Mohs Micrographic Surgery
Mohs micrographic surgery (MMS) is one of the most advanced techniques in skin cancer and reconstructive surgery. It is designed to remove malignant skin cancers, such as basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), while preserving as much surrounding healthy tissue as possible.
Standard surgical excision checks only a random sample of the margin (<1%) using vertical "bread-loaf" sections. Mohs surgery, by contrast, allows 100% of the lateral and deep margins to be examined, using a mapped, orientated frozen-section system while the operation is still under way.
Mohs Surgery vs. Standard Excision
In standard skin cancer excision, the surgeon removes the lesion with an estimated safety margin (for example 4–6 mm), and the specimen is sent to the lab and sectioned vertically. Up to 99% of the margin is never examined, so tumors with complex hidden roots carry a high risk of recurrence.
How the Pathology Techniques Compare:
| Criterion | Standard Excision | Mohs Micrographic Surgery (MMS) |
|---|---|---|
| Margin examined | <1% (random tissue sections) | 100% of lateral and deep margins |
| Time to margin result | 3–7 days after surgery | 15–30 minutes, during the procedure |
| Cure rate (BCC/SCC) | 90–95% (primary), 80% (recurrent) | 99% (primary), 94–99% (recurrent) |
| Healthy tissue preserved | Low (wide safety margin required) | Maximal (only tissue containing tumor cells is removed) |
| Sectioning technique | Vertical slicing | Horizontal 360° en face sections |
When Mohs Surgery Is Recommended at Haute-Derma
Not every skin cancer needs Mohs surgery. At Haute-Derma, Mohs is recommended strictly according to the Appropriate Use Criteria (AUC) of the American College of Mohs Surgery (ACMS):
High-Risk Anatomical Sites (Area H):
- Area H (high risk): The face, especially the central face (nose, eyes, lips, ears, temples), hands, feet and genitals. These areas need maximum preservation of healthy tissue to avoid cosmetic deformity and loss of function.
- Area M (moderate risk): Earlobes, forehead, scalp, neck and shins.
High-Risk Tumor Characteristics:
- Tumors that have recurred after excision, radiotherapy or laser treatment.
- Large tumors (>2 cm on the trunk, >1 cm in area M, any size in area H).
- Aggressive histologic subtypes: sclerosing (morpheaform) and micronodular BCC, poorly differentiated SCC, and tumors with perineural invasion.
- Tumors with poorly defined clinical borders.
The Standard 5-Step Mohs Procedure
Mohs surgery is a closed-loop, stage-by-stage process of five linked steps in which the surgeon also acts as the pathologist:
Removing the First Layer (Surgical Excision): A 45° beveled incision is made around the visible edge of the tumor.
Mohs Tissue Mapping: The specimen is oriented (12, 3, 6 and 9 o'clock) and its margins are color-coded with dye.
Embedding and Frozen Sectioning: The margin is flattened, embedded and frozen in a cryostat at −20 °C, then cut into sections 4–6 µm thick.
Reading the Map Under the Microscope (Microscopic Evaluation): Any remaining tumor cells are pinpointed on the map.
Further Targeted Excision or Wound Closure (Clearance or Reconstruction)
- If tumor remains: another layer is taken from EXACTLY that location (stage 2).
- If the margins are clear (negative margin): the wound is repaired with a reconstructive closure or skin graft.
Tissue Mapping and Cryostat Processing
The success of Mohs depends on translating a 3D specimen into a 2D map with complete accuracy:
- 45-degree beveled edge: Cutting the edge at a 45° angle allows the deep and lateral margins to lie in a single horizontal plane when the specimen is flattened for embedding.
- Tissue inking: Water-insoluble tissue marking dyes are used to label anatomical orientation:
- Red: 12 to 3 o'clock.
- Blue: 3 to 6 o'clock.
- Green: 6 to 9 o'clock.
- Yellow: 9 to 12 o'clock.
- En face frozen sectioning: The underside of the specimen is pressed flat on a chuck, covered with OCT embedding medium and frozen at −20 to −25 °C in the cryostat. Sections just 4–6 µm thick are cut and stained with H&E.
Reconstruction After Clear Margins (Post-Mohs Reconstruction)
Only once the intraoperative sections confirm that all margins are completely free of cancer cells does the Haute-Derma doctor reconstruct the defect. The method depends on its size, depth and location:
- Secondary intention healing: For small, deep defects in concave areas (the concha of the ear, the inner corner of the eye), minimizing visible scarring.
- Primary closure: For small defects, aligned with the relaxed skin tension lines (RSTL).
- Full-thickness skin graft (FTSG): Used for larger defects on the eyelids or nasal ala that cannot be closed directly and lack enough surrounding tissue for a flap (secured with a tie-over bolster).
- Local flaps: Rotation, advancement and bilobed flaps are preferred because they preserve the color, thickness and texture of facial skin.
Clinical and Economic Benefits and Risk Management at Haute-Derma
- Avoiding repeat surgery: Confirming clear margins in a single session spares patients the repeat operations that can follow when delayed pathology reports positive margins.
- Best cosmetic outcome: Because every millimeter of healthy skin is preserved, scars are noticeably smaller than after conventional surgery, protecting the movement of the eyes, nose and mouth.
- Very low recurrence rate: The 5-year recurrence rate for primary BCC after Mohs surgery is <1%, compared with 5–10% after standard excision.
