SANTA ANA PUEBLO, N.M. Evaluation of Mohs specimens sent to the American Society for Mohs Surgery for peer review has offered an opportunity to critique various styles of processing Mohs specimens and of planning second stages.
Some styles fail to comply with the basic principle of Mohs surgery. Undermining-type stage II reexcisions, straight-down cuts through the midline of stage II, and squared-off edges are examples of types of stage II reexcisions "that can turn the Mohs proof of tumor extirpation to pretense," Dr. Sharon F. Tiefenbrunn said at a meeting of the American Society for Mohs Surgery (ASMS).
Mohs surgery "is a proven technique with a 95%99% cure rate in the published literature," she said, but the rules "used to plan reexcision and to evaluate adequacy of the slides from these reexcisions have not been published."
She discussed her "rules" for Mohs surgery based on patterns that emerged during the analysis. "Most of these rules are fairly obvious and are stated for completeness," said Dr. Tiefenbrunn, a dermatologic surgeon who practices in St. Louis and is the current president of the ASMS.
Rules for stage I include the following:
▸ The complete skin edge must be included on the slides and examined. (The ASMS uses 90% as its standard of completeness.)
▸ The complete deep margin (90%) must be present on the slide.
▸ Make sure tissue remains intact on the wafers, without shredding.
▸ Make sure structural details are visible.
▸ Use adequate stain.
▸ Use visible ink for orientation and to ensure complete examination of sectioned specimens.
▸ Tumor must be identified and mapped accurately.
The rules for stage II include:
▸ If the skin edge is missing on stage I, it must be taken on stage II.
▸ If deep tissue is missing on stage I, it must be taken on stage II. "We see a few cases where these simple mandates slide by," she said.
▸ Stage II must include the tumor in stage I. "Early wafers must be unequivocally clear to call the stage clear if tumor is seen deep in the block," she said.
▸ Respect the Heisenberg uncertainty principle. "We must overlap in all directions from the edge of the tumor and we must connect to the previous layer and confirm with inked margins," said Dr. Tiefenbrunn.
If the connection to the defect left by the previous stage is not examined, a larger overlap will be needed.
She pointed out that taking 20 turns of the cryostat before putting a section on the slide means that approximately 200 micrometers of tissue from the sample are lost. This amounts to 0.2 mm, a space in which tumor can travel undetected. If 95% of all possible angles of spread of the tumor are to be examined, an overlap of about 2.5 mm is needed.
"Failure to follow the above rules can turn proof to pretense," she said.
Common errors in Mohs surgery occur in planning the second stage, particularly inadequate overlap and lack of concern about observing the line of connection of stage II to stage I. "Undermining-type reexcisions in which the dermal rim of tissue is undermined and a vertical or beveled incision is made circumferentially around the defect connecting to this plane, frequently accompanied by vertical incision extending radially from the defect to complete this excision, [are] a common error," she said.
"This section is then processed so that only one of the four planes of the surgical margin is examined. The exclusion of the deep margin and the radially cut ends of the specimen from exam can allow tumor to be left behind. Proper process of an undermining stage II is difficult for the technician. Even if this type of reexcision is processed properly, there is inadequate overlap on the deep margin," she said.
"Straight-down cuts, such as the type made when stage II is reexcising half of the stage I defect, are another source of error," she said. "If a straight-down cut is used and not adequately flattened, this 2.5 mm of necessary overlap doubles the every-0.2 mm of space between the defect and the depth of the reexcision. This needed overlap can easily exceed the size of the defect."