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Mohs procedure
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Mohs surgery is performed 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 hematoxylin and eosin (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, cut on the cryostat and placed on 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 essential 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 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 stains and labels the sections for a microscoping exam. The sections can be processed in one piece (using relaxing incisions at multiple points, or hemisectioned like a "Pac-Man" 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 prevent errors introduced by soft hard to handle tissue, or from accidental dropping or mislabeling of specimen. Multiple sectioning prevent compression artifacts, separation of tissue, and other logistical problems with handling large thin sheets of frozen skin.
Some physicians believe that frozen section histology is the same as Mohs micrographic surgery, and 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 those 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 will become. False negatives occur when a pathologist reads cancer excision as "free of residual carcinoma", even though cancer might be present in the wound and missed because of the random sampling. In reality, most pathology labs examine only 3 to 8 sections of the "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 since tissue processing is very difficult in practice. The alternative to Mohs surgery is when a pathologist requests the processing to be done by "cutting through the block". Again, this method approaches that of Mohs surgery, but still is not as good. Cutting through the block will result in the random discarding of many slices, but does greatly decrease the incidence of "false negative" reports. Dr. Mohs perfected a simple and efficient way to "flatten" and examine the entire surgical margin.
History
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Originally, Dr. Mohs used an escharotic agent made of zinc chloride and bloodroot (contains the alkaloid sanguinarine that is derived from the root of the plant Sanguinaria canadensis). This paste is very similar to "Hoxsey's paste". Harry Hoxsey, a lay cancer specialist was developing a herbal tonic and paste designed to treat internal and external cancers. Hoxsey recommended applying paste to the affected area and within days to weeks, the area would necrose (cell death), separate from surrounding tissue and fall out. Dr. Mohs applied a very similar paste after experimenting with a number of compounds to the wound of his skin cancer patients. They were to leave the paste on the wound overnight, and the following day, the skin cancer and surrounding skin would be anesthetic, and ready to be removed. The specimen was then excised, and the tissue examined under the microscope. If cancer remains, more paste was then applied, and the patient would return the following day. Later, frozen section histopathology and local anesthetic allowed the procedure to be performed the same day, with less tissue destruction, and similar cure rate. The term "chemosurgery" remains today, and is used synonymously with Mohs micrographic surgery.
Future applications of Mohs surgery
Mohs surgery can be applied to any relatively non-aggressive locally invasive tumors with contiguous growth pattern (i.e. no skipped growth, or metastasis). Today, most Mohs procedures are performed by dermatologists. However, pathologists, plastic surgeons, and otolaryngologists have been trained and are ulitizing Mohs surgery in their practice as well.
Associations
The American Society for Dermatologic Surgery founded in 1970 is the largest organization of board certified dermasurgeons with over 5000 members who perform dermatologic surgeries including Mohs micrographic surgery.
The American Society of Mohs Surgery is the largest organization of Mohs surgeons in the United States with over 850 members.
The American College of Mohs Surgery is the organization that sets standards of care for fellowship trained Mohs surgeons.
The American Board of Medical Subspecialities is in the process of reviewing Mohs micrographic surgery as a separate subspecialty. Mohs surgery has not been recognized as a separate subspeciality and there is no certifying exam or board for Mohs Surgery at this time.
The American Academy of Dermatology is the largest organization of board certified dermatologists, many of whom perform dermatologic and Mohs micrographic surgery. With a membership of over 15,000, it represents virtually all practicing dermatologists in the United States and Canada and has specific member information regarding those performing Mohs micrographic surgery.
The Association of Academic Dermatologic Surgeons has board certified dermasurgeon professors who have faculty appointments at major teaching hospitals and universities and are engaged in training medical students and residents in the practice of dermatologic surgery and Mohs micrographic surgery.
References
- Bowen G, White G, Gerwels J (2005). "Mohs micrographic surgery.". Am Fam Physician 72 (5): 845–8. PMID 16156344.Full text
External links
- American cancer society statement about Mohs surgery
- American Society of Mohs Surgery
- American College of Mohs Surgery
- "Mohs math – where the error hides"
- American Melanoma Foundation information
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