Ultraviolet Radiation Exposure and Risk of Malignant Lymphomas

The incidence of malignant lymphomas has been increasing rapidly, but the causes of these malignancies remain poorly understood. One hypothesis holds that exposure to ultraviolet (UV) radiation increases lymphoma risk. We tested this hypothesis in a population-based casecontrol study in Denmark and Sweden. Methods: A total of 3740 patients diagnosed between October 1, 1999, and August 30, 2002, with incident malignant lymphomas, including non-Hodgkin lymphoma, chronic lymphocytic leukemia, and Hodgkin lymphoma, and 3187 population controls provided detailed information on history of UV exposure and skin cancer and information on other possible risk factors for lymphomas. Odds ratios (ORs) with 95% confi dence intervals (CIs) were calculated by logistic regression. Statistical tests were two-sided. Results: Multivariable-adjusted analyses revealed consistent, statistically signifi cant negative associations between various measures of UV light exposure and risk of non-Hodgkin lymphoma. A high frequency of sun bathing and sunburns at age 20 years and 5 -10 years before the interview and sun vacations abroad were associated with 30% -40% reduced risks of non-Hodgkin lymphoma (e.g., for sunbathing four times a week or more at age 20 versus never sunbathing, OR = 0.7, 95% CI = 0.6 to 0.9; for two or more sunburns a year at age 20 versus no sunburns, OR = 0.6, 95% CI = 0.5 to 0.8). These inverse associations increased in strength with increasing levels of exposure (all P trend .01). Similar, albeit weaker, associations were observed for Hodgkin lymphoma. There were no clear differences among non-Hodgkin lymphoma subtypes, although associations were stronger for B-cell than for T-cell lymphomas. A history of skin cancer was associated with a doubling in risks of both non-Hodgkin and Hodgkin lymphoma. Conclusions: A history of high UV exposure was associated with reduced risk of non-Hodgkin lymphoma. The positive association between skin cancer and malignant lymphomas is, therefore, unlikely to be mediated by UV exposure. J Natl Cancer Inst 2005;97:199 -209 More than 10 years ago, Zheng et al. ( 1 ) hypothesized that the increasing incidence of non-Hodgkin lymphoma, observed worldwide through decades, may be due, in part, to an increasing trend in ambient ultraviolet (UV) radiation levels and sun exposure habits. Since then, many studies have provided indirect support for this hypothesis, although others have not ( 2 , 3 ) . The supportive evidence includes observations of increased risks of non-Hodgkin lymphoma and chronic lymphocytic leukemia after diagnoses of UV-related skin cancers and, conversely, an increased risk of skin cancer after a diagnosis of lymphoma ( 4 -15 ) . In addition, ecologic studies have demonstrated parallel time trends