Scars from Mohs micrographic surgery for facial skin cancers may be longer than patients expect,  according to new research published in JAMA Network Open.

In this cross-sectional study of 101 patients, 83.2 percent of those receiving Mohs micrographic surgery for skin cancer underestimated their scar size. Scars were a median of 2.2 times larger than patients expected, compared with 1.1 times larger for physicians’ estimates, the study showed.

“This study’s findings suggest that patients with facial skin cancers have unrealistic expectations regarding scars that measure, on average, less than half the length of the actual postoperative scars,” the researchers conclude. “Surgeons appear to accurately estimate the length of most surgical scars and have an opportunity to set realistic patient expectations about scar length before surgery.”

Hooman Khorasani, MD Chief of Dermatologic and Cosmetic Surgery at Icahn School of Medicine at Mount Sinai in New York City, reviewed the new study. “It is well documented that Mohs surgery is the most tissue-sparring procedure for the removal of skin cancer. It is estimated that for every millimeter (mm) of tissue sparred, there is a net reduction of 3 mm of scar length,” he says. “This study emphasizes the need to set accurate expectations for patients prior to surgery.”

He adds that in his experience the length of the final scar is on average 6-times the size of the original visible tumor, he says. “This has to do with the fact that most tumors have roots under the skin; thus, on average the amount of skin that needs to be removed is twice the size of the tumor that is visible to the naked eye.”

Additionally, most of these defects are closed in a linear fashion. “It is simply a geometrical rule that one has to convert a circle into an ellipse [and] the ratio between the diameter of the circle to the length of the ellipse is 3:1.”

For example, he says, if the size of a basal cell carcinoma (BCC) is 5mm, by the time the BCC roots are removed, the area of the skin removed would be 10mm. “Once the circle is converted into the ellipse, the length of the ellipse would be 30 mm,” he explains. “This is simply a mathematical equation and has nothing to do with the skill of the surgeon. Surgeon’s can control the quality of the scar but not the length of it.”