Mohs surgery is a highly effective treatment for certain types of skin cancer. It is an exacting procedure in which the dermatologist performs both surgical excision of the skin cancer and microscopic examination of the surgical margins to ensure that all skin cancer cells have been removed. The excised tissue from the patient is brought to our in-house Mohs Laboratory by the surgeon or medical assistant. Our Mohs tech then logs in the patient’s information which includes name, case #, site, and diagnosis.
The tissue received is usually oval or round in shape. In order to relax the tissue so that the epidermis lays flat, we make “relaxing cuts” that bring down the margins of the tissue; which is called grossing. After grossing the specimen needs to be inked for orientation, with several colors; blue, red, yellow, or green. Orientation is similar to a compass, which helps the surgeon read the slides as if they’re reading them on the actual site.
The tissue is carefully placed in a mold, manipulating the epidermis to lay flat, and then placed in our Cryostat that is set -24 to -28° C. With the tissue in the mold, we squeeze a generous amount of (TFM) Tissue Freezing Medium over the tissue and top it with a chuck. The cryostat and set degrees helps make a “block” by freezing the tissue and TFM. Sometimes we will apply liquid nitrogen which helps speed up the freezing process. After about 3-5 minutes, depending on the size of specimen, the tissue is placed in the chuck holder, in the cryostat, and is ready to be cut.
When cutting, we work inside the cryostat, known as the cryochamber. With the chuck already in place, the first step is to “rough cut” the specimen in order to get a flat and even plane. Once that is completed we start cutting small sections of 3-5 microns, depending on the thickness of the tissue. To put into perspective, the thickness of human hair is about 50-100 microns. An average of three slides are made up of 3 sections of tissue on each slide. Most surgeons like to see at least 8 sections in total.
When all slides are done, they are placed in a series of chemicals, also known as H&E staining (Hematoxylin & Eosin). Hematoxylin stains the nucleus of the cell and Eosin (referred to as a counter stain) which stains nearly everything that the hematoxylin does not. This process takes about 5 minutes.
The stained slides with the corresponding paperwork is then given to the surgeon to be read under a microscope. The diagnosis can be clear or if there is more tumor seen the surgeon will then remove another layer from just the site where the cancerous cells are present, which is documented as a stage II and the process is repeated.
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