Mohs micrographic surgery is a tissue-sparing, precise method of skin cancer removal named in honor of the surgeon who developed the technique, Frederick Mohs. It is a surgical approach that offers high cure rates for the treatment of a variety of skin cancers, including basal cell carcinomas (BCC) and squamous cell carcinomas (SCC). The main advantage of Mohs surgery is that it offers precise microscopic control of the entire tumor margin while maximizing conservation of healthy tissue.
This technique was developed by Dr. Mohs in the 1930’s. The procedure was originally named “chemosurgery,” since the technique involved the application of a chemical fixative (zinc chloride) to the in-situ tumor. After 24 hours of in-situ fixation, the tumor was excised and microscopically examined. The process was repeated until the tumor was completely removed. Over the following decades, Mohs surgery shifted away from using zinc chloride fixation in favor of processing fresh tissue that was frozen and sectioned in a cryostat microtome. This technique offered several advantages compared to the original chemosurgery technique, including faster processing times (15 to 30 minutes), decreased patient discomfort, and increased tissue conservation.
Mohs surgery is appropriate for skin cancers with a high risk of recurrence and when tissue conservation is essential. It is performed by removing a thin margin of tissue circumferentially around and deep to the clinical margins of a skin tumor. The specimen is typically removed with a 45-degree bevel to facilitate tissue processing. It is then rapidly frozen and sectioned in a cryostat microtome, allowing for quick tissue processing (about 15 to 30 minutes). Sectioning the tissue in a horizontal direction allows virtually 100% of the tissue margin (peripheral and deep margins) to be examined under the microscope. The process is repeated until the tumor has negative histologic margins.
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.
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.
Mohs surgery is particularly suitable for areas of the body in the "H" area:
Higher-risk patient characteristics include:
Tumor characteristics include:
Aggressive features that are high risk for recurrence of BCC:
Aggressive features of SCC:
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).
There are no absolute contraindications to Mohs surgery in patients deemed suitable for surgery in general.
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.
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.
The technique of Mohs surgery is as follows:
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.
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.
Mohs surgery has had a high degree of clinical success.