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Mohs Surgery for Skin Cancer

Mohs surgery is named after its developer, Dr. Frederic E. Mohs. In the 1930’s, Dr. Mohs developed a method that “fixed” (preserved) and stained tissue directly on the patient. A chemical paste was applied to the tumor and was allowed to penetrate to a few millimeters depth. This required 6-24 hours, after which, the area was surgically removed and the entire peripheral and deep margin was examined for residual tumor. This process was repeated over days until all margins were found to be free of tumor. Defects resulting from Mohs surgery were generally allowed to granulate (fill-in) on their own and not sewn closed. This method had the disadvantages of being painful and time consuming (only one “stage” could be done per day) and the granulated wound often produced an ugly scar. Today, this technique has been modified; no chemicals are applied directly to the patient, multiple stages can be performed in hours, and the defect can be closed immediately if desired. The original concept of examining the entire peripheral and deep margin of the excised specimen remains.Background:When a routine excision is done, the specimen is processed by “breadloafing.” Like a loaf of bread is sliced, sections are taken vertically through the specimen at 3-4mm intervals and these sections are examined under the microscope. For practical reasons, only a few sections or “slices” are examined. Most skin cancers grow like the roots of a tree with extension of the tumor to the sides and downward. With breadloafing, tumor roots can be missed if they happen to grow in the 3-4mm gap between sections. In fact, less than 1% of the true peripheral and deep margin is examined by this method. This limitation is addressed by Mohs surgery.Technique:It is beyond the scope of this article to explain the Mohs procedure precisely step-by-step. However, the following are critical points that lead to the examination of theoretically 100% of the peripheral and deep surgical margin and conservation of surrounding normal tissue. After a pre-operative consultation has been completed and informed consent obtained:- The lesion is numbed with local anesthesia.- Tumor that can be seen or felt is removed with a curette or scalpel.- A thin, saucer-like layer of tissue is excised just a few millimeters around and underneath the resulting defect.- This layer is divided into pieces and inked. - A map is drawn that shows each piece in relation to the patient. - Each piece is frozen, mounted in a waxy substance and very thinly sectioned horizontally starting from the underside of the specimen. This is done by the Mohs histotechnician.- The slides are processed and examined by the Mohs surgeon. If tumor roots are seen on the slide, they extend beyond the underside of the tissue that was excised, and therefore, more tumor remains in the patient. The tumor site on the slide is matched to the previously drawn map and the precise location on the patient is identified. The process is then repeated and another saucer-like layer is removed, but only from the area that showed residual tumor. In this way, surrounding normal tissue is conserved.When no residual tumor is seen, the tumor is deemed completely removed and the defect can be repaired if desired. Often, the Mohs surgeon can repair the defect immediately. Alternatively, when the defect is more complex, closure can be coordinated with other physicians who specialize in reconstructive surgery such as plastic surgeons, facial plastic surgeons, otolaryngologists, or ophthalmologists. Cancers treatable by Mohs surgery:Mohs surgery has been used to excise many different types of skin cancer. However, BCCs and SCCs, the two most common forms of skin cancer, are the cancers most often treated by Mohs surgery. The treatment of melanoma by Mohs surgery is controversial at this time.Indications:Mohs surgery is a specialized technique that is not indicated for the treatment of every BCC or SCC and is most appropriately used under certain, well-defined circumstances. The majority of BCCs and SCCs can be treated with very high cure rates by standard methods including electrodessication and curettage (ED&C), local excision, cryosurgery (freezing) and irradiation. These are fairly quick and easy and can often be performed in the local physician’s office. Large or recurrent lesions or those that arise at sites where recurrence is more likely or where tissue conservation is important are most aptly treated with Mohs surgery. This includes many areas of the face. Training:The Mohs surgeon must be proficient in all aspects of the procedure including anesthesia, anatomy, surgery, pathology and reconstruction. Knowledge of these and other aspects of the procedure and most importantly, surgical judgement, are usually not acquired during the course of residency training and often require additional specialized fellowship training. Summary:Mohs surgery is a specialized surgical technique that results in high cure rates when used to remove skin cancers such as BCCs and SCCs. It is especially useful in lesions that, due to their size, location or other factors are at higher risk of recurrence if treated by standard methods or arise in areas where tissue conservation is important. It is not indicated for use in all cancers of the skin. Mohs surgery is labor intensive and requires a large support staff and specialized training in the technique.

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