Sun Exposure and Skin Cancer Risk

David Polsky, MD, PhD, Professor of Dermatology and Pathology;
Alfred W. Kopf, MD, Professor of Dermatologic Oncology; Director, Pigmented Lesion Section; The Ronald O. Perelman Dept of Dermatology; New York University School of Medicine; NYU Langone Medical Center

Q: Summer sun season is upon us. What role, if any, does sun exposure have in the causation of cutaneous: 1) Actinic keratosis? 2) Squamous cell carcinoma? 3) Basal cell carcinoma? 4) Malignant melanoma? How completely should humans try to eliminate sun exposure lifelong?
A: The potential health effects (both benefits and harms) of sun exposure have been the subject of much controversy over the last 100 years. In the early 20th century when dietary sources of vitamin D were more limited, sun exposure was thought to promote both bone health and optimal health overall. Early epidemiologic studies, and later laboratory investigations demonstrated a definitive causal link between ultraviolet light exposure (i.e. the damaging wavelengths from sunlight) and skin tumors. Unfortunately, the genie was out of the bottle, and everybody wanted to be tan due to the mistaken belief that it was a sign of good health.
Interestingly, the contribution of sun exposure in different skin cancers varies.
Actinic keratosis: These extremely common skin growths are potentially pre-cancerous pre-cursors of squamous cell carcinoma. They occur on the same skin sites as squamous cell carcinoma (described below) and have a high rate of ultraviolet light-induced mutations.
Squamous cell carcinoma: This malignant tumor has the most direct relationship between sunlight and cancer. These growths occur on the face, arms and hands; skin sites that have received high cumulative levels of sun exposure. In addition, epidemiologic studies found a strong association of squamous cell carcinoma and outdoor occupations. Ultraviolet light-induced mutations are seen in more than 80% to 90% of these growths.
Basal Cell Carcinoma: This skin cancer is also linked to sun exposure, but not as directly as squamous cell carcinoma. The rate of ultraviolet light-induced mutations is lower in basal cell carcinoma than squamous cell carcinoma, and some studies suggest that skin cells’ ability to repair ultraviolet light-induced DNA damage is critical in preventing basal cell carcinoma. In contrast to squamous cell carcinoma, intensive sun exposure in childhood and adolescence may play an important role in basal cell carcinoma.
Cutaneous melanoma: While most cases are clearly caused by sunlight, melanoma is a heterogeneous disease. Some forms are associated with long-term chronic sun exposure; the more common form is associated with indoor occupations and intermittent, intense sun exposure (i.e. sunburns) in childhood and/or adulthood. Rare forms that occur on mucosal surfaces and the palms and soles are unlikely to be associated with sun exposure. Besides sun exposure related risk factors, having a large number of moles and/or atypical/dysplastic moles/nevi is even more important in identifying individuals with an increased chance of developing melanoma.
So how completely should humans try to eliminate sun exposure lifelong, and what are the most effective methods? Clearly sun exposure contributes to all forms of skin cancer so individuals, especially those with fair skin should protect themselves. Patients with light skin and hair color have higher risks because they have relatively low levels of the skin pigment melanin that protects skin cells from sun-induced DNA damage. Patients with lower levels of melanin sustain more DNA damage per minute of sun exposure than darker skinned individuals. To reduce skin damage caused by ultraviolet light, protective clothing should be the major strategy (e.g. long sleeves, long pants, hat, etc.). Nowadays, specialized lightweight sun protective clothing is available for different types of activities (e.g. water sports, golf, hiking, etc.). For skin that cannot be covered by clothing, use a high SPF, broad-spectrum sunscreen. SPF means sun protection factor, a measure of how well a sunscreen blocks ultraviolet B radiation, the wavelengths that cause sun burn. While SPF numbers roughly translate into how many times longer you can stay in the sun without burning, most people apply less than half the amount used to develop the numbers. Also sunscreens may lose effectiveness throughout the day. Experts typically recommend an SPF of at least 30 to insure a minimal level of protection. ‘Broad spectrum’ sunscreens have ingredients that also block ultraviolet A radiation which contributes to skin cancer and photoaging but doesn’t cause sunburns. Apply sunscreen generously in the morning and at midday to insure adequate sun protection throughout the day. Finally, planning outdoor activities to avoid the midday sun between 10AM and 4PM, is an effective strategy but may be impractical.
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