來源: Cancer Prevention Overview (PDQ®) - National Cancer Institute
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The major sources of population exposure to ionizing radiation are medical radiation (including x-rays, computed tomography [CT], fluoroscopy, and nuclear medicine) and naturally occurring radon gas in the basements of homes. Limiting unnecessary CT scans and other diagnostic studies, as well as reducing radiation exposure doses, are important prevention strategies.[7,8] (Refer to the PDQ summaries on Breast Cancer Prevention; Breast Cancer Screening; Skin Cancer Prevention; and Lung Cancer Prevention for more information.)
Exposure to ionizing radiation has increased during the last 2 decades as a result of the dramatic increase in the use of CT. Exposure to ionizing radiation associated with CT is in the range where carcinogenesis has been demonstrated.[9,10] Repeat exposure to radiation from medical imaging will further increase cancer risk, because risk is proportional to exposure. One study found that half the subjects who were exposed to radiation from medical imaging underwent repeat imaging within 3 years. Overall, 0.2% of the nearly 1 million subjects followed for 3 years received doses above 50 mSv.[11]
One approach to estimate the potential contribution of exposure to ionizing radiation from medical imaging is to develop statistical models based on the estimated cancer risks associated with a range of dose levels. For example, one estimate of the CT scans performed in the United States in 2007 predicted that 29,000 (95% uncertainty limits of 15,000–45,000) cancers might result in the future. One-third of the projected cancers were caused by CT scans done on individuals aged 35 to 54 years. This estimate was derived from risk models based on organ-specific radiation doses from national surveys, frequency of CT scans in 2007 by age and sex from survey and insurance claim data, and the National Research Council’s "Biological Effects of Ionizing Radiation” report.[9]
Data are now emerging from studies large enough to directly estimate the cancer risk associated with CT scans. For example, in a cohort of 10.9 million Australians, electronic medical records were used to document the diagnostic CT scans of youths who received the CT scans when they were aged 0 to 19 years. This cohort was then linked to the National Death Index and Australian Cancer Database.[12] Compared with those who did not have a CT scan, those who had at least one CT scan were statistically significantly more likely to be diagnosed with cancer as they were followed into young adulthood (RR = 1.24; 95% confidence interval [CI], 1.20–1.29; average follow-up in those who had a CT was 9.5 years). A statistically significant dose-response relationship was observed, with cancer risk increasing with each additional CT scan. Thus, the findings of cohort studies with directly measured CT scans now substantiate the statistical models and document the real-world cancer risks associated with exposure to ionizing radiation via medical imaging.
