Mohs surgery


Mohs surgery, developed in 1938 by general surgeon Frederic E. Mohs, is microscopically controlled surgery used to treat both common and rare types of skin cancer. During the surgery, after each removal of tissue and while the patient waits, the tissue is examined for cancer cells. That examination dictates the decision for additional tissue removal. Mohs surgery is the gold standard method for obtaining complete margin control during removal of a skin cancer. This method allows for the removal of skin cancer with a very narrow surgical margin and a high cure rate.
The cure rate with Mohs surgery cited by most studies is between 97% and 99.8% for primary basal-cell carcinoma, the most common type of skin cancer. Mohs procedure is also used for squamous cell carcinoma, but with a lower cure rate. Recurrent basal-cell cancer has a lower cure rate with Mohs surgery, more in the range of 94%. It has been used in the removal of melanoma-in-situ, and certain types of melanoma.
Other indications for Mohs surgery include dermatofibrosarcoma protuberans, keratoacanthoma, spindle cell tumors, sebaceous carcinomas, microcystic adnexal carcinoma, merkel cell carcinoma, Paget's disease of the breast, atypical fibroxanthoma, and leiomyosarcoma. Because the Mohs procedure is micrographically controlled, it provides precise removal of the cancerous tissue, while healthy tissue is spared. Mohs surgery can also be more cost effective than other surgical methods, when considering the cost of surgical removal and separate histopathological analysis. However, Mohs surgery should be reserved for the treatment of skin cancers in anatomic areas where tissue preservation is of utmost importance.

Uses

Skin cancer can be categorized into two groups: melanoma, which is considered more severe, and nonmelanoma skin cancer which includes basal cell carcinoma and cutaneous squamous cell carcinoma.
Mohs micrographic surgery is used for high-risk nonmelanoma skin cancers located in cosmetically critical or sensitive areas like the face, ears, scalp, neck, genitalia, hands and feet where tissue conservation is of utmost importance. It is also indicated when the tumor is recurrent, aggressive, large, or painful which tells us there is invasion of the nerve or vasculature.
Some cases of melanoma, such as early, surface-level melanoma or thin invasive melanoma, can be treated with Mohs surgery. This is especially considered in areas where tissue sparing is essential. In these cases special immunohistochemical staining is used to visualize the melanoma cells, evaluate the margins, and ensure the cancer has been completely removed. More evidence today is linking Mohs surgery with lower recurrence rates of melanoma in these cases. 
This approach is also used in treating rare skin cancers. For example, dermatofibrosarcoma protuberans, a slow growing cancer that begins in the deeper layers of the skin, as well as cancers arising from hair follicles, oil glands or sweat glands would benefit from Mohs surgery as these are cases where margin clearance is essential.
In summary, the Mohs micrographic surgery criteria are as follows:
  • Recurrent or high risk basal cell and cutaneous squamous cell carcinomas in anatomically or cosmetically sensitive areas.
  • Specific cases of early stage or surface level melanoma, and thin invasive melanoma in similarly sensitive sites.
  • Other rare skin cancers where all the margins need to be visualized.

    Contraindications

Mohs surgery is generally contraindicated when the criteria summarized in the “Uses” section above are not met. For example, the tumor is small, low-risk, has well-defined margins, and is in a non-critical area.
The standard protocol for Mohs surgery requires the surgeon to both remove the tissue and interpret the pathology. The procedure is not considered Mohs surgery if the removed tissue is sent and read by a pathologist instead of the surgeon performing the procedure. In this case, it is considered a standard excision and should be documented as such. Another doctor interpreting histopathology is incompatible with Mohs surgery.
Relative contraindications include instances where the risks of surgery outweigh the benefits such as in patients with co-morbidities, in cases where the defect caused by surgery would need complex reconstruction beyond the scope of the surgery, or when patient factors such as severe bleeding predispositions or being unable to tolerate local anesthesia would affect the prognosis.
Increased postoperative complications are associated with immunosuppressed patients and elderly patients. These are not absolute contraindications, but the risks of this procedure should be weighed against the benefits for each individual patient.

Risks and complications

The overall risk of Mohs micrographic surgery complications is very low. Reported adverse event rates are between 0.7% and 2.6% according to large multi-center prospective studies. Infection of the surgical site, hematoma formation, bleeding, and suboptimal wound repair with dehiscence or ischemic necrosis are the most reported adverse events. Complication rates leading to permanent damage or requiring hospitalization are below 0.1% and no deaths have resulted from this procedure.
Certain patient factors may make complications more likely. These include immunosuppression and use of anticoagulants or anti-platelets. Increased risk is not independently associated with older age. There are surgical characteristics which also may increase the risk of complications including cancer location, large tumors with depth, and tumors requiring advanced flap or graft repairs.

Specific risks/complications

  • Infection: About 0.9-2.5% of patients who undergo Mohs surgery will develop an infection. This is the most frequently documented adverse event with higher rates in patients who are immunosuppressed. To reduce this risk antiseptics and antibiotics may be used, but the absolute benefit is modest.
  • Impaired wound healing: Dehiscence and necrosis rates are approximately 1-2%. Risk factors include larger excisions, flap/graft use, and immunosuppressed patients. Complication rates decrease when the wound heals with primary closure or secondary intention.
  • Hematoma formation and bleeding: This is most commonly seen in patients on anti-platelet or anticoagulant therapy and occurs in 0.9-1.5% of cases. Despite this, patients are not routinely asked to stop these medications as most experience no complications and withdrawal is generally not justified.
  • Scarring: 5-7% of cases, especially larger tumors and complex repairs, result in hypertrophic scarring.
Less commonly seen is postoperative swelling, rash, and disturbance of skin sensation. Pain is generally mild.
Overall, Mohs surgery is generally safe with risks/complications being minor and manageable.

Technique

In 2012, the American Academy of Dermatology published appropriate use criteria on Mohs micrographic surgery in collaboration with the following organizations: American College of Mohs Surgery; American Society for Mohs Surgery; and the American Society for Dermatologic Surgery Association. More than 75 physicians contributed to the development of the Mohs surgery AUC, which were published in the Journal of the American Academy of Dermatology and .
The Australasian College of Dermatologists, in concert with the Australian Mohs Surgery Committee, has also developed evidence based guidelines for Mohs Surgery.
The Mohs procedure is a pathology sectioning method that allows for the complete examination of the surgical margin. It is different from the standard bread loafing technique of sectioning, where random samples of the surgical margin are examined.
Mohs surgery is performed in four steps:
  • Surgical removal of tissue
  • Mapping the piece of tissue, freezing and cutting the tissue between 5 and 10 micrometres using a cryostat, and staining with hematoxylin and eosin or other stains
  • Interpretation of microscope slides
  • Possible reconstruction of the surgical defect
The procedure is usually performed in a physician's office under local anesthetic. A small scalpel is utilized to cut around the visible tumor. Unlike a normal surgical excision, a Mohs surgery cut is performed at a beveling between 10 and 45 degrees to allow visibility of all skin layers during pathological diagnosis. A very small surgical margin is utilized, usually with 1 to 1.5 mm of "free margin" or uninvolved skin. The amount of free margin removed is much less than the usual 4 to 6 mm required for the standard excision of skin cancers. 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/pathologist who examines the sections for cancerous cells. If cancer is found, its location is marked on the map and the surgeon removes the indicated cancerous tissue from the patient. This procedure is repeated until no further cancer is found. The vast majority of cases are then reconstructed by the Mohs surgeon. Some surgeons utilize 100 micrometres between each section, and some utilize 200 micrometres between the first two sections, and 100 micrometres between subsequent sections.

Blood thinners

The trend in skin surgery over the last 10 years has been to continue anticoagulants while performing skin surgery. Most cutaneous bleeding can be controlled with electrocautery, especially bipolar forceps. The benefit gained by ease of hemostasis is weighed against the risk of stopping anticoagulants; and it is generally preferred to continue anticoagulants.

Recovery

A majority of patients generally report feeling minimal discomfort after Mohs surgery with the most discomfort being reported the day of surgery and day after. After the first two days, the pain steadily reduces over the next week. The surgery is overall well-tolerated and has a fast recovery. Many patients don’t require analgesics or pain medication, but of those who do, acetaminophen is usually enough to manage pain. Only a few go on to need a prescription pain medication. When multiple areas are operated on at once or when the scalp is involved, pain tends to be increased.
Gentle cleansing and coverage of the wound with antibiotic ointment or petrolatum is recommended until sutures are removed in instances of primary closure or local flap repairs. Depending on varying factors such as tension of the wound, anatomic site, and personal patient factors, this usually occurs 5-14 days after the operation. On the other hand, immobilization and non-adherent dressings may be required for skin grafts. This is more intensive postoperative care, however better outcomes in select locations, like on the nose or foot, are associated with delayed grafting.
For certain areas of the body, like the concave surface of the ear, secondary intention healing may be the best option for healing as it is associated with good cosmetic outcomes and lower complications in the appropriate cases. SIH involves leaving the wound to heal naturally, by itself, with no intervention. The wound heals while the patient monitors for healthy tissue formation and keeps the area clean with dressing changes.
To reduce the risk of bleeding patients are generally advised to not carry over 10 pounds of weight and avoid excessive physical activity for 2-3 days after the procedure. Minor complications rarely occur and patients are usually able to continue their normal activity within days of the operation.