Residency · Residency · Dermatology
Mohs Micrographic Surgery: Principles and Indications
Introduction
Mohs micrographic surgery (MMS) is a precise surgical technique for the removal of skin cancer that provides the highest cure rate while maximizing tissue conservation. Developed by Dr. Frederic Mohs in the 1930s and refined to the fresh-tissue technique by Dr. Perry Robins and others, MMS allows complete microscopic margin assessment through systematic horizontal (en face) frozen sectioning of excised tissue.
Fundamental Principles
The Mohs Concept
The defining feature of Mohs surgery is complete circumferential and deep margin assessment (CCPDMA), meaning that 100% of the surgical margin is examined microscopically. By contrast, standard bread-loaf histologic sectioning examines only 0.1% of the true surgical margin, leaving the vast majority unexamined. The surgeon serves as both the surgeon and pathologist, enabling real-time correlation of histology with the surgical wound. Tissue is mapped and color-coded so that any residual tumor can be precisely located and selectively re-excised.
Tissue Processing
The current standard is the fresh-tissue technique, in which tissue is excised, mapped, color-coded with dyes, and processed as frozen sections. Sections are cut horizontally (en face) from the deep margin upward, allowing the entire peripheral and deep margin to be viewed on a single slide. Staining is typically with H&E or toluidine blue for frozen sections. Immunohistochemistry (MART-1/Melan-A, SOX-10, cytokeratin) can be applied to frozen or rush permanent sections for difficult tumors where standard staining may be insufficient.
<image>Step-by-step diagram of the Mohs micrographic surgery process showing tumor debulking, first stage excision with 1-2mm margins, tissue mapping with color-coding, cryostat sectioning in the en face plane, microscopic examination, and selective re-excision of residual tumor</image>
Indications for Mohs Surgery
Appropriate Use Criteria (AAC)
The joint guidelines from the AAD, ACMS, ASDSA, and ASMS define appropriate use based on tumor characteristics and anatomic location.
Tumor-Related Factors
MMS is indicated for high-risk histologic subtypes including morpheaform/sclerosing BCC, infiltrative BCC, micronodular BCC, desmoplastic SCC, and tumors with perineural invasion. Recurrent tumors previously treated by any method and incompletely excised tumors with positive margins on prior excision are strong indications. Large tumors (diameter greater than 2 cm on trunk/extremities or greater than 1 cm on head/neck) and tumors with poorly defined clinical borders also warrant MMS.
Patient-Related Factors
Immunosuppressed patients including organ transplant recipients, those with CLL, and HIV-positive individuals benefit from the highest cure rates MMS provides. Patients with genetic syndromes such as basal cell nevus syndrome (Gorlin) or xeroderma pigmentosum, and those with prior radiation to the site, are also appropriate candidates.
Anatomic Considerations
The H-zone of the face carries the highest appropriateness for MMS, encompassing the periorbital, nasal, periauricular, perioral, and temple regions. Digits, genitalia, and the pretibial region are sites where tissue-sparing is critical. The ears and lips are high-risk sites with limited reconstructive options.
Indications by Tumor Type
Basal Cell Carcinoma
BCC is the most common indication for MMS. The 5-year cure rate is 99% for primary BCC and 94 to 95% for recurrent BCC, compared to standard excision cure rates of approximately 95% for primary and approximately 80% for recurrent tumors. MMS is particularly valuable for aggressive subtypes such as morpheaform, infiltrative, and micronodular BCC.
Squamous Cell Carcinoma
MMS is appropriate for high-risk SCC with perineural invasion, poor differentiation, size greater than 2 cm, or in immunosuppressed patients. The 5-year cure rate is 97% for primary SCC. Desmoplastic SCC may require immunostains (AE1/AE3) on frozen sections due to its subtle histologic features.
Melanoma In Situ and Lentigo Maligna
For melanoma in situ, a "slow Mohs" or staged excision approach uses rush permanent sections and immunostains (MART-1, SOX-10). This is particularly useful for lentigo maligna on the face, where standard 5 to 10 mm margins would be disfiguring. Standard frozen section assessment of melanocyte density is less reliable, so rush permanent sections are preferred.
Other Tumors
Dermatofibrosarcoma protuberans (DFSP) is an ideal MMS indication due to its tentacle-like subclinical extensions, with MMS achieving a 99% cure rate versus 80% with wide local excision. Microcystic adnexal carcinoma exhibits aggressive local behavior with perineural invasion. Extramammary Paget disease often has ill-defined subclinical extension. Some centers use MMS with immunostains (CK20) for Merkel cell carcinoma.
| Tumor | Mohs Cure Rate | Standard Excision Cure Rate | Special Considerations |
|---|---|---|---|
| BCC (primary) | 99% | ~95% | Most common MMS indication |
| BCC (recurrent) | 94–95% | ~80% | Strong MMS indication |
| SCC (primary, high-risk) | 97% | Variable | May need IHC for desmoplastic variants |
| Melanoma in situ/LM | High (staged) | Variable | "Slow Mohs" with MART-1/SOX-10 |
| DFSP | ~99% | ~80% | Extensive subclinical extension |
| MAC | >95% | 40–60% (high recurrence) | Deep PNI common |
| Sebaceous carcinoma | High | Variable | Map biopsies for pagetoid spread |
<image>Anatomical face diagram showing the H-zone (periorbital, nasal, periauricular, perioral, temple regions) highlighted in red indicating highest-risk zones where Mohs surgery is most strongly indicated</image>
Surgical Technique
Step-by-Step Process
The procedure begins with clinical assessment and marking to identify visible tumor and mark clinical margins. Debulking with a curette defines the clinical extent of the tumor (the "saucerization" curette step). The first stage excision removes tissue with 1 to 2 mm margins at a 45-degree angle to facilitate flat tissue processing. Scoring and mapping involves marking the tissue with orientation marks, applying color dyes, and creating a precise tissue map. Cryostat processing embeds, freezes, and cuts the tissue in en face sections at 4 to 6 micrometers. During microscopic examination, the surgeon-pathologist evaluates all margins and precisely locates any residual tumor on the map. Selective re-excision removes only the area with residual tumor, and this cycle repeats until margins are clear. Once tumor-free, the wound undergoes reconstruction.
Reconstruction Options
Secondary intention healing is ideal for concave surfaces such as the medial canthus, temple, and conchal bowl. Primary linear closure is used where tissue laxity permits. Local flaps including rotation, advancement, and transposition flaps (rhombic, bilobed, nasolabial) offer excellent cosmetic results. Full-thickness skin grafts from preauricular or postauricular donor sites provide good tissue matching. Delayed reconstruction may be appropriate when perineural invasion requires adjuvant radiation.
Limitations and Contraindications
MMS is not indicated for routine low-risk tumors in non-critical locations where standard excision is appropriate. For melanoma, standard frozen sections are unreliable for melanocyte assessment, requiring modified techniques. Tumors with discontinuous growth such as sebaceous carcinoma skip lesions may not be fully mapped by the technique. The cost and time demands of MMS are substantial, and appropriate use criteria ensure cost-effectiveness.
<image>Microscopic image of a Mohs frozen section showing a tongue of infiltrative basal cell carcinoma extending into the deep dermis at the surgical margin, highlighted with toluidine blue staining</image>
Key Clinical Pearls
MMS examines 100% of the surgical margin compared to less than 1% with standard bread-loaf sectioning, a difference that fundamentally explains its superior cure rates. The H-zone of the face represents the highest-risk anatomic sites where MMS is most strongly indicated. For recurrent BCC, MMS achieves a 94 to 95% cure rate compared to approximately 80% with re-excision. DFSP is an ideal MMS indication due to its tentacle-like subclinical extensions, with cure rates approaching 99%. "Slow Mohs" with rush permanent sections and MART-1 immunostain is the preferred approach for lentigo maligna and melanoma in situ.
References
- Tolkachjov SN, Brodland DG, Coldiron BM, et al. Understanding Mohs micrographic surgery: a review and practical guide for the nondermatologist. Mayo Clin Proc. 2017;92(8):1261-1271.
- Ad Hoc Task Force, Connolly SM, Baker DR, et al. AAD/ACMS/ASDSA/ASMS 2012 appropriate use criteria for Mohs micrographic surgery. J Am Acad Dermatol. 2012;67(4):531-550.
- Rowe DE, Carroll RJ, Day CL. Mohs surgery is the treatment of choice for recurrent (previously treated) basal cell carcinoma. J Dermatol Surg Oncol. 1989;15(4):424-431.
- Etzkorn JR, Sobanko JF, Elenitsas R, et al. Low recurrence rates for in situ and invasive melanomas using Mohs micrographic surgery with immunostains. J Am Acad Dermatol. 2015;72(6):1066-1072.
