National Cancer Institute NCI Cancer Bulletin: A Trusted Source for Cancer Research News
December 13, 2011 • Volume 8 / Number 24

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Spotlight

Institute of Medicine Releases Report on Breast Cancer and the Environment

Breast Cancer and the Environment: A Life Course Approach cover The Institute of Medicine released a report on December 7 reviewing the current evidence on breast cancer and the environment.

On December 7, the Institute of Medicine (IOM) released a report called Breast Cancer and the Environment: A Life Course Approach at the 2011 San Antonio Breast Cancer Symposium. The scientific advisory board of the breast cancer organization Susan G. Komen for the Cure commissioned the report, which involved 20 months of research by a committee of 15 experts in the field.

The committee was asked "to review current evidence on the contribution of environmental exposures to breast cancer, either alone or in combination with genetic factors, and to review the challenges in conducting this kind of research," explained committee chair Dr. Irva Hertz-Picciotto of the University of California, Davis, at a press conference announcing the report's release. The Komen advisory board also asked the committee to explore "evidence-based actions that women could take to reduce their risk and to recommend research for the future," she continued.

The committee defined "environmental exposures" as all factors that are not directly inherited through DNA. It found strong evidence for several commonly identified risk factors for breast cancer, including ionizing radiation, estrogen-progestin hormone therapy in postmenopausal women, and being overweight after menopause. The conclusions also identified physical activity as potentially protective against breast cancer and outlined a set of actions women can take that may reduce their risk. 

"Some of the strongest evidence that the committee reviewed was the increased risk of breast cancer in association with ionizing radiation. Since medical diagnostic x-rays are an increasingly common source of radiation, especially in young women, the committee emphasized the importance of limiting exposure to diagnostic x-rays to situations of proven benefit and the monitoring of cumulative doses of radiation," said Dr. Barry Kramer, director of NCI's Division of Cancer Prevention and a member of the IOM committee.

Actions that May Reduce the Risk of Breast Cancer

  • Avoid inappropriate exposure to medical radiation.
  • Avoid combination menopausal hormone therapy, unless medically appropriate.
  • Avoid or stop smoking.
  • Limit or eliminate alcohol consumption.
  • Maintain or increase physical activity.
  • Maintain healthy weight or achieve healthy weight to reduce postmenopausal risk.
  • Limit or eliminate workplace, consumer, and environmental exposure to chemicals that are plausible contributors to breast cancer risk while considering risks of substitutes.
  • If at high risk of breast cancer, consider use of chemoprevention.

From Breast Cancer and the Environment: A Life Course Approach

The committee emphasized that the concern over medical radiation exposure mainly focuses on high-dose procedures, such as some types of CT scans. Other common medical procedures, such as mammograms and dental x-rays, use low doses of ionizing radiation, and the committee stressed that they are not recommending that women avoid these low-dose procedures when medically recommended.

"Evidence suggests that women may have substantial opportunities to reduce their risk of breast cancer…[though] the potential risk reduction that these actions could bring for any individual woman will vary, and they may be small or…moderate," said Dr. Picciotto.

Women also run the risk, explained the committee members, that an action taken to reduce the risk of breast cancer may inadvertently raise the risk of other disease. Some of these trade-offs are known; for example, the drug tamoxifen, which can decrease the risk of breast cancer, also increases the risk of endometrial cancer. Other trade-offs are not yet understood, such as whether avoiding exposure to a chemical suspected of having cancer-causing properties, such as bisphenol A (BPA), could lead to exposure to other chemicals with equal or greater risks.

Directions for Research

The report emphasizes that there are many challenges to studying breast cancer risk, including the possibility that some exposures that may influence risk occur early in a woman's life, even in utero. The report recommends examining exposures at all stages of life, not just during adulthood, as well as taking a multidisciplinary approach to studies of breast cancer risk. It also emphasizes developing new methods for epidemiologic research, for accurately assessing exposure to potential risk factors, and for testing the cancer risk of chemicals and other substances.

Because data from human studies are often unavailable or inconclusive, the committee did not find strong evidence linking many suspected chemicals to breast cancer risk; only exposures to benzene, 1,3-butadiene, and ethylene oxide had "possible association[s]" with increased breast cancer risk. Several other chemicals, including BPA, had "biological plausibility as a hazard," but no human studies have confirmed an association with cancer risk.

"Traditionally, many of the cancer bioassays that have been done [in animal models]…started exposures early in life, but not at a very young age nor in utero or during the major period of breast growth," said Dr. David Eaton, a committee member from the University of Washington. "More modern-day testing…does take into account the life stage, but it's important to recognize that many of the chemicals tested in the past did not include early-life exposures that might be important."

"Some … vulnerabilities to environmental exposures only occur in very early periods of life, like during development," agreed committee member Dr. Cheryl Lyn Walker from Texas A&M Health Science Center.

"If we think about the ideal study, it would be to develop a cohort of individuals from birth, and follow them across their [lives]," added Dr. Robert Hiatt, a committee member from the University of California, San Francisco. Since such cohort studies are unlikely to be done for cost and other reasons, better integration between animal models and human epidemiologic studies are needed, he said.

Other Advisory Efforts Under Way

The National Institute of Environmental Health Sciences (NIEHS), in collaboration with NCI, is also examining future directions for research on environmental and genetic factors related to breast cancer. In September 2010, the congressionally mandated 19-member Interagency Breast Cancer and Environmental Research Coordinating Committee (IBCERCC) began to review all breast cancer research efforts conducted or supported by federal agencies.

"Both [the IOM and IBCERCC] reports will include a review of the state of the science regarding breast cancer and the environment and recommendations for future research, but the IBCERCC was called upon to make recommendations about how to coordinate and facilitate research efforts on environmental factors and breast cancer risks across federal agencies and with other research partners," explained Dr. Debbie Winn, deputy director of NCI's Division of Cancer Control and Population Sciences, who is leading NCI's effort on the IBCERCC.

"We are thoroughly examining prior reports, especially authoritative reviews and evaluations such as this IOM report, as we develop our recommendations," she added. The IBCERCC hopes to release its results by early summer 2012.

Sharon Reynolds

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