SAN DIEGO Since most Mohs surgeons want at least a 1-mm clear margin around a basal cell or squamous cell carcinoma, it is crucial to know how many slices were taken between frozen sections mounted for review, Dr. John Campbell said at a meeting sponsored by the American Society for Mohs Surgery.
Dr. Campbell, a San Diego pathologist with an interest in Mohs surgery, said that planning and consistency in how a block of tissue is sectioned by the cryostat's microtome is of the utmost importance to performing good Mohs surgery.
By averaging out the amount of tissue that disappears when the microtome slices a block of tissue, it appears that about 3 mum of tissue are lost for every 10 mum in most microtomes, Dr. Campbell said at the meeting.
Therefore, he recommends that the microtome be set to make slices every 7 mum, and that every 20th section get mounted on the slide. When the mounting is that consistent, then it is easy to know exactly how many clear sections one must see before declaring a margin clearin this case, five to six sections.
Dr. Campbell instructs the cryostat technicians he works with that he wants to see the first mounted tissue section within the first 100 mum of tissue whenever possible.
To be considered a proper and countable section, a section should have epithelium visible around 90% of the section edge.
Sometimes, though, when it is not possible to get a section without a hole in the middle, or skin all the way around the edge, it does not hurt to have incomplete sections mounted for the physician to at least see, he noted.
Although some surgeons claim to need less than a 1-mm clear margin around a tumor and that the amount of clear tissue necessary depends on the tumor type, 1 mm of clear tissue is a good, fairly conservative option for margins, Dr. Campbell said.