Mohs is named after its inventor, Dr. Frederic Mohs, who first described the technique in 1941. He combined horizontal sectioning with tissue mapping. Dr. Mohs observed that skin cancers often have contiguous growth with unpredictable extension (subclinical extension) that can spread deeply or laterally from the clinically evident lesion. Because skin cancer can spread extensively beneath the skin and still not be detected by inspection or palpation, what is visible to the patient and physician may just be the “tip of the iceberg.”
With Mohs surgery, cancer is detected by removing tissue, in a controlled and cautious manner, layer by layer much like a very small archeological project. As each layer of skin and tissue is analyzed, the Mohs surgeon, with the use of margin control, can see where the cancerous tissue ends and the healthy tissue begins. This allows the surgeon to identify and remove all of the cancerous tissue with as little loss of healthy tissue and as minimal scarring as possible.
Margin control is a principle used by surgeons to increase the cure rate by histologically checking the edge of the resection or specimen for cancer. A margin is the distance between the edge of the resection and the edge of the cancer. Two methods are commonly used to check the margins – vertical sectioning and horizontal sectioning.
Vertical sectioning is the predominant method used by pathology labs for checking margins. Unfortunately it only checks about 1% of the margin. This method is used because it is fast and is adequate for most skin cancer resections.
Horizontal sectioning is the method used by Mohs labs to check the margins. This method is more time consuming but it checks 100% of the margins. This thorough examination is why Mohs surgery has the highest cure rate for skin cancer compared to any other modality.