Mohs Micrographic Surgery

Anatomy and Physiology

The tissue-sparing properties of Mohs micrographic surgery make it particularly useful in areas of functional and aesthetic importance such as the head and neck area, anogenital area, hands, and feet.

Indications

Mohs surgery is appropriate for skin cancers with a high risk of recurrence and when tissue conservation is essential. The Mohs Appropriate Use Criteria (AUC) guidelines were developed to assist clinicians in determining if a specific tumor would be appropriately managed by Mohs surgery. A Mohs AUC mobile phone app is available for download to mobile devices. These criteria were based on areas of the body, patient characteristics, and tumor characteristics.[5]

Mohs surgery is particularly suitable for areas of the body in the "H" area:

• Central face, eyelids/canthi, eyebrows, nose, lips, chin, ear, and periauricular area
• Genitalia
• Hands, feet, ankles, and nail units
• Nipples/areola

Higher-risk patient characteristics include:

• Immunocompromised
• Genetic syndromes (basal cell nevus syndrome, xeroderma pigmentosum)
• Prior radiated skin
• Patient with history of high-risk tumors

Tumor characteristics include:

• Positive margin on recent excision

Aggressive features that are high risk for recurrence of BCC:

• Aggressive histologic subtype: morpheaform, infiltrating, micronodular
• Perineural involvement
• Metatypical/keratotic

Aggressive features of SCC:

• Poorly or undifferentiated (characterized by a high degree of nuclear polymorphism, high mitotic rate, or low degree of keratinization)
• Perineural/perivascular
• Spindle cell
• Breslow depth 2 mm or greater
• Clark level IV or greater

 While the Mohs AUC can be helpful in determining if a specific lesion is appropriately managed with Mohs surgery, it does not exclude the validity of alternate modalities in treating the same lesion (e.g. curettage, electrodesiccation & curettage, or excision).  [4][5]

Contraindications

There are no absolute contraindications to Mohs surgery in patients deemed suitable for surgery in general.

Equipment

Mohs micrographic surgery requires equipment for the operating room as well as for the lab in which tissue is processed and examined microscopically. The operating room requires good lighting and an adjustable table to provide optimal visualization and access to the tumor. Surgical equipment is relatively simple, consisting of a scalpel, fine forceps, scissors, gauze, and an electrosurgical device for coagulation. Reconstruction can be achieved with an expanded tray that includes needle holders, scissors, fine forceps, skin hooks, and a scalpel.

The Mohs histology laboratory consists of microtomes that freeze tissue and then allow cutting of very thin slices of tissue to mount on glass slides. The slides are then placed in an automated stainer or may be stained by hand. This process may require a vent hood to minimize exposure to chemicals involved in the staining process. Completed slides are then read by the Mohs surgeon under light microscopy to determine if tumor remains in the tissue. Many Mohs labs also have special stainers and reagents to allow immunohistochemical staining of tissue.

Personnel

The procedure requires the surgeon and at least one assistant in the surgical suite.  In addition, at least one histotechnician is needed in the Mohs laboratory for tissue processing.

Technique or Treatment

The technique of Mohs surgery is as follows:

1. The tumor is first outlined prior to injection with a local anesthetic. After anesthetized, any visible tumor is removed or “debulked,” with a curette, flexible blade, or scalpel. 
2. Prior to removal, the tissue layer is carefully oriented by placing small superficial etch marks with a scalpel (often at 3 o’clock, 6 o’clock, 9 o’clock, and noon) around the tissue layer and corresponding in-situ skin. 
3. A thin margin of tissue is then removed circumferentially around and deep to the debulked tumor defect. This “layer” of tissue is removed with a beveled angle of approximately 45 degrees, which facilitates tissue processing (see below).
4. Once removed, the tissue layer is often cut into halves or quadrants and then marked with colored dyes to facilitate precise mapping of the tumor. The tissue is then pressed flat, so the epidermal edge occupies the same tissue plane as the deep margin. The “beveled” edge acquired tissue removal facilitates this flattening process.  
5. The tissue is then cut and processed in a horizontal direction so that virtually 100% of the peripheral and deep margin can be examined on the same tissue section under the microscope. This is in contrast to the traditional vertical, or “breadloafed,” tissue processing which examines only a small portion of the tumor margin.[6]
6. If residual tumor is identified under the microscope, then the Mohs map is marked and the corresponding in-situ tissue is precisely removed from the patient in that portion that was found to still have tumor. This process is repeated until the tumor is histologically negative, thus ensuring complete tumor removal with maximum conservation of healthy tissue.
7. Once the tumor has been removed, a variety of techniques are used to close the defect, including primary closure, flaps, grafts, and second intention healing. A recent tabulation of Mohs stages per case for experienced Mohs surgeons showed a median of about 1.7 stages per tumor to clear. Obviously, that number can be much higher for more complicated cases.[7]

Tissue stains most commonly used for Mohs surgery are hematoxylin and eosin (H&E) and toluidine blue. While the majority of Mohs surgeons use H&E routinely, a significant minority prefer toluidine blue for processing basal cell carcinoma, since mucopolysaccharides and hyaluronic acid that are associated with BCC stain metachromatically with a magenta coloration.[8][9][10]

The Mohs procedure depends upon the presence of continuous tumor growth (no "skip" areas) to be maximally effective. Fortunately, this characteristic is present in most cancers that occur on the skin.

Clinical Significance

Mohs surgery has had a high degree of clinical success.

• Mohs surgery reports excellent 5-year cure rates for non-melanoma skin cancers (NMSC), in particular basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). Examples of 5-year cure rates include:  Primary BCC (99%), recurrent BCC (94.4%), primary SCC (92-99%), and recurrent SCC (90%). [11][12][13]
• Mohs surgery also can be used to treat other less common tumors, including dermatofibrosarcoma protuberans, microcystic adnexal carcinoma, extramammary Paget disease, Merkel cell carcinoma, and sebaceous carcinoma.[14] More recently, with the availability of reliable immunohistochemical stains, Mohs micrographic has also shown great usefulness in treating some forms of malignant melanoma, including lentigo maligna, lentigo maligna melanoma, and thin melanomas. [15]

Enhancing Healthcare Team Outcomes

The procedure requires the operative surgeon and nurse to work together in the surgical suite. In addition, at least one histotechnician is needed in the Mohs laboratory for tissue processing. A coordinated team approach provides the best results for patient care. [Level V]