Tanning Bed Use Is Not Associated with Internal Cancer Risk: Evidence from a Large Cohort Study

We previously reported that use of tanning beds increased the risk of all three types of skin cancers (basal cell carcinoma, squamous cell carcinoma, and melanoma) by following up 73,494 female nurses for 20 years in the Nurses' Health Study II (NHSII; ref. 1). Our findings were further confirmed by two recent meta-analyses for both melanoma and nonmelanoma skin cancers (2, 3). However, there are arguments that use of tanning beds may be a good source of vitamin D, which has potential health benefits (4). Here, we report our investigation on tanning bed use in relation to the incidence of total internal cancers as well as individual major cancers in the same cohort. We carefully adjusted for outdoor sun exposure, UV index of residence, and dietary and supplemental vitamin D intake in addition to the known risk factors for cancers.

Our study population consisted of participants in the NHSII, a prospective cohort study established in 1989 and enrolled 116,678 female registered nurses ages 25 to 42 and residing in the United States. Details of this cohort have been described previously (5). The protocol for this study was approved by the Institutional Review Board at Brigham and Women's Hospital and the Harvard School of Public Health (Boston, MA).

In the 2005 questionnaire, we collected information on the frequency of tanning bed use during high school/college and at ages 25 to 35 (none, 1–2 times/y, 3–5 times/y, 6–11 times/y, 12–23 times/y, and 24+ times/y). We obtained information about outdoor UV exposure, dietary and supplemental vitamin D intake, and other risk factors for cancer from the follow-up questionnaires. More details were described in our previous publications (6).

Eligible cases consisted of women with incident cancers diagnosed any time after the baseline up to the 2009 follow-up cycle. Only pathologically confirmed invasive cancer-related cases were included, except for breast cancer, which included both invasive and in situ cases.

All participants in this analysis were U.S. non-Hispanic Caucasians. We also excluded the participants with missing information on tanning bed use or with self-reported cancers at baseline. We grouped women into four categories based on their self-reported frequency of tanning bed use (none, 1–2 times/y, 3–5 times/y, and ≥6 times/y) and created a continuous variable for linear trend test using the median value of each category. Participants contributed person-time data from the baseline in June 1989 to the first report (followed by confirmation) of a primary cancer, death, or the end of follow-up, whichever came earlier. We used age-adjusted and multivariate Cox proportional hazards models to calculate the HRs and 95% confidence intervals (CI) of total cancer and each type of major cancer with more than 100 cases. All of the statistical analyses were carried out using Statistical Analysis System software (version 9.1.3; SAS Institute). All P values were two-sided.

During the 20-year follow-up of 73,358 female nurses from 1989 to 2009, a total of 4,271 cancer-related cases (excluding skin cancers) were diagnosed. The first primary cancers for which at least 100 cases were diagnosed were breast cancer (n = 2,779), thyroid cancer (n = 306), colorectal cancer (n = 186), non-Hodgkin lymphoma (n = 185), and endometrial cancer (n = 100). In Table 1, we present the basic characteristics of participants according to the frequencies of tanning bed use based on information collected in baseline questionnaire in 1989. People who used tanning beds more often either during high school/college or from ages 25 to 35 were more likely to be current smokers and oral contraceptive users. No substantial difference was found for other cancer risk factors across the different categories.

As a result, no association was found between tanning bed use and total cancer risk (Table 2). Both age- and multivariable-adjusted HRs were 0.99 (95% CI, 0.95–1.04) for an average of every 4 times/y use during high school/college and at ages 25 to 35. Separately for the use during high school/college and at ages 25 to 35, no association was found for the use during either period of time (multivariable-adjusted HR, 1.02; 95% CI, 0.97–1.07 for an average of 4 times/y use during high school/college; and multivariable-adjusted HR, 0.99; 95% CI, 0.96–1.02 for an average of 4 times/y use at ages 25–35). In addition, none of the individual major cancers (with at least 100 cases) was associated with tanning bed use in our cohort (Table 2).

We further conducted a secondary analysis restricted to incident cancer-related cases diagnosed after their report of tanning bed use in 2005 (n = 1,315). The results were very similar to those of our primary analysis using overall cases (multivariable-adjusted HR for total cancers, 0.94; 95% CI, 0.86–1.03 for an average of 4 times/y use during high school and at ages 25–35). No substantial difference was found for the risks of individual major cancers, nor did we find any significant association between dietary and supplemental vitamin D intake and cancer risk in the multivariable models (data not shown).

In this large, well-characterized cohort study, we found no inverse association between tanning bed use and the risk of internal cancer after following 73,358 female nurses for 20 years. Our finding provided epidemiologic evidence to dispute against the claims by the indoor tanning industry that tanning bed use may has potential health benefits on cancer prevention by promoting vitamin D production. Of note, we have previously reported significantly increased risk of skin cancers with a dose–response relationship for the use of tanning beds in the same cohort (1), which further suggested an overall harmful effect of tanning bed use.

One limitation of our study is that we collected information on tanning bed use in the 2005 questionnaire cycle, which may introduce survival bias. To address this issue, we have conducted a secondary analysis restricted to the incident cancer-related cases diagnosed after report of tanning bed use in the 2005 questionnaire cycle; the results were very similar to those of our primary analysis using overall cases (multivariable-adjusted HR, 0.94; 95% CI, 0.86–1.03 vs. multivariable-adjusted HR, 0.99; 95% CI, 0.95–1.04 for every 4 times/y use).

In conclusion, our data do not suggest any association between the use of tanning beds and internal cancer risk. On the basis of the strong evidence of increase in risk of skin cancers (including melanoma) and no evidence of reduction in risk of internal cancers, it is important to warn the public against indoor tanning and increase regulations on the tanning industry.

No potential conflicts of interest were disclosed.

Conception and design: M. Zhang, D.J. Hunter, A.A. Qureshi, J. Han

Development of methodology: M. Zhang, F. Song, J. Han

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): M. Zhang, F. Song, D.J. Hunter, J. Han

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): M. Zhang, F. Song, D.J. Hunter, A.A. Qureshi, J. Han

Writing, review, and/or revision of the manuscript: M. Zhang, D.J. Hunter, A.A. Qureshi, J. Han

Administrative, technical, or material support (i.e., reporting or organizing data, constructing databases): M. Zhang, F. Song, D.J. Hunter, J. Han

Study supervision: M. Zhang, J. Han

The authors thank Dr. Walter Willett for his insightful comments and Tricia Li for her statistical and programming support. The authors thank the participants in the Nurses' Health Study II for their dedication and commitment. The authors also thank the participants and staff of the Nurses' Health Study II for their valuable contributions, as well as the following state cancer registries for their help: AL, AZ, AR, CA, CO, CT, DE, FL, GA, ID, IL, IN, IA, KY, LA, ME, MD, MA, MI, NE, NH, NJ, NY, NC, ND, OH, OK, OR, PA, RI, SC, TN, TX, VA, WA, WY.

The NHSII cohort is supported by NIH grant CA176726.