Sunscreen protection of moles (aka melanocytic nevi)…

Melanocytic nevi (moles) are considered melanoma precursors, and people with many melanocytic nevi are at increased risk for melanoma. Sun exposure, as well as being an etiologic agent for melanoma, also produces clinical, histological, and dermatoscopic changes in melanocytic nevi. Two study groups investigated whether topical sunscreens and opaque barriers prevented alterations in melanocytic lesions.

Carrera and colleagues covered half the area of 23 dysplastic nevi with either an opaque barrier or a broad-spectrum sun protection factor 50 sunscreen and exposed the lesions to ultraviolet B light at twice the minimal erythema dose. Seven days after exposure, erythema, pigmentation, and scaling were increased compared with preexposure. These effects were at least partially prevented in sunscreen- or barrier-treated portions. Ultraviolet (UV)-induced changes were identified on dermatoscopic examination in some nevi, including diffuse pigmentation, blurring of the pigment network, dotted vessels, and erythema. Some nevi also had changes in the size of dots and globules and increases in regression structures. Surprisingly, many dermatoscopic changes were also found in the protected nevi halves. Only blurring of the pigment network was significantly less common in protected areas. On histological examination, unprotected portions were more likely to have marked lentiginous melanocytic hyperplasia, suprabasal solitary melanocytes, and prominent and elongated melanocytic dendrites. Unprotected areas also stained more intensely for melanocytic markers HMB-45 and Melan-A, although changes in markers were observed even in protected lesion halves. Similar findings were reported by Massone and colleagues.

Comment: Unquestionable evidence links sun exposure and tanning bed use to melanoma, and sunscreens reduce these effects. Results of both studies indicate that subclinical changes (i.e. changes in the moles that are not noticeable to the naked eye) occur in nevi exposed to sunlight and that sunscreens and physical barriers protect melanocytic nevi from some, but not all, UV-related changes. Changes in the protected nevi halves may represent indirect effects of exposure on unprotected skin cells adjacent to the nevi or result from exposure to segments of the visible or infrared spectra not filtered by sunscreens. The significance of these changes and how long they persist are unknown. From the dermatologist’s perspective, the findings suggest that biopsies performed in recently sun- or tanning bed–exposed skin may produce worrisome histologic findings. These results reinforce the deleterious effects of sun exposure on nevi.

— Craig A. Elmets, MD

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