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Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2
similar; the ERR/Gy for external exposures appears to be lower, but this result is based on only four cases exposed.
Incidence rates for stomach cancer vary considerably throughout the world, with particularly high rates in Japan. Many countries have seen decreases in incidence and mortality over the past 50 years or so, believed in large part to be due to healthier diets with increased fruits and vegetables and less salt.
Of all the studies reviewed on medical uses of radiation, five provide dose-specific estimates of ERR and/or EAR. Table 7-6 and Figure 7-6 summarize the results from these studies. In the figure, results are shown for all studies as well as restricted to studies in which the average dose to the active bone marrow was less than 1 Gy.
Among the studies of populations with external radiation exposure and/or 226Ra, the estimates of ERR/Gy range from negative (in the hemangioma study) to 1.3 Gy−1 in the study of benign breast disease. The confidence intervals are wide, and they all overlap, indicating that these estimates are statistically compatible. An ERR of 1.32 Gy−1 (not significantly different from zero) was seen among patients treated for hyperthyroidism with 131I.
Radiation and Circulatory Diseases
Although radiation exposure is well established as a risk factor for cancer, a clear understanding of the relationship between radiation exposure and other diseases is lacking. It has been postulated that the cardiovascular system is resistant to radiation-induced injury (Stewart and others 1995). However, it appears that tissue damage may occur as a result of both therapeutic (Stewart and Fajardo 1984) and A-bomb radiation exposure (Villeneuve and Morrison 1997; Shimizu and others 1999). Capillaries represent the most radiosensitive component of the cardiovascular system, with characteristic changes including detachment of endothelial cells and thrombosis. Arterial changes resulting from radiation exposure depend on vessel size, with small and medium-sized arteries undergoing changes in all vessel layers, and large arteries appearing to be relatively radioresistant, although radiation exposure may predispose larger vessels to the development of atherosclerosis (Louis and others 1974).
Radiation exposure has also been implicated in the development of cerebrovascular injury (O’Connor and Mayberg 2000). Specific conditions postulated to arise from irradiation include vasculopathy, intracranial aneurysm formation, cerebral radiation necrosis, intracranial atherosclerosis, and stroke (Trivedi and Hannan 2004).
Both animal and human studies have identified intimal thickening, lipid deposition, and adventitial fibroses of the
TABLE 7-6 Risk Estimates for Cancer Incidence and Mortality from Studies of Radiation Exposure: Stomach Cancer
NOTE: The number of cases and controls (or population size in cohort studies) as well as the mean dose and range relate to exposed persons only. Empty cells indicate data not available from publication. LB = lower bound; UB = upper bound of CI.