FIGURE 7-3 Distribution of study-specific estimates of ERR/Gy for breast cancer according to average age at exposure.

Protraction of low-dose-rate exposure in the hemangioma cohort may account for the reduced risk following exposures in infancy in this cohort, although analyses within this study do not indicate a significant association.

Preston and colleagues (2002b) carried out a pooled analysis of eight cohorts to estimate radiation-induced breast cancer risk and evaluate the role of modifying factors. The analyses included studies of the following populations: Japanese atomic bomb survivors (Thompson and others 1994), the original and extended Massachusetts TB fluoroscopy cohorts (Boice and others 1991b), the New York acute post partum mastitis cohort (Shore and others 1986), the Rochester infant thymic irradiation cohort (Hildreth and others 1989), the Swedish benign breast disease cohort (Mattsson and others 1993), and the Gothenburg and Stockholm skin hemangioma cohorts (Lindberg and others 1995). The analyses included 1502 breast cancer cases among 77,527 women, about half of whom were exposed to radiation, with 1.8 million person-years of follow-up. No simple unified summary model adequately described the excess risk in all of these studies.

The excess risks for the thymus, tuberculosis, and atomic bomb survivor cohorts showed similar temporal trends, depending on attained age in the ERR model and on both age at exposure and attained age in the EAR model. The excess rates appeared to be similar in these cohorts, with a combined EAR estimate of 9.9 per 104 PY per gray (95% CI 7.1, 1.4) at age 50, suggesting similarity of risks following acute and fractionated low-dose-rate exposure. The ERR/Gy was greater among Japanese atomic bomb survivors; this difference may be partly attributed to the lower background rates of breast cancer in Japan.

The excess rates were higher for the mastitis and benign breast disease cohorts with EAR estimates of 15 (95% CI 7.7, 24) and 32 (95% CI 21, 47) per 104 PY per gray, respectively, suggesting that women with some benign breast conditions may be at an elevated risk of radiation-induced breast cancer.

The hemangioma cohorts showed lower risks (EAR: 5.1 per 104 PY per gray; 95% CI 1.3, 11), suggesting a reduction of risks following protracted low-dose-rate exposures.

Thyroid Cancer

Thyroid cancer is one of the less common forms of cancer. Its incidence is relatively high before age 40, it increases

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