Mohs Surgery Step by Step: Exactly What Happens During the Procedure

What Is Mohs Surgery and Why Is It Done in Stages?

Mohs micrographic surgery was invented by Dr. Frederic Mohs in the 1930s and has been refined into one of the most precise cancer treatments in all of medicine. The reason it is done in stages is deceptively simple: skin cancers are invisible to the naked eye at their edges. A tumor that looks like a 1 cm spot may have microscopic tentacles of cancer extending several millimeters in all directions underneath normal-looking skin. You cannot see these extensions—you can only find them by looking at the tissue under a microscope after it is removed.

In standard excision, a surgeon guesses where the edges are by taking a set margin (say, 4 mm all around) and sends the whole thing to a lab. The lab examines a few cross-sections—but actually views only about 1–2% of the margin. In Mohs surgery, the entire margin is examined. Every millimeter of the undersurface and edges is checked. This is only possible because the Mohs surgeon personally processes and reads the tissue right there in the office.

The Mohs Technique: A Step-by-Step Walkthrough

Step 1: Pre-Surgical Preparation

You arrive at the clinic and the surgical site is identified. Your surgeon examines the area and discusses the plan with you. Photography is taken for documentation. The site is cleaned with an antiseptic solution and draped with sterile covers.

Step 2: Local Anesthesia

A local anesthetic—almost always lidocaine with epinephrine—is injected around (not into) the tumor. You will feel a brief stinging sensation that lasts about 15–30 seconds. Within 5–10 minutes, the area is completely numb. You will feel pressure and movement during the procedure but not pain. The epinephrine also causes local blood vessels to constrict, reducing bleeding and improving visibility during surgery.

Step 3: Visible Tumor Debulking (Optional but Common)

If the tumor is raised or clearly visible, the surgeon may first use a curette (a small, spoon-like scraping tool) to remove the visible bulk of the tumor. This helps define the edges of the tumor and provides a preliminary tissue sample, but it is not the definitive cancer removal—that comes with the Mohs layer.

Step 4: Excision of the First Mohs Layer

The surgeon uses a scalpel to cut a thin, saucer-shaped layer of tissue. The key feature is the angle: the cut is made at approximately a 45-degree angle to the skin surface (rather than straight down). This bevel allows the edge and the undersurface of the specimen to be flattened into a single plane—which is what makes it possible to examine the entire margin on a single microscope slide.

The layer is kept very thin—just a millimeter or two—to minimize the amount of healthy tissue removed.

Step 5: Color-Coding and Tissue Mapping

This step is the intellectual core of Mohs surgery. The excised tissue is divided into sections, and each section is marked with a specific color of dye—for example, blue on one side and red on the other, or different dyes for each quadrant. The surgeon draws a detailed map of the wound showing exactly which color corresponds to which location on the patient’s body.

This color-coding system means that when the surgeon later sees cancer cells on a slide, they can look at the color in that area of the slide and immediately know exactly where on the wound the cancer is still present. They can then go back and remove tissue precisely from that location—and only that location.

Step 6: Tissue Processing (Cryostat Sections)

The tissue sections are taken to the in-office laboratory. A technician carefully embeds each section so that its deep surface and edges face upward (this is the critical orientation that allows 100% margin examination). The tissue is then frozen with liquid nitrogen and cut into extremely thin slices using a cryostat machine—a refrigerated, precision microtome.

The thin slices are placed on glass slides and stained with hematoxylin and eosin (H&E), the standard pathology stain that colors different cell types different shades. This process takes approximately 30–45 minutes, which is why you wait in the waiting room between stages.

Step 7: Microscopic Examination by the Surgeon

The Mohs surgeon sits at the microscope and systematically examines every slide—looking at the entire undersurface and edge of the removed tissue. They are looking for cancer cells at or near the margins of the tissue. If none are seen, the margins are clear and surgery is complete. If cancer cells are seen anywhere, the surgeon notes their location on the map and circles the affected area.

In Mohs surgery, the surgeon examining the tissue is the same person who removed it—meaning they have direct spatial knowledge of exactly where each piece of tissue came from. This eliminates the communication errors that can occur when specimens are sent to a separate outside laboratory.

Step 8: Targeted Re-Excision (If Needed)

If cancer cells are found, only the specific area where they were detected is re-excised. The surgeon returns to the patient, re-anesthetizes the specific section of the wound where the map shows residual cancer, and removes another thin, precisely targeted layer from that area only.

This new tissue goes through the same color-coding, processing, and microscopy cycle. This is called Stage 2. The process continues until the final stage shows no cancer cells anywhere in the margins. Most Mohs procedures are completed in 1–3 stages; the average is approximately 1.5 stages.

Step 9: Wound Reconstruction

Once the surgeon confirms that all margins are clear, attention turns to repairing the wound. The wound is typically repaired on the same day by the Mohs surgeon or a reconstructive specialist. Options include:

• Primary closure: Simply stitching the wound closed with sutures—possible for smaller wounds with movable surrounding skin
• Skin flap: Nearby skin is rearranged and moved to cover the wound—allows the wound to be closed even when direct closure would distort the surrounding features
• Skin graft: Skin is taken from a donor site elsewhere on the body and transplanted to cover the wound—used when a flap is not possible or practical
• Secondary intention: The wound is left open to heal on its own with daily wound care—works well for certain locations (especially concave areas) and avoids a second scar at a donor site

The choice of repair depends on wound size, location, and your individual anatomy. Your surgeon will discuss the options with you and explain their recommendation before proceeding.

After Surgery: Before You Leave

Before you go home, your nurse will:

• Dress the wound and show you how to change the dressing at home
• Give you written instructions for wound care (usually involves daily cleaning and petroleum jelly application)
• Review activity restrictions (typically no strenuous exercise for 1–2 weeks)
• Tell you when to return for suture removal and follow-up
• Explain warning signs that warrant calling the office (increasing pain, fever, pus, or wound opening)

When to See a Dermatologist

• You have been diagnosed with a skin cancer and want to discuss whether Mohs surgery is right for your situation
• You have a skin lesion that has changed, grown, bled, or failed to heal—schedule an evaluation
• After Mohs surgery, your wound shows signs of infection or other complications
• You have a history of skin cancer and are due for a full-body skin check

Frequently Asked Questions

Why does Mohs surgery take all day?

Each stage requires 30–45 minutes of laboratory processing time while you wait. If your cancer is clear after one stage, you might be done in 2–3 hours. If multiple stages are needed, the procedure takes longer. Additionally, reconstruction of the wound after all cancer is removed can take an additional 30–90 minutes depending on complexity. Bring snacks, entertainment, and comfortable clothing—most offices have a dedicated waiting area where patients spend time between stages.

Is Mohs surgery more accurate than regular pathology?

In terms of margin examination, yes—significantly. Standard “bread-loaf” sectioning used in conventional pathology examines only about 1% of the surgical margin. Mohs surgery examines close to 100% of the margin using en face frozen sections. This is why the cure rates for Mohs are consistently higher than those for standard excision, particularly for aggressive or recurrent tumors.

What if I feel pain during a stage?

Tell your surgeon immediately. The local anesthetic can wear off between stages, and it is very easy to add more. You should never feel pain during a Mohs procedure. There is no limit to how much additional anesthetic can be given—your surgeon will always re-numb an area before proceeding if needed.

Can I eat before Mohs surgery?

Yes. Unlike general anesthesia procedures, you can eat and drink normally before Mohs surgery. In fact, eating a good meal before your appointment is a good idea given the length of the day. Take your regular medications as usual unless your doctor has specifically told you to hold something (like a blood thinner).

Trusted Resources

• American Academy of Dermatology – Mohs Surgery
• Skin Cancer Foundation – Mohs Surgery
• American College of Mohs Surgery – Patient FAQs
• Mayo Clinic – Mohs Surgery

Always consult a board-certified dermatologist or Mohs surgeon for diagnosis and treatment recommendations specific to your situation.