Breast cancer risk in residents of Belarus exposed to Chernobyl fallout while pregnant or lactating: standardized incidence ratio analysis, 1997 to 2016

Background Pregnancy and lactation may constitute radiation-sensitive reproductive periods due to rapid cell proliferation and concentration of radioiodine in the lactating breast. However, there are limited epidemiological data among women exposed to radiation during these periods. Methods We examined incidence of breast cancer in a cohort of 3214 women who were pregnant and/or lactating within 2 months (26 April to 30 June 1986) of the Chernobyl accident and residing in contaminated regions of Belarus. There were 33 breast cancers identified through linkage with the Belarusian National Cancer Registry. Breast cancer rates among women who were either pregnant and/or lactating were compared with population rates by calculating standardized incidence ratios (SIR) and 95% confidence intervals (CI) adjusting for attained age, oblast, urban/rural residence and calendar year. Results Among women who were lactating, we found a greater than 2-fold increased risk of breast cancer compared with the general population, SIR = 2.49 (95% CI: 1.55, 3.75). In contrast, women who were pregnant were not at increased risk (SIR = 0.84 95% CI: 0.46, 1.38). The SIR was highest in women who were exposed at a younger age and at the earliest time period since the accident, though stratified analyses had limited sample sizes. Conclusions We found a significantly increased risk of breast cancer among women residing in contaminated areas of Belarus who were lactating at the time of the accident and may have had elevated exposure to radioiodine, when compared with the general population. Studies of breast cancer with individual radiation dose estimates among women exposed during lactation are warranted.