Page 10 Complete Your CE Test Online - Click Here gamma rays, and other forms of high-energy radiation. Ionizing radiation is defined as radiation with enough energy to remove tightly bound electrons from their orbits causing atoms to become charged or ionized. Ions formed in the molecules of living cells can go on to react with and potentially damage other atoms in the cell. At low doses (e.g., those associated with background radiation and limited X-rays), the cells repair the damage rapidly. At moderate doses, the cells may be permanently mutated or die from their inability to repair the damage. Cells that are damaged and unrepaired but that do not die may go on to produce abnormal cells when they divide. In some circumstances, these altered cells may become cancerous or lead to other abnormalities (e.g., birth defects). Defects in the ability to repair damage caused by ionizing radiation may influence the how much radiation exposure increases cancer risk. There is extensive evidence linking exposure to ionizing radiation with the development of cancer. In particular, ionizing radiation is linked to cancer that involves the hematological system, breast, lungs, and thyroid. The National Research Council of the National Academies Committee to Assess the Health Risks from Exposure to Low Levels of Ionizing Radiation put out the Biologic Effects of Ionizing Radiation VII, which is the most widely cited source on the topic. In this report, three lines of evidence were cited documenting the links between ionizing radiation exposure and cancer: 1. The first line of evidence comes from studies of the development of cancer among Japanese atomic-bomb survivors. Even survivors who were exposed to lower doses of radiation were at a significantly increased risk of developing cancer. 2. The second line of evidence comes from epidemiological studies of medically irradiated populations (in which people were medically irradiated for both malignant and benign diseases). Following high- dose radiation therapy for malignant disease, the risk of secondary malignancy is relatively high. The relatively common use of radiation for benign disease between 1940 and 1960 resulted in a substantial relative risk (RR) of developing cancer. 3. The third line of evidence comes from an increased risk of cancer- specific mortality associated with exposure to medical ionizing radiation, for both the recipients of diagnostic X-rays and X-ray personnel. This is one reason that public health interests have worked to lessen the radiation levels for many diagnostic X-rays over the years (with some exceptions), and why radiology staff wear dosimeters to monitor actual exposure to radiation. The report concluded, after a comprehensive review of the medical literature, that no dose of radiation should be considered completely safe, and attempts should be made to keep radiation doses as low as possible. Of note, there are some experts who believe that there might be very low radiation exposure levels (thresholds) below which no harm might be done. It is difficult to measure long-term effects from very low radiation exposures, and it would be unethical to do clinical trials in which people are deliberately exposed to low levels that could possibly cause harm. However, scientific consensus has established the Linear No-Threshold (LNT) model as the most practical one to guide policy and safety measures regarding ionizing radiation exposures. The LNT model assumes that even very low levels of ionizing radiation might cause harm and that the dose relationship is linear: simply stated, the higher the exposure, the higher the risk. The major sources of population exposure to ionizing radiation are medical radiation (including computed tomography [CT], fluoroscopy, nuclear medicine, and regular X-rays) and naturally occurring radon gas in the basements of homes (see “Radon”). Limiting unnecessary CT scans and other radiation-based diagnostic studies as well as reducing radiation exposure doses for these studies are important prevention strategies. (See Table 3.) Exposure to ionizing radiation has increased during the last two decades as a result of the dramatic increase in the use of CT scanning. The National Cancer Institute reports that exposure to ionizing radiation associated with CT is in the range where carcinogenesis has been demonstrated. Repeat exposure to radiation from medical imaging (mainly CT scans, but angiography and a few other techniques can use as much or more radiation) further increases cancer risk, assuming that risk is proportional to exposure. One study found that half the subjects who were exposed to radiation from medical imaging underwent repeat imaging within three years. Overall, 0.2% of the nearly one million subjects followed for three years received doses above 50 millisieverts (mSv). For comparison, the International Commission on Radiological Protection recommends a limit of 50mSv per year for people who work with artificial ionizing radiation, with a five-consecutive-year maximum of 100mSv. For reference, 1mSv is 1/1000 of a sievert, which is an International System of Units measure for equivalent dose – a measure of biological damage to living tissue from radiation exposure. The U.S. measures often use the rem as the basic unit of equivalent dose: 100 rems equals one sievert; a millirem (mrem) is 1/1000 of a rem; and 1mSv is 100 mrems. Table 3. Comparative exposure to ionizing radiation Exposure or procedure Estimated exposure for an adult* Bone densitometry. 0.001mSv Air travel per 1,000 miles flown. 0.01mSv Spine X-ray. 1.5mSv Head CT. 2mSv Background radiation, average yearly U.S. exposure.** 3.1mSv CT scan of abdomen and pelvis. 10mSv PET (positron emission tomography)/ CT scan. 25mSv Radiation therapy for cancer treatment. 20,000–60,000mSv *These are typical for average-sized adults but will vary based on size and practices. **This will be somewhat lower for people at sea level and higher at higher altitudes; it also varies by radon emission from the ground. This total adds the average cosmic radiation and radon gas exposures. 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 that looked at CT scans performed in the U.S. in 2007 predicted that 29,000 (95% confidence interval: 15,000 to 45,000) cancers might result. 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 Biologic Effects of Ionizing Radiation VII report. 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. 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, 1.20 to 1.29; average follow-up in those who had a CT was 9.5 years). A 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. Evidence-Based Practice: Biphasic and repeat CT scanning can increase a person’s cancer risk over time. Although CT may be an essential part of follow-up monitoring in many cancer patients, it should be kept to a minimum, especially in younger people. MRIs and lower radiation scans might be acceptable substitutes in some cases.