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Mohs Surgery, created by a general surgeon, Dr. Fredrick E. Mohs, is microscopically controlled surgery that is highly effective for common types of skin cancer, with a cure rate cited between 97 to 99% for basal cell carcinoma, the most common type of skin cancer, and for squamous cell carcinoma. It has been used in the removal of melanoma-in-situ, but only by a few physicians. Because the Mohs procedure is micrographically controlled, it provides precise removal of the cancerous tissue, while healthy tissue is spared. For this reason, Mohs surgery results in a significantly smaller surgical defect and an improved cosmetic result compared to other surgical methods of skin cancer treatment.
The Mohs procedure is recommended for skin cancer removal in anatomic areas where maximum preservation of healthy tissue is desired for cosmetic and functional purposes (the face, eyelids, nose, ear, fingers, genital area), for cancers with indistinct margins, and for recurrent cancers in scar tissue. It is especially indicated for lesions that have recurred following prior treatment, or for lesions in anatomic areas that have the greatest likelihood of recurrence (eg., the side of the nose). Mohs surgery is relatively expensive when compared to other surgical modalities alone, but has been shown to be less expensive compared with other modalities for aggressive tumors or tumors in high risk locations due to the inherent high risk of recurrence in these tumors and potential future associated costs. For this reason, it is used generally for recurrent tumors, indistinct tumors, or tumors in areas such as the face, where sparing normal tissue around the skin cancer is paramount.
Mohs procedure
Mohs surgery is performed by a Mohs skin cancer surgeon in four steps:
- Surgical removal of tissue (Surgical Oncology)
- Mapping the piece of tissue, freezing and cutting the tissue between 5 and 10 micrometres using a cryostat, and staining with H&E or other stains (including T. Blue)
- Interpretation of microscope slides (Pathology)
- Reconstruction of the surgical defect (Reconstructive Surgery)
After each surgical removal of tissue, the specimen is processed (look here for a clay animation of tissue processing), cut on the cryostat and placed on a slides, stained with H&E and then read by the Mohs surgeon who examines the sections for cancerous cells. If cancer is found, its location is marked on the map (drawing of the tissue) and the surgeon removes the indicated cancerous tissue from the patient. This procedure is repeated until no further cancer is found.
The mapping, combined with the unique "smashing the pie pan" method of processing, is the essance of Mohs surgery. If one imagines an aluminum pie pan as the blood covered surgical margin, and the top of the pie is the crust covered surface of the skin - the surgeons goal is to flatten the aluminum pie pan into one flat sheet, mark it, stain it, and examine it under the microscope. The mapping is simply how one stain and label the sections for microscoping exam. The sections can be processed in 1 piece (using relaxing incisions at multiple points, or hemisectioned like a "pacman" figure), cut in halves, cut in quarters, or cut in multiple pieces. Single piece processing is acceptable for small cancers, and multiple piece sectioning facilitate processing and prevent artifacts. Single piece sectioning prevents errors introduced by soft, difficult to handle tissue, accidental dropping, or mislabeling of the specimen. Multiple sectioning prevents compression artifact, separation of tissue, and other logistical problems associated with handling a large thin sheet of frozen skin.
Some physicians believe that frozen section histology is the same as Mohs micrographic surgery, yet it is not. Standard histology processing is a random tissue sampling technique, examining less than 10% of the total surgical margin (imagine pulling 5 slices of bread out of a whole loaf of sliced bread and examining only 5 slices to visualize the whole loaf). In Mohs processing, the entire surgical margin is examined (imagine one who examined the entire outside crust of the same loaf of bread). In statistical terms, the more slices of bread one examines, the lower the "false negative" rate. "False negatives" occur when a pathologist reads a cancer excision as "free of residual carcinoma", yet cancer might continue to be present in the wound. Therfore, the cancer is missed because of the random sampling. In reality, most pathology lab examine only 3 to 8 sections of this "loaf" in their margin determination. While a diligent pathologist can cut and process a standard excision to get the same margin control as Mohs surgery, it is seldom done, as tissue processing is very difficult in practice. The alternative to Mohs surgery is when a pathologist request the processing to be done by "cutting through the block". This method approaches the accuracy of Mohs surgery, but still falls short. Dr. Mohs perfected a simple and efficient way to "flatten" and examine the entire surgical margin.