Background: Increased risk of skin cancer by indoor tanning has drawn public attention. However, there are arguments that tanning bed use increases vitamin D production, which may therefore prevent internal cancers. Methods: We followed 73,358 female nurses for 20 years (1989-2009) in the Nurses' Health Study II and investigated the frequency of tanning bed use during high school/college and at ages 25 to 35 in relation to the incidence of total cancers (excluding skin cancers). We used multivariate Cox proportional hazards models to estimate the HRs and 95% confidence intervals (CI) of total cancers and each individual major cancer with more than 100 cases. Results: During follow-up, a total of 4,271 internal cancer-related cases were diagnosed. No association was found between tanning bed use and risk of total cancers (multivariable-adjusted HR, 0.99; 95% CI, 0.95-1.04 for every 4 times/y use on average during high school/college and at ages 25-35). In addition, no association was found for the risk of any individual major cancers, such as breast cancer, thyroid cancer, colorectal cancer, non-Hodgkin lymphoma, or endometrial cancer. Conclusions: Our data do not suggest any association between the use of tanning beds and risk of internal cancers. Impact: On the basis of the strong evidence of increase in skin cancer risk and no evidence of reduction in internal cancer risk by tanning bed use, it is important to warn the public against indoor tanning.
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