The Bottom Line

Mohs micrographic surgery achieves cure rates of 98–99% for basal cell carcinoma and squamous cell carcinoma by examining 100% of the surgical margins during the procedure. The surgeon removes thin layers of tissue one at a time, maps them precisely, and checks every edge under a microscope before removing more. This means cancer is fully eliminated — and only cancerous tissue is removed, preserving as much healthy skin as possible.

What Makes Mohs Surgery Different

Most types of cancer surgery send tissue to a pathology lab after the procedure, where a technician examines a small sample of the margins — not all of them. Results come back days later. If cancer cells were missed at the edges, a second surgery is needed.

Mohs surgery works differently. The Mohs surgeon is also trained as a pathologist. Right in the office, during the procedure, they examine 100% of the tissue margins immediately after each removal. There is no sampling, no days-long wait, and no uncertainty. Cancer is not declared fully removed until the surgeon has confirmed it under the microscope themselves.

This combination of precision and completeness is why Mohs surgery has the highest cure rates of any treatment for basal cell carcinoma (BCC) and squamous cell carcinoma (SCC): 98–99% for primary tumors. For recurrent or previously treated skin cancers, cure rates are still 94–96%.

Who Is Mohs Surgery For?

Mohs surgery is recommended when the stakes of incomplete removal are highest. Key indications include:

  • Skin cancers on cosmetically or functionally important areas: face, eyelids, nose, lips, ears, scalp, hands, feet, and genitals
  • Tumors with aggressive growth patterns or indistinct borders
  • Large tumors (over 2 cm) or rapidly growing cancers
  • Recurrent cancers that have come back after prior treatment
  • Cancers in areas with limited skin available for repair (meaning tissue conservation matters most)
  • Patients who are immunosuppressed and at higher risk for aggressive behavior

The Surgical Technique: How It Actually Works

Mapping the Tumor

Before any cutting begins, the surgeon carefully examines the tumor and surrounding skin, often using a dermatoscope (a magnifying device). The boundaries of the visible tumor are marked. A detailed map — essentially a diagram of the site drawn to scale — is prepared. This map will be used throughout the procedure to identify exactly where cancer cells are found in subsequent layers.

Stage 1: Curettage and First Excision

The surgeon first scrapes the tumor surface with a curette — a small spoon-shaped instrument. Cancer cells are softer and more fragile than normal skin, so they come away more easily. This defines the clinical extent of the tumor and guides where to excise. The surgeon then removes the visible tumor along with a 1–2 mm margin of surrounding tissue, angling the blade at roughly 45 degrees to produce a saucer-shaped specimen. This shape is important for processing.

Tissue Processing: Color-Coded Mapping

The removed tissue is oriented and divided into sections. Each section is marked with colored dyes — typically two or three colors — at specific, recorded positions on the map. This color-coding is what makes it possible to later identify exactly which part of the wound needs more tissue removed.

The sections are then processed using a technique called frozen section histology. The tissue is rapidly frozen, cut into very thin slices (5 microns thick), and stained. The entire undersurface and all edges are mounted on microscope slides. This is the critical difference from standard pathology: the sections are oriented so that the entire margin is visible, not just a cross-section through the middle.

Microscopic Margin Analysis

The surgeon reviews every slide under the microscope, scanning the full circumference and deep margin of the excised tissue. They look for tumor cells at or near the edges. If the margins are completely clear — meaning no cancer cells are seen at any edge — the surgery is complete and wound repair begins.

If cancer cells are present at a specific margin, the surgeon marks that location on the map. Only that portion needs to be re-excised. This is what makes Mohs so tissue-sparing: instead of cutting a larger margin all the way around, only the involved area is targeted.

Subsequent Stages

Additional layers are taken, processed, and examined in exactly the same way. Each successive stage removes tissue only from the areas where cancer was found in the previous stage. This continues until all margins are clear. The median number of stages is 1–2. About 70% of cases are complete in one stage; most of the rest require two or three. More than three stages are uncommon.

Why 100% Margin Assessment Matters

Standard excision pathology (called "bread-loafing") samples cross-sections of the tissue — typically reviewing only 1–3% of the actual margin surface. Mohs processes the tissue in a way that allows the entire undersurface and edges to be visualized, not just slices through the middle. This is why Mohs catches residual cancer that standard excision would miss, particularly in tumors with irregular, finger-like extensions into normal tissue — common in morpheaform basal cell carcinomas and some squamous cell carcinomas.

Reconstruction After Mohs

Once surgical clearance is confirmed, the wound is repaired. The reconstruction plan is guided by wound size, location, and available surrounding tissue:

  • Primary linear closure: Direct side-to-side closure with sutures — simplest and fastest for smaller wounds
  • Local flaps: Nearby tissue is mobilized and moved to cover the defect. Advancement flaps, rotation flaps, and transposition flaps are tailored to the anatomy of each site
  • Full-thickness skin grafts: Skin from a donor site (often behind the ear or from the inner arm) is used when local tissue is insufficient
  • Second-intention healing: Some anatomically concave areas (inner corner of the eye, the ear bowl) heal remarkably well without stitches, producing excellent cosmetic results

For defects involving the eyelid, nose, or other complex structures, reconstruction may be performed by an oculoplastic surgeon, plastic surgeon, or oral surgeon — sometimes at a separate visit if planning time is needed.

Outcomes and Long-Term Follow-Up

Cure rates for Mohs surgery exceed those of any other skin cancer treatment. For primary basal cell carcinoma, the 5-year recurrence rate after Mohs is about 1%. For primary squamous cell carcinoma, it is about 2–3%. Compare this to standard excision, which has recurrence rates of 5–10% for primary BCC on the face.

After Mohs surgery, regular skin checks are essential. People who have had one skin cancer have a significantly higher chance of developing additional cancers. Most dermatologists recommend follow-up skin exams every 6–12 months for at least 5 years.

When to See a Dermatologist

  • You have been diagnosed with basal cell carcinoma or squamous cell carcinoma and want to know if Mohs is right for your tumor
  • You have a skin cancer on your face, ears, nose, eyelids, lips, or scalp
  • You have a recurrent skin cancer that has come back after prior treatment
  • You are immunosuppressed and have a skin cancer that carries higher recurrence risk
  • You notice a new growth, a sore that does not heal, or a changing spot on your skin

Frequently Asked Questions

How is Mohs surgery different from standard excision?

Standard excision removes the tumor with a preset margin (usually 4–6 mm) and sends it to an outside lab where technicians sample only a small portion of the edges. Results take 3–7 days. Mohs examines 100% of the surgical margin immediately during the procedure. The surgeon is the pathologist. This eliminates the sampling gap and allows same-day confirmation that all cancer has been removed.

Does Mohs leave a smaller scar than other surgeries?

Usually, yes — at least in terms of the initial wound size. Because Mohs removes only as much tissue as is necessary (guided by microscopy, not by preset margins), defects tend to be smaller than with standard wide excision. However, the reconstruction technique has a much larger impact on the final scar appearance than the excision itself.

Can Mohs surgery be used for melanoma?

Mohs surgery is primarily used for basal cell carcinoma and squamous cell carcinoma. It can be used for certain subtypes of melanoma (particularly lentigo maligna melanoma) when modified with special staining techniques, but it is not the standard approach for most melanomas. Your dermatologist will recommend the best treatment for your specific diagnosis.

Is Mohs covered by insurance?

Yes, Mohs surgery is typically covered by Medicare and most commercial insurance when it is performed for appropriate indications. Your dermatologist's office can verify your coverage and provide prior authorization if needed before scheduling.

References

  1. Shriner DL, McCoy DK, Goldberg DJ, Wagner RF. Mohs micrographic surgery. J Am Acad Dermatol. 1998;39(1):79–97.
  2. Mosterd K, Krekels GAM, Nieman FHM, et al. Surgical excision versus Mohs' micrographic surgery for primary and recurrent basal-cell carcinoma of the face. Lancet Oncol. 2008;9(12):1149–1156.
  3. Leibovitch I, Huilgol SC, Selva D, et al. Cutaneous squamous cell carcinoma treated with Mohs micrographic surgery. J Am Acad Dermatol. 2005;53(2):253–260.
  4. American Academy of Dermatology. Mohs Surgery — Appropriate Use Criteria. J Am Acad Dermatol. 2012;67(4):531–550.
  5. Mohs FE. Chemosurgery: microscopically controlled surgery for skin cancer — past, present, and future. J Dermatol Surg Oncol. 1978;4(1):41–54.

Trusted Resources

Always consult a board-certified dermatologist for personalized advice about your skin cancer diagnosis and treatment options.