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

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.