Mohs micrographic surgery is a highly refined outpatient technique developed to treat common forms of skin cancer, such as basal cell carcinoma and squamous cell carcinoma. The procedure is defined by its ability to examine tissue margins immediately, ensuring that 100% of the surgical edge is analyzed for cancer cells. This precise, layer-by-layer removal targets only cancerous tissue, sparing the maximum amount of surrounding healthy skin. This approach provides the highest cure rates while minimizing the size of the resulting wound and subsequent scarring.
The Initial Mohs Layer
The procedure begins after the area is numbed with a local anesthetic. The Mohs surgeon first removes any visible portion of the tumor, a step often referred to as debulking. Once the mass is gone, the surgeon removes the first thin layer of surrounding tissue, which represents the true surgical margin. This initial layer is typically excised with a narrow peripheral margin, often only 1 to 2 millimeters of skin around the visible tumor site.
The tissue is removed as a thin saucer- or disc-shaped specimen to maximize the surface area for microscopic review. A scalpel is used to remove this layer at a slight angle, often between 30 and 45 degrees, creating a beveled edge. This angle helps the tissue lay flat for processing, allowing the surgeon to examine the deepest and outermost edges of the specimen simultaneously.
The Layer-by-Layer Examination Process
Once the initial tissue layer is removed, the surgeon creates a detailed map of the wound site, correlating anatomical landmarks with the specimen’s orientation. The excised tissue is marked with colored dyes at specific points (e.g., 12, 3, 6, and 9 o’clock positions) to maintain its exact orientation relative to the patient’s body. This mapping ensures the surgeon can return to the precise location if cancer is detected.
The specimen is immediately taken to an on-site laboratory, where it is flash-frozen using a cryostat. Freezing allows the tissue to be cut into ultra-thin sections, mounted onto glass slides, and stained (typically H&E) to make cellular structures visible. This rapid processing, known as frozen sectioning, is a defining feature of Mohs surgery.
The Mohs surgeon, who also functions as the pathologist, then examines the prepared slides under a microscope, checking 100% of the peripheral and deep margins. If no cancer cells are found, the margin is considered clear, and the removal phase is complete. If cancer cells are identified, the surgeon uses the map and ink markings to pinpoint the exact area where residual tumor remains. The surgeon removes only another thin layer of tissue from the positive location. This process is repeated until microscopic examination confirms that the margins are entirely clear of cancer, which determines the final depth of the procedure.
Variables Influencing Tissue Removal Depth
Mohs surgery does not have a predetermined depth; the procedure stops when the cancer is fully removed. The final depth is directly proportional to the subclinical extent of the tumor, and the number of layers required is influenced by several factors.
The histological subtype is a major determinant; aggressive forms (e.g., morpheaform, infiltrative, or micronodular basal cell carcinoma) often have deeper and less predictable roots. These subtypes require more stages to achieve clear margins compared to superficial or nodular types.
The anatomical location also affects depth, particularly in areas where cancer spreads along tissue planes or where the skin is thin. Lesions in high-risk areas like the nose, ears, or the periocular region are associated with a higher number of surgical stages.
Tumor history and size also play a role. Recurrent lesions often have complicated, deep extensions and require more layers for complete removal. Larger tumors (greater than two centimeters in diameter) are more likely to have a substantial subclinical component, necessitating greater tissue removal depth.
Post-Procedure Wound Management
Once the Mohs surgeon confirms that the margins are cancer-free, attention shifts to the resulting wound, known as the surgical defect. The management method depends on the size, depth, and location of the final defect, aiming for the best functional and cosmetic outcome.
For smaller or linear defects, the surgeon often performs a primary closure by stitching the edges of the wound together. For larger or more complex wounds, a skin flap procedure may be necessary, involving the strategic movement of neighboring healthy tissue into the defect.
Other options include a skin graft, where skin is borrowed from a distant site and secured over the wound. Alternatively, defects in concave areas (like the ear or inner corner of the eye) may be left to heal through secondary intention, allowing the body to fill in the wound naturally.
Following the procedure, patients receive specific instructions for wound care. This generally involves keeping the area clean and moist with a protective ointment, and changing the dressing daily. Elevation of the surgical site and avoiding strenuous activity or heavy lifting are also recommended to minimize swelling and reduce tension.