A Mayo Clinic physician and colleagues have defined the normal number of melanocytes that are present in Caucasians’ sun-exposed skin.

Until now, there has not been a criterion to distinguish sun damage from early (in situ) melanoma. Results of the study, which shed light on this undefined area in skin cancer, are available in the July issue of Archives of Dermatology.

“There are many uncertainties in medicine. In many instances these uncertainties lead to overly aggressive treatments,” says Ali Hendi, M.D., Mayo Clinic dermatologic surgeon and lead investigator of the study. “We didn’t have an accurate way to distinguish cancer from over-exposed but normal skin, and this study was designed to find the missing link.”

There are two basic types of skin cancer, non-melanoma, which includes basal cell and squamous cell cancers, and melanoma. Melanoma is the deadliest, reports the American Cancer Society, accounting for only 4 percent of diagnosed skin cancers, but nearly all of the deaths. It originates in melanocytes, the cells that produce melanin, which colors skin, hair and eyes.

Dr. Hendi’s team thought that pathologists and Mohs surgeons may err on the side of over-diagnosing melanoma in sun-damaged skin, leading to additional and unnecessary surgery, complications or deformity. Mohs surgery, named for Frederic E. Mohs, M.D., who developed the technique, allows incremental removal of skin cancers, reducing removal of tissue to just the diseased areas. Mohs surgeons remove a thin section of the tumor, review its pathology under a high-powered microscope and are able to stop tissue removal as soon as the edge of the diseased tissue is reached. Because of this assumption of overly cautious surgery, the researchers decided to develop criteria by which cell changes that can indicate noncancerous sun damage can be distinguished from those which indicate melanoma.

In the study, disease-free tissues samples were obtained from 132 randomly selected Caucasian patients undergoing Mohs surgery for non-melanoma skin cancers of the face and neck. The samples were cut from the edges of the excised area, and consisted of normal, noncancerous skin. They found that in normal sun-exposed skin, the number of melanocytes in a 0.5 millimeter (mm) diameter was 15.6 (as opposed to undamaged skin that might have five to seven). Adjacent melanocytes, another indicator used by pathologists to diagnose early melanoma, were present in varying degrees in all but 11 percent of the samples. Finally, melanocytes were found to descend along the hair follicles, a finding previously attributed only to melanoma in sun-damaged skin. The researchers say these findings are significant because many surgeons remove tissue until they reach undamaged cells with “normal” melanocyte distribution.

Dr. Hendi predicts the study’s findings will be valuable to doctors who diagnose and treat melanoma. “To be able to look in the microscope and have a measurement by which to determine successful removal of melanoma in situ is something we’ve hoped for quite a while,” he says. “In many cases, surgeons can stop removing tissue much sooner, which will result in less trauma to the skin.” Dr. Hendi’s team recommends doctors use the study’s findings as their new baseline for melanoma diagnosis and tissue removal.

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