Breast cancer is the number one cancer diagnosis for women in the United States. While progress has been made in the understanding of genetic and lifestyle factors that influence the risk for this common type of cancer, much work remains. Exposure to solar ultraviolet (UV) radiation, is recognized as a possible lifestyle factor that may modify breast cancer risk. However, evidence on the association between ambient UV radiation and breast cancer from epidemiological studies is mixed. While some studies have shown an inverse association, others have found no relationship. Endogenous production of vitamin D is a hypothesized mechanism through which UV radiation exposure may influence breast cancer risk. A potential partial explanation for the inconsistencies in previous study findings may be related to the challenges in accurately estimating sun exposure over a long period of time. New work that utilized a higher resolution UV exposure model from Dr. Trang VoPham, assistant member in the Division of Public Health Sciences, and colleagues was recently published in the journal Environmental Epidemiology.
The authors used an improved method to assess historical UV exposure. Dr. VoPham explained, “We were able to combine a recently developed UV exposure model with residential addresses collected every two years since 1989 among a group of over 100,000 US female nurses. This UV exposure model predicts UV levels across the US over time at a high spatial and temporal resolution (see Figure).” Although this is a relatively new tool to be used in modeling UV exposure, it has already received global recognition, “…this UV model was highlighted as new innovative science to study the effects of UV on human health by the United Nations Environment Programme,” said Dr. VoPham.
Study participants were followed from baseline (1989) through 2013. Invasive breast cancer cases were assessed via biennial questionnaires through which the participants self-reported. Subtypes of breast cancer cases were assessed through additional analyses of breast tissue samples to determine hormone receptor status. The primary analysis relied on UV exposure modeling for the years between baseline and 2013. For secondary analyses, the authors also assessed early life UV radiation exposure based on self-reported state of residence at birth and at 15 and 30 years of age.
Dr. VoPham summarized the major results from the study, “This study showed that higher exposure to ultraviolet (UV) radiation, which is associated with circulating levels of vitamin D, during adulthood is not associated with the risk of developing breast cancer. However, higher UV exposure earlier in life (such as at age 30) may be associated with a lower risk of developing a particular subtype of breast cancer called estrogen receptor-negative (or ER-) breast cancer.” Additional subgroup analyses did not identify any significant associations between UV exposure in adulthood and breast cancer risk by menopausal status, body mass index, physical activity, race, total vitamin D intake, and several additional factors that influence UV exposure.
While this work shines new light on understanding the potential association between long-term solar UV exposure and breast cancer risk in females, it also provokes new research questions. Dr. VoPham described plans for the next steps, “These findings warrant further investigation to understand how UV exposure during early life, which represents a critical period for breast development, may be more relevant to breast cancer risk compared to exposure later in life. In particular, future research could try to better estimate UV exposure levels by incorporating fine-scale information regarding exactly where and how much time individuals spend at home, work, and other locations.” Dr. VoPham also indicated that future work should be expanded to studying these research questions in minority populations as the current study was conducted in a predominantly white population.
This work was supported by the Susan G. Komen Foundation and the National Institutes of Health.
Fred Hutch/UW Cancer Consortium member Dr. Trang VoPham contributed to this research.
VoPham T, Bertrand KA, DuPre NC, James P, Vieira VM, Tamimi RM, Laden F, Hart JE. 2019. Ultraviolet radiation exposure and breast cancer risk in the Nurses’ Health Study II. Environmental Epidemiology. doi: 10.1097/EE9.0000000000000057.
VoPHam T, Hart JE, Bertrand KA, Sun Z, Tamimi RM, Laden F. Spatiotemporal exposure modeling of ambient erythemal ultraviolet radiation. Environ Health. doi: 10.1186/s12940-016-0197-x.