicians at the UCSF Dermatologic Surgery and Laser Center are fellowship-trained Mohs surgeons and members of the American College of Mohs Surgery. These advanced fellowships have an extremely competitive selection and review the process and provide an intense year-long program that gives unmatched clinical training in the treatment of skin cancer and reconstruction. In addition, the physicians at UCSF are all national experts in evaluating and managing skin cancer and skin tumors and have trained numerous physicians across the country. We are the only ACGME-approved Mohs surgery and Procedural Dermatology program in the SF Bay Area. Our training program is one of the most sought-after by dermatologists in the US looking to get advanced skin cancer surgical training. In short, the care you receive at the UCSF Dermatologic Surgery and Laser Center is unmatched.

How long does Mohs surgery take?

Mohs surgery removes skin cancer layer by layer. Since the skin is immediately evaluated under the microscope after each stage, Mohs surgery takes longer than traditional excisional surgery. However, this process results in the highest cure rates with the smallest amount of normal tissue being removed. As a result, it is impossible to predict how long the surgery will take. Typically, it takes about 60-90 minutes to evaluate each layer of skin. If multiple layers are needed, then the surgery may take several hours. The size, location, and type of cancer all play a factor in determining how long the case may take. In addition, the reconstruction time may vary based on the complexity of the tumor and size of the defect.

A consultation with your Mohs surgeon to evaluate the specifics of your skin cancer, may help to estimate exactly what will be required and the time involved. Feel free to bring a book, computer (we have Wi-Fi), or family/friends to help pass the time. In addition, patients are able to get something to eat while they are waiting between stages.

I don't see anything left at my biopsy site. Do I still need to have my skin cancer treated?

Yes! The biopsy only removes a small portion of the skin cancer. After the biopsy heals, the surface of the skin may look normal. However, skin cancers often have roots that are not visible to the naked eye and can extend beyond the biopsy site. If left untreated, these residual cancer cells will continue to grow. The remaining tumor can spread deeper into the skin and invade nearby structures (like muscles, nerves, vessels, etc.) Over time, cancer eventually reappears and the surgery required at this point is much more extensive and challenging than if it had been performed right away. Even if you don't see anything left, treating cancer after the biopsy is the safest and best course of action.

I have been diagnosed with skin cancer. What are my treatment options?

Skin cancer is the most common malignancy in the US. There are many different kinds of skin cancer, including basal cell carcinoma, squamous cell carcinoma, melanoma, and many other rare and more unusual kinds. The physicians at UCSF are leading experts in the management and treatment of all kinds of skin cancer. There are a variety of different options available, including Mohs micrographic surgery. The specific options that is best suited for your skin cancer can be reviewed during a consultation.

What are the advantages of Mohs micrographic surgery?

The advantages of Mohs surgery are:

  • Highest cure rates of any technique for skin cancers
  • Best treatment for recurrent skin cancers that failed other treatments
  • Minimizes removal of normal skin tissue around the skin cancer site
  • Resulting reconstruction limits the amount of scaring compared to other treatments
  • Reconstruction can be performed the same day
  • Fewer risks with local anesthesia compared with general anesthesia
Why is it called Mohs surgery? Does Mohs stand for something?

Dr. Frederic Mohs, a professor of surgery at the University of Wisconsin, developed this surgical technique in the 1930s. Over the years, it has undergone many refinements and has come to be known as "Mohs micrographic surgery" or simply "Mohs surgery" in honor of Dr. Mohs.

Will Mohs surgery leave a scar?

Yes. Any surgical procedure to remove skin cancer will leave a scar. The advantage of Mohs surgery is that the cancer is removed layer by layer, with a careful analysis under the microscope after each section. Therefore, this technique results in the smallest possible surgical defect and minimizes the amount of normal skin removed. By keeping the surgical defect as small as possible, the Mohs surgeons at UCSF can reconstruct the wound in a way that gives you the best possible cosmetic outcome. Scars can often be placed in pre-existing lines and wrinkles on your skin so they are hidden. In many cases, the scars are hardly noticeable after everything is healed.

Scars can take up to a year to mature and evolve. Right after surgery, scars are often bumpy and red as part of the normal wound-healing process. There is an increase in blood vessels which bring wound healing factors to the surgical site to help the wound heal, thus producing the redness in color. Over the months following surgery, the redness fades and the scar softens. The Mohs surgeons at the UCSF Dermatologic Surgery and Laser Center will follow you closely during the post-operative period to ensure that you get the best results possible. We have a variety of lasers and other interventions that we can use to maximize the cosmetic results of your surgery.