Although the use of sunscreen has been advocated as a preventive measure against skin cancer, no studies have demonstrated that regular use of sunscreen can prevent basal or squamous cell skin carcinomas. Beta-carotene and other antioxidant micronutrients have also been advocated as preventive measures. The theory is that these compounds may act by lessening damage to DNA by free radicals or by influencing cell differentiation or proliferation. Green and colleagues conducted a community-based study to assess the ability of sunscreen and beta-carotene to prevent the development of skin cancers in residents of an Australian town.

In 1992, adults 20 to 69 years of age were invited for skin-cancer screening. Those willing to participate in the four-year study were randomly assigned to one of four treatment groups. One group took 30 mg of beta-carotene daily and used factor-15 broad-spectrum sunscreen; patients in the second group also used sunscreen but were provided with placebo tablets. The other two groups were asked to continue their usual practices of applying sunscreen. One of these groups was given beta-carotene daily and the other placebo tablets. All participants were rescreened every three months. Compliance was monitored by measuring sunscreen used and tablets returned. In addition, skin concentrations of beta-carotene were measured by photometry in patients taking tablets.

Of the 1,621 participants, 27 percent had a history of skin cancer. The four groups were similar in all important variables including age, sex, skin type, sun exposure, evidence of sun damage to the skin and beta-carotene levels. By the end of the study, 238 (15 percent) of participants had withdrawn from the study. Of those remaining, 75 percent assigned to sunscreen use reported applying sunscreen at least four days per week. In the groups not assigned to sunscreen, 74 percent reported no use or use on only one or two days per week. Compliance with tablets appeared to be 70 to 80 percent as assessed by pill counts. Skin photometry measurements indicated significantly increased levels in patients assigned to beta-carotene tablets.

Between 1992 and 1996, 256 participants developed 789 new skin cancers. Cancers diagnosed in the first year of the study were excluded from the analyses because these were believed to represent latent lesions developing before the intervention. The incidence of basal cell carcinoma was similar in groups allocated to sunscreen and other groups. The incidence of squamous cell carcinoma on exposed areas was significantly reduced in groups using daily sunscreen. Beta-carotene did not appear to significantly alter the incidence of skin cancers. The incidence of basal cell carcinomas was slightly lower and that of squamous cell carcinoma slightly higher in patients taking beta-carotene, but these differences were not statistically significant. The number of patients with symptoms attributed to tablets was similar (64 and 65, respectively) in the placebo and beta-carotene groups. A small number of participants (fewer than 25) complained of contact allergy, skin greasiness, eye irritation or other adverse effects of sunscreen use.

The authors conclude that regular use of sunscreen appears to prevent development of cutaneous squamous cell carcinoma but has little effect on basal cell carcinoma. Dietary supplementation with beta-carotene had no effect on the development of either type of skin cancer.