Since melanomas are most curable when caught early, it’s understandable that physicians might be aggressive in removing tissue suspected of harboring melanoma cells. However, normal sun-damaged skin can look a lot like melanoma under the microscope. For example, its melanocytes (the pigment cells, where melanomas can develop) increase in number, density, and "confluence" (the tendency to come in contact with one another), much the way they do in melanomas. Unfortunately, guidelines for just how numerous, dense, or clustered melanoma cells are have been lacking, so physicians sometimes have had to make subjective judgments, removing some tissue that turns out to be normal.

Now, new research may help physicians distinguish early melanomas from skin that is merely sun-damaged. After carefully examining the number of melanocytes per 0.5 mm of skin, as well as other traits in the skin of 132 patients with sun damage, investigators at the University of Pittsburgh Medical Center and the Mayo Clinic in Jacksonville, FL, determined that increased number and confluence of melanocytes alone is not enough to diagnose melanoma. Instead, they found, these factors must be accompanied by nesting (four or more melanocytes clustered together) or vertical stacking (also known as "pagetoid spread" – an invasion of the melanocytes into the upper epidermis from below) of the melanocytes. If an increased number of melanocytes is accompanied by any of these three factors, a melanoma diagnosis can be made much more firmly.

"The uncertainties inherent in diagnosing melanoma have sometimes led to overly aggressive treatments. Our data should assist the physician in distinguishing increased melanocyte density in sun-damaged skin from that found in certain early melanomas," explained study leader Ali Hendi, MD, at the Mayo Clinic. "The dermatopathologist now has the criteria needed to avoid overdiagnosing melanoma in sun-exposed skin, thereby avoiding some unnecessary biopsies and surgeries, as well as potential complications and deformity."

The research was partially funded by a grant from The Skin Cancer Foundation.