Mohs Micrographic Surgery: Principles

Frederic E. Mohs developed the technique in the 1930s at the University of Wisconsin using zinc chloride paste to fix tissue in situ before excision (chemosurgery). In the 1970s, Tromovitch and Stegman demonstrated that fresh tissue (unfixed) could be processed with equivalent accuracy, transforming the technique into the fresh-tissue Mohs surgery used today. The defining feature of Mohs surgery is the dual role of the surgeon: the Mohs surgeon serves as both the surgeon who removes the tumor AND the pathologist who examines the margins. This integration eliminates the communication gap inherent in standard surgical pathology and allows real-time, intraoperative margin control.

The clinical tumor margin is identified and marked. Curettage is performed to define the extent of the tumor. The soft, friable tumor tissue is distinguishable from the firm, gritty normal dermis. This tactile assessment helps define the true tumor boundary, which often extends beyond the visible clinical margin. The tumor is then debulked (curetted away) to the level of surrounding normal skin. This debulking step is informational, not therapeutic. It reveals the approximate tumor footprint and guides the subsequent excision.

A thin, saucer-shaped layer of tissue is excised around the curetted defect with 1–2mm margins and beveled (45-degree) edges. The beveled edge is critical. It allows the tissue to be flattened during processing so that the entire peripheral and deep margin can be examined on a single horizontal section. Orientation marks are made with score marks (nicks) at specific positions, and the tissue is divided into sections. Each section is color-coded with tissue dyes (typically 2+ colors). A precise tissue map is drawn correlating each section’s position and orientation to the patient’s wound.

The excised tissue sections are taken to the on-site Mohs histology laboratory for processing: 1. Relaxing incisions are made at the beveled edges to allow the tissue to flatten completely 2. The flattened sections are mounted on a cryostat chuck with the peripheral and deep margins facing the cutting surface 3. The tissue is frozen to −20°C to −30°C using the cryostat 4. Thin sections (4–8 microns) are cut from the block face 5. Sections are mounted on glass slides and stained (H&E or toluidine blue) The result: a horizontal section that displays the entire peripheral margin and the deep margin on a single slide. This is the fundamental advantage of Mohs over bread-loaf histology.

The Mohs surgeon personally examines every slide under the microscope, scanning the entire margin for residual tumor. Any tumor identified is precisely mapped onto the tissue map, indicating its exact location and extent. If the margins are clear (no residual tumor identified on any section): the procedure proceeds to reconstruction. If residual tumor is identified: an additional stage is taken, targeting ONLY the areas where tumor was found on the map. This selective re-excision is what makes Mohs tissue-sparing. Clear areas are not re-excised.

Standard histologic processing (bread-loaf technique) cuts the excised specimen into vertical cross-sections at 2–3mm intervals. Each cross-section is examined on a glass slide. The critical limitation: each section is approximately 4–6 microns thick, and sections are spaced 2–3mm apart. This means that only a tiny fraction. Approximately 0.1%. Of the true surgical margin is actually examined. Tumor can exist between the sections and remain undetected. This is sampling error, and it is the fundamental weakness of bread-loaf histology for margin assessment.

Mohs micrographic surgery uses complete circumferential peripheral and deep margin assessment (CCPDMA). The tissue is oriented with the beveled edges flattened and sectioned horizontally, allowing the ENTIRE peripheral margin and the ENTIRE deep margin to be visualized on a single section. This means 100% of the surgical margin is examined, compared to 0.1% with bread-loaf. There is no sampling gap and no possibility of missing tumor between sections.

The evidence base for Mohs cure rates is strong, with large prospective and retrospective series demonstrating consistently superior outcomes compared to standard excision.

The AUC evaluates clinical scenarios based on: • Anatomic location (Area H, M, or L) • Tumor characteristics (size, subtype, recurrence status, perineural invasion) • Patient factors (immunosuppression) Each scenario is rated on a 1–9 scale: 7–9 = Appropriate, 4–6 = Uncertain, 1–3 = Inappropriate. Area H: central face, eyelids, eyebrows, periorbital, nose, lips, chin, mandible, preauricular/postauricular, temple, ear, genitalia, hands, feet, ankles, nail unit. Area M: cheeks, forehead, scalp, neck, jawline, pretibial. Area L: trunk, extremities (excluding pretibial, hands, feet).

The AUC provides specific appropriateness ratings for common clinical scenarios:

For every Mohs case, the medical record should document: • Tumor location with specific Area designation (H, M, or L) • Tumor size (measured with ruler) • Clinical description (borders, fixation) • Histopathologic subtype from biopsy • Primary vs. recurrent status • Immunosuppression status • Prior treatment history at the site • Perineural invasion if present on biopsy This documentation ensures compliance with AUC criteria and provides the clinical rationale for Mohs selection.