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Health Risks from Exposure to Low Levels of Ionizing Radiation: Beir VII Phase 2
in the above cohort has been published (Sadetzki and others 2002), but there were no risk estimates. A more recent study (Sadetski and others 2005) conducted a survival analysis using Poisson regression to estimate the excess relative and absolute risks for brain tumors. After a median follow-up of 40 years, ERRs/Gy of 4.63 and 1.98 (95% CI 2.43, 9.12 and 0.73, 4.69) and EARs/Gy per 104 PY of 0.48 and 0.31 (95% CI 0.28, 0.73 and 0.12, 0.53) were observed for benign meningiomas and malignant brain tumors, respectively. The risk of both types of tumors was positively associated with dose. The estimated ERR/Gy for malignant brain tumors decreased with increasing age at irradiation from 3.6 for exposures below the age of 5 to 0.5 for exposures at ages 10 or above (p = .04), while no trend with age was seen for benign meningiomas. The ERR for both types of tumor remains elevated 30-plus years after exposure.
Modan and colleagues (1989) reported on an additional 5-year follow-up (until 1986) of the Israeli tinea capitis cohort. While the previously observed increases in the incidence of head and neck tumors (mainly brain, CNS, and thyroid tumors) continued, an elevated risk of breast cancer was observed for the first time in this cohort, based on 13 new cases in 1982–1986. The estimated average dose to the breast was low—0.016 Gy. The increase was seen only among women who were 5–9 years of age at the time of radiation exposure (10 cases). No estimate of risk per dose is presented.
Ron and colleagues (1989) reported on the risk of thyroid cancer following irradiation in childhood for tinea capitis, based on an extended follow-up (until 1986). The dose reconstruction method is based on the approach described above for the brain and CNS study. To adjust the dose for possible head movement during treatment, individual dose estimates were multiplied by a factor of 1.5 as suggested by results of dosimetric studies. Average doses to the thyroid were 0.13, 0.09, and 0.06 Gy, respectively, for children aged less than 5, 5–10, and 10–15 years at the time of exposure. Overall, 98 thyroid tumors were identified among the exposed and 57 among the two control populations. An estimated dose of 0.09 Gy was related to a fourfold increase (95% CI 2.3, 7.9) in the risk of thyroid cancer and a twofold increase in benign tumors. The dose-response was consistent with linearity. The risk diminished with age at exposure, and the RR appeared to be constant over time. The ERR was estimated to be 30 Gy−1 and the EAR was 13 per 104 PY per gray.
Modan and coworkers (1998) also reported a 4.5-fold increase in the incidence of malignant salivary gland tumors (p < .01) and a 2.6-fold increase of benign tumors in subjects irradiated for tinea capitis. A clear dose-response association with both cancer and benign tumors was demonstrated. No estimate of risk per dose level was presented.
In New York, about 2200 children who received X-ray treatment for tinea capitis during the 1940s and 1950s and a comparable group of 1400 treated without X-rays were followed by mail questionnaire to evaluate the incidence of skin cancer (Shore and others 1984). The average length of follow-up was 26 years. Delivered doses ranged from 3 to 6 Gy depending on the portion of the scalp, with lower doses to the skin of the face and neck (0.1–0.5 Gy). In the irradiated group, 41 subjects had a diagnosis of basal carcinoma of the scalp or face, compared to 3 in the control group. The prevalence of multiple lesions was high in the exposed group. The minimum latent period was long (about 20 years); skin cancers were more pronounced on the face, where the potential for exposure to ultraviolet is higher, and were restricted to Caucasians although one-quarter of the study population was African American. No estimate of risk per dose is presented.
Enlarged Thymus Gland
Patients in Rochester, New York who received X-ray treatment between 1926 and 1957 in infancy (before 6 months of age) for an enlarged thymus gland and their nonirradiated siblings have been followed up periodically through the use of a mail questionnaire (Shore and others 1985, 1993a, 1993b; Hildreth and others 1985). Information on X-ray treatment factors was extracted from medical records and supplemented by interviews with the treating physicians. These, along with anatomic measurements for infants, allowed estimation of doses to various nearby organs. The thyroid doses were estimated by irradiating a radiological phantom of an infant. The irradiated group had a statistically significant increase of both benign and malignant thyroid tumors (Shore and others 1985) and extrathyroid tumors (Hildreth and others 1985), particularly benign tumors of the bone, nervous system, salivary glands, skin, and breast (women only) and malignant tumors of the skin and breast.
In the most recent paper on thyroid cancer, which reports on follow-up to 1986, the cohort included 2657 exposed subjects and 4833 unexposed siblings with at least 5 years of follow-up (Shore and others 1993a). The average duration of follow-up was 37 years. Thyroid doses could be estimated for 91% of the subjects. The thyroid dose distribution was skewed, ranging from 0.03 to more than 10 Gy, with a mean of 1.4 Gy and a median of 0.3 Gy. There were 37 pathologically confirmed thyroid cancers among the irradiated group and 5 among the sibling controls. A linear dose-response was found in this cohort with an ERR of 9.0 at 1 Gy (90% CI 4.0, 24.0). An increased risk was seen even at low doses, with a significant positive slope in the dose range 0–0.3 Gy, based on four exposed cases. The risk ratio decreased over time, but was still highly elevated 45 years after exposure. There was no evidence of a decrease in the absolute excess risk over time (EAR varying from 2.1 per 104 PY per gray 5–14 years postirradiation to 6.0 per 104 PY per gray after 45 years or more). Analyses of interactions suggested that all Jewish subjects and women with older ages at menarche