SAN ANTONIO – Women may reduce their risk of breast cancer by avoiding unnecessary medical radiation throughout life, avoiding the use of combined estrogen-progestin hormone therapy after menopause, avoiding smoking, limiting alcohol intake, and increasing physical activity level, according to a report by the Institute of Medicine’s Committee on Breast Cancer and the Environment.
The committee was convened in response to a request by Susan G. Komen for the Cure to review evidence on the contribution of environmental exposures to the development of breast cancer. "Environment" was broadly interpreted to include all nongenetic contributors to breast cancer development, from growth patterns to chemical and microbial exposures to social and cultural practices across the lifespan.
The most consistent evidence backs a link between breast cancer and combined hormone therapy, exposure to ionizing radiation, excess weight after menopause, and alcohol consumption. The evidence regarding smoking and breast cancer is less consistent, with some studies showing a causal relationship and others showing limited evidence of a relationship, according to the findings, which were reported during a press briefing held in conjunction with the San Antonio Breast Cancer Symposium.
Evidence is particularly conflicting with regard to physical activity, personal use of hair dyes, and exposure to non-ionizing radiation such as that emitted by microwave ovens and other electrical devices.
Possible associations with even less persuasive evidence include secondhand smoke exposure, nighttime shift work (possibly through disruptions to circadian rhythm), and exposures to benzene, ethylene oxide, and 1,3-butadiene. Exposure to bisphenol A (BPA) presents a "plausible hazard" for which little data exist.
In general, environmental factors found to have any possible link with breast cancer development were associated with less than a doubling of risk.
For an individual woman, the potential risk reduction from avoiding environmental factors would vary, and "may be small or may be moderate," committee chair, Irva Hertz-Picciotto, Ph.D., a professor at the school of medicine at the University of California, Davis, said during the press briefing.
Nonetheless, the impact of risk factor avoidance could be important at a population level, according to the report.
The IOM committee focused on initial breast cancer occurrence, taking into account changes in the breast over a woman’s lifetime, as well as the potential influence of the timing of certain exposures. Diagnosis, treatment, and screening practices were not addressed.
The committee analyzed evidence amassed by the International Agency for Research on Cancer, the World Cancer Research Fund International, and other authoritative organizations. Those data were supplemented by reviews and original research reports from the peer-reviewed literature.
Evidence reviewed primarily focused on exposure during adulthood so the committee was unable to address the effects of various exposures across the life course. Also, many chemical exposures have never been studied in regard to their association with breast cancer.
Furthermore, the contribution of genetic factors and potential gene-environment associations are difficult to assess, the committee conceded.
Topics considered high priority for further research include the role of shift work, endocrine activity, and genotoxicity. Furthermore, research is needed on the "biologic significance of life stages at which environmental risk factors are encountered, what steps may counter their effects, when preventive actions can be most effective, and whether opportunities for prevention can be found for the variety of forms of breast cancer," according to the report.
The report provides a number of strategies for counseling women about how to best prevent breast cancer, according to Dr. Robert Hiatt, a committee member and deputy director of the Comprehensive Cancer Center at the University of California, San Francisco. For example, doctors need to address with their patients the issue of ionizing radiation exposure.
The IOM report was supported by a contract between the National Academy of Sciences and Susan G. Komen for the Cure. Individual authors had no conflicts of interest, according to NAS protocols.