tant modifiers of risk. The excess risks appear to be higher in populations of women treated for benign breast conditions, suggesting that these women may be at an elevated risk of radiation-induced breast cancer. The hemangioma cohorts showed lower risks, suggesting a possible reduction of risks following protracted low-dose-rate exposures.

For thyroid cancer, all of the studies providing quantitative information about risks are studies of children who received radiotherapy for benign conditions. A combined analysis of data from some of these cohorts with data from the atomic bomb survivors and from two case-control studies of thyroid cancer nested within the International Cervical Cancer Survivor Study and the International Childhood Cancer Survivor Study provides the most comprehensive information about thyroid cancer risks. For subjects exposed below the age of 15, a linear dose-response was seen, with a leveling or decrease in risk at the higher doses used for cancer therapy. The pooled ERR was 7.7 Gy−1 and the EAR 4.4 per 104 PY per gray. Both estimates were significantly affected by age at exposure, with a strong decrease in risk with increasing age at exposure and little apparent risk for exposures after age 20. The ERR appeared to decline over time about 30 years after exposure but was still elevated at 40 years.

Little information on thyroid cancer risk in relation to 131I exposure in childhood was available. Studies of the effects of 131I exposure later in life provide little evidence of an increased risk of thyroid cancer following 131I exposure after childhood.

For leukemia, ERR 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. Estimates of EAR are also similar across studies, ranging from 1 to 2.6 per 104 PY per gray. Little information is available on the effects of age at exposure or of exposure protraction.

For stomach cancer, the estimates of ERR/Gy range from negative to 1.3 Gy−1. The confidence intervals are wide and they all overlap, indicating that these estimates are statistically compatible.

Finally, results of two studies of patients having undergone radiotherapy for HD or breast cancer suggest that there may be some risk of cardiovascular morbidity and mortality for very high doses and dose-rate exposures. The magnitude of the radiation risk and the shape of the dose-response curve for these outcomes, if an effect exists, are uncertain.

In conclusion, studies of medically irradiated populations provide information on the magnitude of risk estimates (mainly in the medium- to high-dose range) and on the effects of factors, such as exposure pattern and age at exposure, that may modify risk. Further studies of medically exposed populations are needed to study possible gene-radiation interactions that may render parts of the population more sensitive to radiation-induced health effects. Studies of populations (particularly children and infants) with lower-to medium-dose diagnostic exposures also are needed because of the increasing use of procedures such as CT and radiological monitoring of infants.

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