VIENNA Successful Mohs micrographic surgery depends on two things: that the tumor is contiguous and that 100% of the surgical margins are examined histologically, Stuart J. Salasche, M.D., said at the 10th World Congress on Cancers of the Skin.
"Recurrences do happen, and if you're doing 1,000, 2,000 cases a year then even small percentages add up to numbers, and each number represents an individual patient who put [himself or herself] in your hands," Dr. Salasche said.
Some recurrences are caused by "housekeeping errors" such as inadequate slide preparation, mapping errors, and poor tissue samples, and can be reduced with repetition and good staff training, he said.
Large tumors in general, and particularly those on the ear or medial canthus of the eye, can be difficult to map, and should be marked carefully with scalpel hatch marks that correspond to color-coded maps for more accurate orientation.
Poor slide preparation can result in false negative margins because of missing epidermis or holes and folds in the tissue where tumor can exist.
False-negative margin situations are frequently caused by noncontiguous tumors. Common culprits are recurrent tumors where residual tumor was left in multiple foci of which only one became clinically apparent. This applies particularly in immunosuppressed patients, he said. Some tumors may inherently have skip areas such as those seen in sebaceous carcinoma and Merkel cell carcinoma.
"The ones that we see most often and cause us the most trouble are tumors that have already been operated on or previously treated," said Dr. Salasche of the Arizona Cancer Center at the University of Arizona in Tucson.
When evaluating recurrent tumors, consider the original treatment modality, the type of repair used, the time from original surgery to clinical recurrence, the aggressiveness of the tumor histology, and whether the area was covered with a graft, he said at the meeting, cosponsored by the Skin Cancer Foundation.
In the approach to a recurrence, all visual tumor and the entire scar should be resected, as if the scar were part of the original tumor. Pay particular attention to squamous cell carcinomas or lesions on the scalp, temple, or forehead, most notably in organ transplant patients, he said.
Inflammation can also mask tumors and is common in elderly populations with chronic lymphocytic leukemia. Tumor masked by the inflammation may go unrecognized by the surgeon, or result in the surgeon chasing the inflammation or subclinical extensions as they track along nerves for great distances, he said. Immunostaining is helpful in these cases.
Another problem is recognizing that basal cell carcinomas probably originate from stem cells that reside in the outer root sheath of the hair follicle, and result in subtle buds of tumor coming off the follicle that can be misread as hair follicles, he said.