Mohs Micrographic Surgery: How It Works

Mohs micrographic surgery is a highly specialized, state-of-the-art technique used for the treatment of complex skin cancers. This procedure was first developed in the 1930s by Dr. Frederick Mohs, a professor of surgery at the University of Wisconsin. Mohs micrographic surgery is distinct from routine surgical excision. With the Mohs technique, surgically removed tissue is carefully mapped, color-coded, and thoroughly examined microscopically by the surgeon on the same day of surgery. During this process, 100% of tissue margins are evaluated to ensure that the tumor is completely removed prior to repair of the skin defect. Mohs micrographic surgery therefore results in the highest cure rate for complex skin cancers while minimizing the removal of normal tissue.

Standard surgical excision allows for delayed examination of approximately 1% of tissue margins. Since only a small percentage of margins are evaluated, residual tumor may be missed. If more cancer cells are found to remain during delayed pathologic examination, a second surgical procedure will be required at a later date.

Mohs surgeons are dermatologists who have performed additional fellowship training to become experts in Mohs micrographic surgery. Fellowship-trained Mohs surgeons are highly skilled in all aspects of this technique, including surgical removal of the tumor, pathologic examination of the tissue, and advanced reconstruction techniques of the skin. All dermatologic surgery faculty at UCSF are Mohs fellowship-trained and members of the American College of Mohs Micrographic Surgery and Cutaneous Oncology (ACMMSCO). This official national organization maintains the high level of training and quality of care of this sub-specialty.

Referral for Mohs micrographic surgery may be made by your physician after a biopsy has been performed to confirm your diagnosis of skin cancer. Your physician’s office may contact our Mohs surgery scheduling nurse at 415-353-9568.


The majority of tumors treated with Mohs Micrographic surgery are complex basal and squamous cell carcinomas. In some circumstances, Mohs surgery can be used to treat less common tumors, including some superficial melanomas.

Skin cancers are complex when:

  • the cancer is in an area where preservation of healthy tissue is critical to maximize function and cosmetic result (eyelids, nose, ears, lips, hands)
  • the cancer is in an area of higher tumor recurrence (ears, lips, nose, eyelids, temples)
  • the cancer was incompletely treated, or was previously treated and is recurrent
  • the cancer is large
  • the edges of the cancer cannot be clearly defined
  • scar tissue exists in the area of the cancer
  • the cancer grows in an area of prior radiation therapy
  • the patient is immunosuppressed (organ transplant, HIV infection, chronic lymphocytic leukemia)
  • the patient is prone to getting multiple skin cancers (including genetic syndromes such as basal cell nevus syndrome and xeroderma pigmentosa

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The Procedure

The Mohs surgical process involves a repeated series of surgical excisions followed by microscopic examination of the tissue to assess if any tumor cells remain. Some tumors that appear small on 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.

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.

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

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 outcome. 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.

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Schematic of Mohs Micrographic Surgery Technique

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Schematic of a Pathologic Examination of Tissue Margins

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UCSF Dermatologic Surgery Faculty