Mohs micrographic surgery has proven itself the standard of care for specific skin cancer tumour removal. It was not always so. An overview of the history of this targeted procedure as well as its technique and indications help elucidate the features that set it apart from other excision methods.

Over the last 30 years, more people have been diagnosed with skin cancer than all other cancers combined1. Skin cancer is also the most common cancer in the United States with an excess of 3.5 million cases diagnosed each year in over two million people2. With these rates skyrocketing annually, focusing on the most effective types of treatment is of utmost importance. While there are multiple treatment options for different types of skin cancers, Mohs micrographic surgery (MMS) has proven itself a uniquely precise and beneficial procedure for the treatment of skin cancer that allows the surgeon to remove the totality of a tumour while preserving as much of the surrounding healthy tissue as possible.

History of Mohs micrographic surgery

This skin cancer treatment was first investigated in the early 1930s by Frederic Edward Mohs and was originally called ‘chemosurgery’. The MMS procedure was not truly developed until 1956, and it was not until the mid-1970s that it was more regularly used. When first introduced, many traditional dermatologists did not see its value, nor did they believe that dermatologists should be surgeons. A small group of physicians, including Drs Perry Robins, Alfred Kopf, and Hugh Brown, along with Dr Mohs, created the first Mohs team at New York University in 1965. This team fashioned the first dermatologic MMS fellowship programme in order to ensure that those physicians performing the procedure were skilled in the treatment3.

The most common type of skin cancer is basal cell carcinoma (BCC), with 2.8 million cases diagnosed every year in the US alone4. Squamous cell carcinoma (SCC) is the second most common with over 700000 US cases a year4,5. These two types of cancers are the most commonly treated with MMS.

Indications for Mohs

According to the American College of Mohs Surgery, the indications for the use of Mohs are as follows:

  • The cancer is recurrent
  • Scar tissue exists in the area of the cancer or in sites of previous radiation therapy
  • The cancer is in an area where healthy tissue must be preserved for maximum functional and cosmetic results (i.e. the eyelids, nose, ears, lips, fingers, and genitals)
  • The cancer is large (>2 cm in diameter) or growing rapidly
  • The edges of the cancer cannot be clearly defined
  • The cancer grows rapidly or uncontrollably, including high risk SCC (infiltrative histology, poorly differentiated).

Only one criterion is sufficient for indicating the use of Mohs.

The procedure and its evolution

Occasionally Mohs is used for melanoma in situ, but it is not preferred for use in thicker melanomas. Many surgeons prefer to do slow Mohs over a period of several days, using permanent paraffin slides rather than the frozen tissue method. With paraffin slides, the tissue is easy to stain and interpret in the case of pigmented melanoma cells. Basal cell and squamous cell carcinomas are easier to stain with haematoxylin and eosin (H&E) stains on frozen sections.

The original technique of chemosurgery was called such due to the chemicals used during the procedure. Dr Mohs developed a paste made of zinc chloride that could be applied to the area of the tumour, allowing him to excise the tissue without inducing bleeding. This technique did not alter the structure of the skin during the process, which took many hours until the tissue was ready for cutting. Once the area was removed, it was sliced into sections, dyed in order to differentiate the sections and intricately mapped to identify the location from which it was removed.

Drs Theodore A. Tromovitch and Samuel Stegman at the University of San Francisco (UCSF) helped in the evolution of this process; instead of using the zinc chloride paste that was time consuming and painful for patients, they began freezing sections of tissue after excision. This frozen tissue method is what is used today and referred to as Mohs micrographic surgery. Once the tumour is removed with a small margin, it is cut, dyed, frozen, and made into slides. This process expedites the procedure as a whole, as the frozen sections can be processed in a short period of time (usually about one hour per layer), which allows a tumour to be removed in one day rather than several. The traditional paraffin embedded layers take 24 hours to process, extending the treatment period over several days. Most patients see an advantage in receiving treatment over a few hours rather than over a period of days. With local anaesthesia there is minimal pain and bleeding associated with Mohs.

After creating the slides, the next step is for the Mohs surgeon to review them in order to identify whether there are any cancer cells remaining in the margin. In Mohs, 100% of the cut edge is reviewed as opposed to other methods of removal, where only a sampling of this edge is studied. This 100% review of the tissue gives the surgeon great confidence about whether the entire lesion is removed. Additionally, the use of colour-coded mapping of the excised tissue specimen has helped improve the technique and procedure.

Another important differentiator is that the Mohs surgeon also acts as the pathologist, which is beneficial because the surgeon then has a clear image of the pathology and mapping when they return to the patient to take another layer, thus allowing them to remove very small amounts of additional tissue. If cancer cells are still present, an additional small margin is taken and the process is repeated until the margins are clear of cancer cells. Once clear, the open wound can be closed by the Mohs surgeon. The area can then be reconstructed to ensure the best cosmetic outcome and the smallest possible scar. It is for these reasons that Mohs is the best option for removing lesions that are in cosmetically sensitive areas or areas that would benefit from removing less tissue for functional reasons, like the eyelids or hands.

Safety and survival rates

A study conducted by the Cutaneous Oncology Research Cooperative indicated that Mohs surgery sets a new standard of safety in skin cancer treatment7. The study had 1,550 patients and demonstrated zero incidences of major complications; only 2.6% had minor complications (bleeding or infection).

In addition to its outstanding safety record, Mohs surgery also provides excellent cure rates due to the removal of all affected cells. According to Dr Isaac Neuhaus, dermatologic surgeon at UCSF’s dermatologic and laser center, Mohs surgery has a 5-year cure rate of 99% for new skin cancers and 95% for recurrent skin cancers. This is significantly higher than other methods.

Protection is best

Although not the only cause of skin cancer, UV exposure is undeniably linked to the development of this disease8. Therefore, comprehensive efforts to avoid unnecessary exposure should be made. Daily use of a broad spectrum sunscreen with an SPF of at least 30 is critical. It should be applied 30 minutes prior to exposure and reapplied every 2 hours thereafter, or after swimming or perspiring. Sun avoidance strategies such as refraining from exposure between the hours of 10 am and 4 pm when UV rays are strongest, and wearing a wide-brimmed hat and protective clothing will certainly help patients minimise their chances of developing skin cancer.

Mohs micrographic surgery has continually demonstrated its benefits as the gold standard in skin cancer care. It has excellent survival rates and leaves the patient with minimal visible scarring.