Director of Cosmetic Surgery,

University of Illinois Chicago

Department  of Dermatology

Oakbrook Center Mall

Oak Brook, Illinois

(630) 928-0240

Jeffrey  Melton, M.D., S.C.,

link to us

drmelton.com

Mohs Surgery is simply a shorter name for the technique Mohs Micrographic Surgery. The the two terms will be used here interchangeably.

Mohs surgery is state-of-the art treatment for skin cancer which offers the highest potential cure rate of any method of treatment. The mohs surgeon serves as surgeon, pathologist and reconstructive surgeon.

The reason for the higher cure rate with Mohs surgery is that the microscopic assesment of whether or not the cancer has all been removed, is far more comprehensive than is the case with other methods.

Even though skin cancers may be visible, some portions of the cancer are often invisible to the naked eye .  "Roots" or "finger-like" extensions may exist beyond the visible portion of the skin cancer.  Non-Mohs treatments are somewhat dependent to heavily dependent upon the ability to see the edges of the cancer.

If all of the cancer cells are not removed completely, the skin cancer will re-grow, requiring re-treatment later.

Mohs surgery offers the most accurate and most complete method of determining whether the edges of the cancer have been completely removed.

Mohs surgery has been used to treat many kinds of skin cancer. The two types which are by far the most commonly treated by Mohs surgery are basal cell carcinoma and squamous cell carcinoma. These are also the two most common types of skin cancer overall.

Situations where Mohs surgery is particularly suitable:

• The skin cancer is located in a cosmetically important or sensitive area, such as around the eyes, nose, ears, lips, or hands.
• Skin cancers which are large, aggressive, or rapidly growing.
• Skin cancers which have recurred, that is, "returned" after previous treatment.
• Skin cancers which have ill-defined or poorly defined edges which are difficult to see.

With skin cancers in the above situations, common (non-Mohs) treatments are often not successful. This is because other methods depend on the naked eye to determine what is to be removed. Due to poorly visible edges, too much tissue may be removed, too little tissue may be removed, or the shape or depth of the removal may be inadequate.

A skin cancer treated by one of the more common methods is more likely to recur or "come back." In these cases, treatment is even less likely to be successful if Mohs surgery is not used for the second treatment.

An example of a skin cancer treeated by Mohs surgery:

          View from above                      from the side

The cancer roots may extend beyond the visible portion of the skin cancer. These roots must be removed, or the cancer will recur. Here, the cancer is in blue, seen from the side, at the level of the skin surface.

Step 1 the visible cancer is removed. A root, not visible to the naked eye, remains in the deeper skin.

Step 2 now viewed from the top, the removed specimen is divided into four pieces and mapped.

Piece 4 is the one with the root on the underside of the specimen. These four specimens are mapped and inked for orientation. They are then turned upside-down and an extremely thin layer is removed from what was the deepest and outermost portion of the removed specimen. The thin layer is mounted on a microscope slide for examination.

Below is a depiction of rotating piece 4, beginning with the view from above looking at the skin surface, and ending with the view of the underside of the removed specimen..

It is from this entire underside that a very thin slice is taken and placed on a microscope slide.  Because skin is flexible, it can be pressed into the desired shape, so the entire  curved undersurface is pressed into a flat pane, imbedded into a gel, and frozen. Once frozen, the solid, formerly curved bottom surface remains in a single flat plane. A very thin slice of this surface is then taken with a large instrument called a cryostat. These thin slices are mounted on microscope slides. The slides then go through a staining process, and are then examined under the microscope. Of course, the process is also done for pieces 1,2, and 3, since the cancer cells still cannot be seen.

Once stained, the blue area of skin cancer cells can be seen under the microscope.

The other three specimens do not have cancer (at the outermost, deepest portions of the removal). The map is marked to indicate the location of any remaining cancer cells.

In this case, the deep area near the center of section 4 is marked in red.

Tissue under the center of area 4 is removed more deeply.

And inked and mapped; slides are prepared and examined once again.

The map is updated to show the removal.  The specimen is mapped and inked;  microscope slides are made, again from the undersurface, and examined. In this case, no skin cancer remains.

If piece 5 showed cancer cells under the microscope, the process would be repeated until no skin cancer cells remained.

Why Mohs surgery is so accurate:

Currently, the only highly reliable accurate method to determine whether a skin cancer has been removed completely is by assessing whether the outermost and deepest edges of the removed tissue is free of cancer cells. This is done by examining mounted slides of removed tissue under the microscope. When the outer edges of the removed tissue is free of cancer cells, it is said that the "surgical margins are clear" or the "surgical margins are free." There are methods other than Mohs surgery for determining this, but these methods are less complete and thus less accurate.

Some hospitals and surgicenters will do "frozen sections," for example. Mohs surgery does use frozen sections also. However, non-Mohs frozen sections (as well as regular standard pathology sections) are three dimensionally different.

For example, one of the most common non-Mohs methods to determine whether the surgical margins are clear is a "breadloafing technique."  Skin cancers are not generally shaped like a loaf of bread, but for the purpose of illustration, imagine the skin cancer specimen that is removed to be shaped like a loaf of bread. The top crust represents the surface of the skin. Several of the slices from the loaf, taken from various points along the loaf, are placed flat on microscope slides and examined. Keep in mind that what is of interest in obtaining clear (of cancer) margins is whether any cancer is present on the outer crust (say, the bottom crust or the side crust) of the loaf. Because if there is cancer on the crust (other than the middle of the top crust), it stands to reason that there is cancer immediately outside the loaf (still in the patient) in that location.

So when the slides are examined under the microscope, there will probably be some cancer cells on the top crust (skin surface) and perhaps in the soft center of the loaf, which is the center of the slice. But if there are cancer cells right at the bottom crust (the deep margin of the removed specimen) or the side crust (the side edge of the removed specimen) then it is known that there is still skin cancer left behind in the patient. It is said that the margins are "positive" (i.e. have cancer).

How standard techniques can miss the cancer:

The breadloafing technique allows the pathologist to determine if there is cancer on the outer edge of the slice, or the surgical margin. The problem is it is only one slice. If say, five slices are examined, what about the other slices in between those five? If cancer is present in the crust of one of the non-examined slices, but the examined slices were clear, then the report would indicate "margins clear" when the really weren't.

One solution would be to examine all of the slices in the loaf. But this is not practical in terms of sheer numbers. In order to be visualized under the microscope, the tissue slices (i.e. bread slices) have to be extremely thin – only 6-10 microns thick. Therefore, a one inch specimen would require over 3,000 sections to look at. In practice, with this breadloafing method, a dozen or more sections are typically looked at. These are assumed to be representative for the other 2,900+ sections. Obviously, skin cancer can and sometimes is present in the "crust" of some of these non-examined sections, even when the report indicates that the "margins are clear."

How Mohs surgery differs:

Once again, assume that the removed cancer specimen has the shape of a loaf of bread. Because the skin cancer is known to be on the skin surface (top crust) and at least slightly below it, the center of the top crust and the soft bread slightly below it is scraped away. This is actually done before the "loaf" is removed. Many skin cancers are very fragile compared to normal skin; the cancer scrapes away easily, but the normal skin does not. So, difference number one is that the depth and shape of the defect made by scraping (actually called curetting) helps the mohs surgeon to better determine what the shape, size and depth the loaf to be removed should be. This actually improves the accuracy of the procedure in itself. In contrast, in the above non-Mohs method, the removal shape is just planned pre-operatively. 

The Mohs surgeon then takes the removed "loaf" with the "scooped-out" top, and splits it vertically, lengthwise into two pieces. Each half is pressed onto a flat surface in such a way that the entire side, end, and bottom crusts are in contact with the flat surface. The flattened specimen is then frozen to preserve its shape. Slices are then taken in such a way that each slice has only crust. Therefore, taken together with the other half of the loaf, the ENTIRE crust is now on microscope slide sections, and each section has nothing BUT crust.  Therefore, in the Mohs method, if there is any cancer in the crust, it will show up on the slide. The reason this can be done is that skin is flexible enough to be bent into the needed flat plane, and will stay flat, once it has been frozen.

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