FOUND IN: Medical radiological procedures, such as X-rays, CT scans, fluoroscopy.
THE VERDICT: Evidence from studies of medical exposures to radiation as well as large-scale tragedies such as the atomic bomb in Japan have demonstrated that radiation can cause cancer.
Ionizing radiation is the emission of energetic particles (alpha, beta, neutron) or rays (gamma and x-rays) from a radioactive isotope–also called a ra-dionuclide. These emissions may knock off an elec-tron in its target, thus resulting in ionization. When something absorbs the energy of the ray or the particle, irradiation occurs. When a living being absorbs it, that individual has received a “dose” of radiation.
Curies, Rads, and Rems
The pioneers of the Nuclear Age invented units for measuring radioactivity. The measure of radio – ac-tive decay–the curie (named for Madame Marie Curie)–is the count, per second, of radioactive emissions, also called “disintegrations.” One curie is that amount of a radioactive material that gives off 37 billion radioactive particles or rays per second. This unit is a fixed standard, and concentrations in curies (or fractions of a curie) per gram or per liter, and per second or per minute, can be verified with proper instrumentation. Translating the curie amount into a potential dose to a living organism is far from precise.
Unlike the curie, which has a clear definition, the units for estimating impacts of radiation on living tissues–rads, rems and millirems–are based on models and assumptions. Estimates of the biological impacts of exposure to specific types of radiation have been based on animal experiments and on a limited number of human experiments. Most estimates of dose are based on data collected from the survivors of the Hiroshima and Nagasaki bombs, even when the given situation is different.
The Rad is used to measure the energy absorbed by tissue that is exposed to radioactivity. In Europe the unit for 100 Rads is called a Gray. The Rem combines the amount of radiation exposure (Rad) with its alleged impact on health. The estimated damage or “biological effectiveness” of the radiation is based on models. In Europe the unit for 100 Rems is called a Sievert. The prefix, “milli,” denotes one thousandth of a unit. For example: one rem equals 1,000 millirems.
The Rem (the unit of radiation dose) is not based upon a standard unit that can be verified. One must know the duration of exposure, amount and type of radioactivity involved, the size of the body that ab-sorbed it, and what that radiation event did to the particular body in question. Even under very controlled conditions like medical uses, it is virtually impossible to derive each of these data points with and degree of certainty.
State of the Evidence on Ionizing Radiation
Ionizing radiation is any form of radiation with enough energy to break off electrons from atoms (that is, 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 common 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 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. Most scientists agree that there is no such thing as a safe dose of radiation (Brenner, 2003; NRPB, 1995). A 2006 National Research Council report confirms this finding, stating that “the risk of cancer proceeds in a linear fashion at lower doses [of ionizing radiation] without a threshold and … the smallest dose has the potential to cause a small increase in risk to humans” (NRC, 2005). Radiation damage to genes is cumulative over a lifetime (Boice, 2001). Repeated low-dose exposures over time may have the same harmful effects as a single high-dose exposure.
Exposure to ionizing radiation is the longest-established and most firmly 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) (Broeks, 2010: Goldberg, 2003; Morgan, 2003; Wright, 2004), and changes in breast cell microenvironments that can lead to damaged regulation of cell-to-cell communication within the breast (Barcellos-Hoff, 2005; Tsai, 2005). Ionizing radiation not only affects cells that are directly exposed, but can also alter the DNA, growth, and cell-to-cell interactions of neighboring cells, a phenomenon referred to as the “bystander effect” (Little, 2003; Murray, 2007b).
Interactions between Ionizing 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 person’s age at exposure, their genetic profile, and possibly a woman’s estrogen levels. As examples:
a. It has been well established in a number of studies of women exposed to military, accidental or medical sources of radiation that exposure in children and adolescents confers greater increased risk than exposure in older women (Boice, 2001).
b. Recent genetic data indicate that women with some gene mutations (such as 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 (Andrieu, 2006; Berrington de Gonzales, 2009a; Pepe, 2012; Turnbull, 2006).
c. Studies using animal tumor cells and in vitro human breast tumor cells have demonstrated that the effects of radiation on mammary carcinogenesis may be additive with effects of estrogens (Calaf, 2000; Imaoka, 2009; Segaloff, 1971). This is of particular concern given the widespread exposure to estrogen-mimicking chemicals (xenoestrogens) in our environment and the multiple sources of ionizing radiation. In a mouse model, radiation exposure increased blood serum estradiol levels and estrogen associated activation of cell-proliferation pathways (Suman, 2012).
Evidence Linking Ionizing Radiation and Cancer
The link between radiation exposure and cancer has been demonstrated in atomic bomb survivors (Goto, 2012; Land, 1995; Pierce, 1996; Tokunaga, 1994). Rates of breast cancer were highest among women in Hiroshima and Nagasaki who were younger than age 15 when the United States dropped atomic bombs there (Land, 1998). Recent analysis of tumor subtypes and tissue DNA from survivors of the atomic bombs indicate that radiation-associated breast tumors are quite aggressive and are associated with increased levels of genomic instability (too many genes, mutations or incomplete replication of genes, etc.), a trait that has been associated with the development of cancer (Oikawa, 2011). In addition, scientists reported a statistically significant association between ionizing radiation exposure and the incidence of male breast cancer in Japanese atomic bomb survivors (Ron, 2005).
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 (Gofman, 1996). Decades of research has confirmed the link between radiation and breast cancer in women who were irradiated for many different medical conditions, including tuberculosis (MacKenzie, 1965), benign breast disease (Golubicic, 2008; Mattson, 1995), acute postpartum mastitis (Shore, 1986), enlarged thymus (Adams, 2010; Hildreth, 1989), skin hemangiomas (Lundell, 1999), scoliosis (Morin-Doody, 2000), Hodgkin’s disease (Bhatia, 2003; Guibout, 2005; Horwich, 2004; Wahner-Roedler, 2004), non-Hodgkin’s lymphoma (Tward, 2006) and acne (El-Gamal, 2006). A dose-response relationship (meaning a higher dose of radiation is related to a higher incidence of breast cancer) was found in women who had been treated with X-rays and who had a family history of breast cancer (Ronckers, 2008). Evidence from almost all conditions suggests that exposure to ionizing radiation during childhood and adolescence is particularly dangerous with respect to significantly increased risk for breast cancer later in life (John, 2007).
Female radiology technologists who had sustained daily exposures to ionizing radiation demonstrated an increased risk of breast cancer if they began working during their teens or, independent of age, worked in the field before the 1940s, when exposure levels were substantially higher than they have been in more recent decades (Doody, 2006; Simon, 2006). The susceptibility of radiologists to later diagnosis of breast cancer may be affected by common variants in particular genes that are involved in the metabolism of circulating estrogens (Sigurdson, 2009). A 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 (Ballard, 2000).
Medical Radiation: Risks and Benefits
Computed tomography (CT) Scans
There is considerable evidence that medical X-rays, which include mammography, fluoroscopy and computed tomography (CT) scans are an important and controllable cause of breast cancer (Gofman, 1999; Ma, 2008). Although there has been a substantial decrease in exposures to ionizing radiation from individual X-rays over the past several decades, there has been a sixfold increase in exposure to medical sources of radiation from the mid-1980s through 2007, with an annual increase of 16 percent, primarily arising from the increased use of CT scans and nuclear medicine (Larson, 2011; Linet, 2012). In 2007, approximately 72 million CT scans were conducted in the United States (Berrington de Gonzales, 2009b). When a CT scan is directed to the chest, the individual receives the equivalent radiation of 30 to 442 or more chest X-rays (Redberg, 2009). Recent modeling estimates that use of chest CTs and CT angiography in 2007 alone will lead to an additional 5,300 cases of lung and breast cancer within the next two to three decades (Berrington de Gonzales, 2009b). Other modeling suggests that 1 in 150 women who are 20 years old when they undergo CT angiograms of the chest, and 1 in 270 women (of all ages) having the procedure, will subsequently develop cancers of the chest, including breast cancer (Smith-Bindman, 2009).
Recent modeling of the long-term effects of cardiac CT angiography, a source of comparably high radiation to the chest, demonstrates a statistically significant increase in risk for breast cancer, especially in pre-menopausal women (Huda, 2011).
Damage from lower-energy sources of X-rays, including those used in mammography, cannot be predicted by estimating risk from models based on higher doses (Heyes, 2009; Millikan, 2005). Recent evidence indicates that the lower-energy X-rays provided by mammography result in substantially greater damage to DNA than would be predicted by these models. Evidence also suggests that risk of breast cancer caused by exposure to mammography radiation may be greatly underestimated (Heyes, 2009).
As with other risk factors for breast cancer, evidence indicates that both age at exposure and the individual’s genetic profile influence the degree of increased risk for disease in women exposed to multiple mammograms. For example, women who had multiple mammograms more than five years prior to diagnosis had an increased risk for breast cancer (Ma, 2008).
This age effect is of particular concern, since it is often recommended that high-risk women, including those with either of the BRCA mutations, foolishly begin annual mammography screening at ages 25 to 30. Further complicating this age-related finding are the data now demonstrating that young women with the very mutations that lead them to begin mammography screenings at earlier ages are actually more vulnerable to the cancer-inducing effects of early and repeated exposures to mammograms. This increased vulnerability has been found in women with BRCA mutations (Berrington de Gonzales, 2009a; Jansen-Van der Weide, 2009) as well as in women with other relatively uncommon variations in genes known to be involved in the process of DNA repair (Millikan, 2005). A recent study found that diagnostic radiation exposure before age 30 increased risk of breast cancer in a dose-dependent manner among women with BRCA mutations (Pijpe, 2012).
The detrimental risks from mammography might also be heightened in older women, whose breast epithelial cells have gone through several decades of cell division. Cells derived from older women’s breast tissue were more sensitive to the DNA-damaging effects of low-energy radiation, increasing the likelihood of later conversion to cancerous cells (Soler, 2009).
In 2009 the U.S. Preventive Services Task Force recommended against the use of routine mammography screening for women under 50 (Nelson, 2009; USPSTF, 2009) and recommended that women 50 to 75 get screened every two years. The Task Force concluded that for women 40 to 49 the benefits of mammograms do not outweigh the harms, which include false-positive results that lead to unneeded breast biopsies and follow up-imaging, and to unnecessary anxiety and distress. Also, the Task Force found that mammograms play an extremely modest role in reducing the likelihood of dying from breast cancer. Among women 40 to 49, who tend to have low rates of breast cancer to begin with, the procedure is responsible for saving very few lives. As women are now facing the need to make their own decisions about whether to undergo routine screening mammography, it is critical that both physicians and women are better educated about mammography’s harms, along with its unproven, perceived potential benefits (Gotzsche, 2009; Jansen-van der Weide, 2010).
Some studies suggest that doctors and patients should carefully evaluate the risks and benefits of radiation therapy for survivors of early-stage breast cancer, particularly older women. Women older than 55 derive less benefit from radiation therapy in terms of reduced rate of local recurrence (Veronesi, 1999) and may face increased risks of radiation-induced cardiovascular complications (EBGTCG, 2000), as well as secondary cancers such as leukemias and cancers of the lung, esophagus, stomach and breast (Mellemkjaer, 2006; Roychoudhuri, 2004). Using the National Cancer Institute’s Surveillance, Epidemiology and End Results (SEER) data, researchers showed a 16-fold increased relative risk of angiosarcoma of the breast and chest wall following irradiation of a primary breast cancer (Huang, 2001).
More recent data indicate that women younger than 45 who received the higher radiation exposure associated with post-lumpectomy radiotherapy (as compared to post-mastectomy radiation) had a 1.5-fold to 2.5-fold increase in later contralateral breast cancer diagnoses. This effect was especially prominent in younger women with a family history of breast cancer (Hooning, 2007; Ng, 2009; Stovall, 2008).
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