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Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2
estimation if (1) the study design was adequate (see Chapter 5 concerning informative study designs and limitations); (2) individual quantitative estimates of radiation dose to the organ of interest were available for the study subjects; (3) if so, the details of the dose reconstruction approach were evaluated; and (4) a quantitative estimate of disease risk in relation to radiation dose—in the form of an estimated relative risk (ERR) or excess absolute risk (EAR) per gray—was provided.
Overall, more than 100 studies of patients receiving diagnostic or therapeutic radiation have evaluated the association between exposure to radiation and risk of cancer at multiple sites (IARC 2000; UNSCEAR 2000b). Studies that provide information about the size of radiation cancer risks are reviewed in detail in this chapter. Articles included in this chapter were identified principally from searching the PubMed database of published articles from 1990 through December 2004. Searches were restricted to human studies and were broadly defined: key words included radiation; neoplasms; cancers; radiation-induced; medical exposures; radiotherapy; diagnostic radiation; and iodine-131. Articles were also identified from UNSCEAR (2000b), from the references cited in papers reviewed, and from direct contacts with some of the main scientists who have been involved with studies of medical exposures in recent years. The data and confidence intervals are those given in the cited papers.
MEDICAL USES OF RADIATION
Medical use of radiation usually occurs under three circumstances: (1) treatment of benign disease, (2) diagnostic examination, and (3) treatment of malignant disease (Table 7-1). Diagnostic imaging using X-rays goes back to the time of Roentgen’s discovery in 1896. Diagnostic procedures, particularly the widespread use of X-rays, continue to be the most common application of radiation in medicine, even as non-ionizing radiation methods—ultrasound and magnetic resonance imaging—have become more generally accepted. Approximately 400 million diagnostic medical examinations and 150 million dental X-ray examinations are performed annually in the United States (Mettler and others 1996). On average, each person receives at least two examinations per year. The annual individual and collective effective doses from diagnostic medical X-rays have been estimated as 0.5 mSv and 130,000 person-Sv (UNSCEAR 2000b).
The range of X-ray techniques used includes radiography, fluoroscopy, CT, interventional radiology, and bone densitometry. These procedures are intended to provide diagnostic information and in principle are conducted with the lowest practicable levels of patient dose to meet clinical objectives. Ranges of typical doses from various medical diagnostic exposures are shown in Table 7-1.
TABLE 7-1 Estimated Range of Effective Doses from Diagnostic Radiation Exposures
Although doses of single procedures are typically low, there is concern that populations of pediatric patients who may need repeated exams over time to evaluate their pulmonary, cardiac, urinary, or orthopedic conditions may receive relatively high cumulative doses. Similarly, adult patients may also require repeated examinations to evaluate fracture healing, or progression of pulmonary disease, or the regression or progression of neoplastic lesions.
In contrast, therapeutic exposures are less frequent, and the dose levels are higher in view of the different purpose. Currently, radiotherapy is used mainly for the treatment of cancer, where the intention is to deliver a lethal dose to malignant tissue within a well-defined target volume, while minimizing the irradiation of surrounding healthy tissue. In the past, high doses of radiation have also been used for the treatment of a number of benign conditions, such as enlarged thymus and ringworm of the scalp (tinea capitis). Doses from radiotherapy to the target organs are generally above 1 Gy (and typically in the range of 50–60 Gy for the treatment of malignant diseases). Radiotherapy involves mainly partial-body irradiation, however; hence very different doses are delivered to different organs or tissues of the body. Doses to distant organs are generally considerably lower (of the order of fractions of a gray), and studies of cancer risk in these organs are therefore potentially informative for the assessment of risks associated with low-level exposure. Further, many of the patients treated with radiotherapy received frac-