Breast Cancer Morbidity in Women Cohort of Techa River Catchment Settlements
Purpose: To assess risk of breast cancer among women resided in the Techa riverside villages, Southern Urals, and chronically exposed to ionizing radiation due to radioactive contamination of the territories by "Mayak" Production Association (PA). Material and methods: In the 1950s of 20 <sup>th</sup> century as a result of "Mayk" PA, a weapon plutonium production facility, activities, a large amounts of radioactive waste were released into the Techa River and became a source of protracted exposure to ionizing radiation for the population of the riverside villages at low to moderate dose rates. The study cohort comprises 9899 females followed-up for 46 years (1.1.1956-31.12.2002) with 268736 person-years at risk and 104 breast cancers. The main information source on breast cancers was a notification form about newly diagnosed cancer from regional oncology dispensary. The data analyses are based on simple parametric excess relative risk (EOR) model. Results: The maximum soft tissue exposure dose due to external y-rays and incorporated <sup>137</sup>Cs in the study group reaches up 0.47 Gy with mean dose 0.04 Gy. Breast cancer incidence significantly increases with age. When considered age-adjusted rates in the study subjects, a significant temporal trend of breast cancer incidence was found that was comparable to the trend observed in the national statistics. The highest risk was registered for nulliparous women. A significant linear dose response (p = 0.01) was shown for the breast cancer incidence in the Techa River female study population with EOR per 1 Gy equal to 5.20 (95 % confidence interval: 0.83; 13.45). Conclusion: We have found a significant association between breast cancer incidence and a number of non-radiation factors such as age, number of the children born and birth cohort (born before and after 1925). Due to a relatively small number of observed cases the point estimate of EOR/ Gy is quite uncertain. Taking into account continuing efforts on improvement and verification of dose estimates in the Techa River population, the assessed risk estimate should be considered as a preliminary one.