The advantages offered by Mohs surgery result from three essential techniques: horizontal tissue sectioning, narrow margins, and surgical mapping.
Horizontal tissue sectioning is the method by which tissue is prepared during Mohs surgery. Visible tumor is removed from the skin and transported to the lab where it is immediately frozen. Because of its inherent plasticity, the technician is able to manipulate the skin edges to lie even with the base of the specimen. The specimen is then cut horizontally, creating a transparent film that reveals the entire base and all skin edges in one layer. Traditional pathologic sectioning is performed in a vertical manner and creates only representative samples of the base . Areas between cuts are never truly examined.
The strict margin control offered by horizontal sectioning gives the Mohs surgeon the freedom to take small surgical specimens. Typical margins in conventional excisions are 5mm beyond visible tumor. In Mohs surgery, these margins are reduced to 2-3 mm.
The trade-off for small surgical specimens is a higher risk of residual tumor at the wound edge. This problem is alleviated by the use of surgical mapping techniques. When residual tumor is present, a second specimen is taken from the involved quadrant only. This process is continued until all margins are free of tumor.