Ionizing Radiation

“More is known about the relationship between radiation dose and cancer risk than any other human carcinogen, and female breast cancer is the best quantified radiation-related cancer.”    –– Charles E. Land

Overview and Mechanisms

Ionizing radiation is any form of radiation with enough energy to break off electrons from atoms (to ionize the atoms). This radiation can break the chemical bonds in molecules, including DNA molecules, thereby disturbing their normal functioning. X-rays and gamma rays are the only major forms of radiation with sufficient energy to penetrate and damage body tissue below the surface of the skin.

Among the many sources of ionizing radiation are traditional X-rays, computed tomography (CT) scans, fluoroscopy and other medical radiological procedures. Sources of gamma rays include emissions from nuclear power plants, scientific research involving radionuclides, military weapons testing and nuclear medicine procedures such as bone, thyroid and lung scans.

In 2005, the National Toxicology Program classified X-radiation and gamma radiation as known human carcinogens. There is no such thing as a safe dose of radiation. A 2005 National Research Council report confirms this finding in stating, “the risk of cancer proceeds in a linear fashion at lower doses of ionizing radiation without a threshold and that the smallest dose has the potential to cause a small increase in risk to humans.” Radiation damage to genes is cumulative over a lifetime. Repeated low-dose exposures over time may have the same harmful effects as a single high-dose exposure.

Exposure to ionizing radiation is the best- and longest-established environmental cause of human breast cancer in both women and men. Ionizing radiation can increase the risk for breast cancer through a number of different mechanisms, including direct mutagenesis (causing changes in the structure of DNA), genomic instability (increasing the rate of changes in chromosomes, therefore increasing the likelihood of future mutations) and changes in breast cell micro-environments that can lead to damaged regulation of cell-cell interactions within the breast. Ionizing radiation not only affects cells that are directly exposed, but it can also alter the DNA, cell growth and cell-cell interactions of neighboring cells, referred to as the “bystander effect.”

Interactions Between Radiation and Other Factors

There are a number of factors that may interact with radiation to increase the potency of its carcinogenic effect. Some of these factors include a woman’s age at exposure, genetic profile and possibly estrogen levels. As examples:

• It has been well established in a number of studies of women exposed to military, accidental or medical sources of radiation that children and adolescents who are exposed are more seriously affected in their later risk for breast cancer than are older women. 
• Recent genetic data indicate that women with some gene mutations (e.g., ATM, TP53 and BRCA1/2) are more likely to develop breast cancer and may be especially susceptible to the cancer inducing effects of exposures to ionizing radiation.
• Studies using animal and in vitro human breast tumor cell culture models have demonstrated that the effects of radiation on mammary carcinogenesis may be additive with effects of estrogens. This is of particular concern given the widespread exposure to estrogen mimicking chemicals in our environment and the multiple sources of ionizing radiation.

Evidence Linking Ionizing Radiation and Breast Cancer Risk

The link between radiation exposure and breast cancer has been demonstrated in atomic bomb survivors. Rates of breast cancer were highest among women who were younger than age 20 when the United States dropped atomic bombs on Hiroshima and Nagasaki. In addition, scientists reported a significant association between ionizing radiation exposure and the incidence of male breast cancer in Japanese atomic bomb survivors.

Use of X-rays to examine the spine, heart, lungs, ribs, shoulders and esophagus also exposes parts of the breast to radiation. X-rays and fluoroscopy of infants irradiate the whole body. Decades of research have confirmed the link between radiation and breast cancer in women who were irradiated for many different medical conditions, including tuberculosis, benign breast disease, acute postpartum mastitis, enlarged thymus, skin hemangiomas, scoliosis, Hodgkin’s disease, non-Hodgkin’s lymphoma, and even treatment for acne. Again, evidence from almost all conditions suggests that exposure to ionizing radiation during childhood and adolescence is particularly dangerous with respect to increased risk for breast cancer later in life.

A recent study of female radiology technologists who had sustained daily exposures to ionizing radiation demonstrated an increased risk of breast cancer for those women who began working during their teens or, independent of age, working in the field before the 1940s, when exposure levels were substantially higher than they have been in more recent decades. And a recent review and analysis of all existing related studies found that women who work as airline flight attendants had increased levels of breast cancer. Factors that could explain this increase may include lifestyle and reproductive histories, as well as increased exposures to cosmic (atmospheric) ionizing radiation.

Medical Radiation: Risks and Benefits

There is credible evidence that medical X-rays (including mammography, fluoroscopy and CT scans) are an important and controllable cause of breast cancer. Although X-rays have been a valuable diagnostic tool for more than a century, the radiation dose has not always been carefully controlled and sometimes has been higher than needed to obtain high quality images. Fortunately, the dose given per X-ray has been drastically reduced over the past several decades and the regulatory oversight of equipment and personnel has increased. In mammography, for example, efforts to reduce the radiation dose to as low as reasonably achievable (ALARA) levels have lowered the radiation dose from an estimated two rads in 1976 to 0.2 rads today, without compromising image quality. Digital mammography can yield doses that are one-third those of conventional mammography.

Patients who ask about the radiation dose involved in any medical procedure are sometimes dismissed with an answer that the dose is similar to the exposure one would get in a cross-country plane flight. This is seldom true, however. An average radiation dose of one rad (or centigray) to the breast is equivalent to the breast irradiation received during about 3,300 hours of flying. Thus, a typical mammogram of 0.2 rads would equal the radiation dose received by the breast in 660 hours of flying, not a single trip.

Although there has been a significant decrease in exposures to ionizing radiation from individual X-rays, the introduction of CT scans in the 1970s greatly increased the radiation dose per medical examination. According to the National Cancer Institute, CT scans “comprise about 10 percent of diagnostic radiological procedures in large U.S. hospitals,” but contribute an estimated 65 percent of the effective radiation dose to the public from all medical X-ray examinations.

Some studies suggest that doctors and patients should carefully evaluate the risks and benefits of radiation therapy for survivors of early breast cancer, particularly older women. Women older than age 55 derive less benefit from radiation therapy in terms of reduced rate of local recurrence and may face increased risks of radiation-induced cardiovascular complications, as well as secondary cancers such as leukemias and cancers of the lung, esophagus, stomach and breast. Using SEER data from the National Cancer Institute, researchers showed a 16-fold increased relative risk of angiosarcoma of the breast and chest wall following irradiation to a primary breast cancer.

Policy and research recommendations for reducing exposure to radiation »

For references, see State of the Evidence 2008.