Dermatology Mohs Micrographic Surgery


Mohs micrographic surgery

I’m actually on vacation this week in California with my family (check out my snapchat TeawithMD to follow along on our road trip to Muir Woods and Sonoma), but last week was my first experience rotating through the Mohs Micrographic Surgery service, and since it is such a special and unique subspecialty within dermatology, I wanted to blog about what it is now that I’ve gotten a taste of it. I’ll still have two more weeks of Mohs micrographic surgery after vacation, so I can continue to answer questions about it on the blog or on social media if there is interest.

Mohs Micrographic Surgery, invented by Dr. Mohs in the 1930’s at the University of Wisconsin, is a special surgical process that involves the removal of skin cancer, mapping of the tissue and skin cancer after the specimen is obtained, and the immediate sectioning and staining of tissues to microscopically check if the margins are clear of cancer. This usually takes around 60 minutes for the tissue to be cut and processed. If the margins are completely free of cancer, then no further stages of surgery are necessary, and the surgeon will close up the defect. If there is still cancer remaining on the slide, then the surgeon will go back to the patient and remove a margin of skin on the patient corresponding to where the remaining cancer was on the tissue slide. The patient goes outside and waits while the tissue that was again removed is cut, stained, and examined under the microscope. The cutting and examining continues until all the margins are free and clear of cancer, at which point the defect from the surgery is repaired, and the patient goes home, knowing that he or she is now cancer free. The patient will usually return to the clinic in 1 – 2 weeks for wound check and suture removal, depending on the type of wound closure that is done and the site of surgery.

Mohs surgery infographic

From the UCSF page about Mohs surgery

The non-Mohs standard type of excision for skin cancers involves cutting out the skin cancer with a margin of normal skin around it, then immediately closing up the wound. The tissue that is removed is then sent out of the office to a pathologist where they cut it “bread-loaf” style, meaning they take specific sections throughout the tissue block to sample the specimen removed and see if there is cancer extending to the margins. This samples only about 1% of the overall margins. Then the tissue slides are read by a dermatopathologist, who will issue a report regarding whether or not the margins are free after a few days’ delay (sometimes a week or more). If the margins are free, then the patient does not require further surgery there; however, if the report shows that there is cancer extending to the margin of the removed tissue, then the patient needs to return back again days to weeks later to have the area excised again.

Mohs Surgery compared to traditional excision

From the UCSF Mohs Surgery page (see more about the horizontal slicing done in Mohs below)

Mohs surgery offers several advantages compared to the traditional method of removing cancerous tissue. First of all, everything happens in one day, meaning the cancer removal, the examining and reading of tissue slides, and the repair all take place before the patient leaves the office; in a traditional excision method, this process takes days to usually weeks. Mohs surgery also gives confirmation of whether or not a cancer is out by the time the defect is sewn up and repaired, since 100% of its margins have already been examined microscopically; in a standard excision, the site of surgery is repaired but you don’t hear back from the pathologists until later so you have a longer waiting period and there’s a possibility you may have to go back for another surgery. Mohs surgeons also cut the tissue in such a way that it examines 100% of the entire surgical margin instead of single sections in a breadloafing pattern in the standard excision that results in only ~1% of margins being examined (see pic above). Mohs surgery allows for the minimal amount of tissue to be removed for the cancer to be clear, which is especially important in cosmetically sensitive areas like the face. Lastly, Mohs surgeons, who do one additional year of fellowship training in Mohs after 3 years of dermatology training, are also trained to do more complicated repairs, so in the case that the cancer extends deeper than it appears and a large section of skin is removed, the surgeon can do flaps and grafts to achieve a cosmetically pleasing result.

Mohs Surgery as Reese's Peanut Butter Cup

A and C are epidermis (top layer of skin) and B and D are the deep margins in the dermis. In Mohs, you cut the specimen out with angled borders like AB and CD in the peanut butter cut, then you flatten out the specimen so AB and CD lie flat with the bottom of the peanut butter cup (think of taking the wrapper off and flattening it into one sheet). Then you slice horizontally from the bottom, and ABDC will all be in one plane, allowing you the surgeon to examine ALL the borders in one section.

One thing that took me a while to understand was how you can evaluate all of the margins in one slice. Basically, I picture the piece of skin excised as a Reese’s peanut butter cup. The top of the peanut butter cup is the epidermis (AC), or the top-most layer of skin, and the bottom of the Reese’s peanut butter cup is the dermis (BD), which is between the epidermis and the subcutaneous fat. The Mohs slice is cut in a 45 degree angle so the tissue slice resembles the shape of a Reese’s peanut butter cup with slanted borders. Then the specimen is flattened and then sliced horizontally (in the direction of the arrow shown) so you can see epidermis and dermis all in one cut. Imagine taking the wrapper of the Reese’s cup off (imagine ABDC is the wrapper) and flattening it so you can see points ABDC all in one plane. Then the histolopathology technician cuts slices horizontally so each slice contains 100% of the borders, so when you examine these slides, you can be sure that you’re not missing any cancer in unexamined slices.

Hope this was an informative introduction to Mohs surgery. As always, feel free to leave questions below.

For more information on Mohs surgery:

  • page on Mohs
  • UCSF page on Mohs

2 Comments on Dermatology Mohs Micrographic Surgery

  1. Angela Nwankwo
    January 2, 2017 at 4:33 pm (1 year ago)

    Do you have to do a formal training in Mohs to be considered a Mohs surgeon or could you just have been exposed to it during your residency?

    • Joyce
      January 2, 2017 at 6:54 pm (1 year ago)

      Hi Angela, great question. The vast majority of dermatologists who do Mohs surgery in their practice have done an American College of Mohs surgery accredited Mohs fellowship, which is a 1 year post-residency training fellowship where all you do is Mohs surgery day in and day out. There are VERY few exceptions in which people who train at surgery-heavy residencies feel comfortable enough to go out and practice without a fellowship. This is more common in rural areas where there are so few Mohs surgeons that it makes sense for dermatologists who are experienced in this field to do Mohs. However, I think in the upcoming years there will be fewer and fewer of the non-fellowship trained Mohs surgeons, since training has become so widely accepted and expected.


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