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Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2
Leukemia is one of the less common malignancies, but substantial epidemiologic and experimental information exists on the leukemogenic effects of ionizing radiation (UNSCEAR 2000b).
Of all the studies reviewed in the “Medical Uses of Radiation” section, six provide dose-specific estimates of ERR and/or EAR. Table 7-5 and Figure 7-5 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. Results shown are for leukemia excluding CLL in all studies except the tinea capitis and uterine bleeding studies.
The ERRs/Gy shown in Table 7-5 range from 0.88 Gy−1 in women who received an average dose to the active bone marrow of 7 Gy from radiotherapy for cervical cancer to 12.4 Gy−1 in subjects treated for ankylosing spondylitis (average dose 4.4 Gy). All other estimates, from studies with average doses ranging from 0.1 to 2 Gy, are relatively close, in the range 1.9 to 5 Gy−1, and are statistically compatible.
Three studies have provided estimates of EAR per 104 PY per gray. Risk estimates between these studies are relatively close, ranging from 1 to 2.6.
In most of the studies included here, the majority of subjects were adults at the time of exposure (with average ages at exposure between 45 and 52 years in the uterine bleeding, benign breast disease, and cervical cancer survivor studies). Only the tinea capitis and hemangioma studies provide information about exposures in childhood. In the hemangioma study—where all subjects were irradiated in infancy—the overall ERR/Gy is similar to that seen in other studies; it is notable, however, that this is driven mainly by a higher ERR for childhood leukemia; the ERR for adult leukemia in this study was very close to zero. In the tinea capitis study, in which all exposures were below age 15, no ERR is shown; the EAR is similar to that seen in the other studies.
In one study (Inskip and others 1993), an effort was made to estimate separately the effects of external exposures, 226Ra, and the combination of the two. Estimates of risk from 226Ra alone or in combination with external radiation are
TABLE 7-5 Risk Estimates for Cancer Incidence and Mortality from Studies of Radiation Exposure: Leukemia Excluding CLL
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.