Between 1920 and 1959, 9675 women were irradiated in infancy for skin hemangioma at Radiumhemmet, Stockholm. They were exposed to low to moderate doses of ionizing radiation. The mean age at first exposure was 6 months and the mean absorbed dose to the breast anlage was 0.39 Gy (range <0.01-35.8 Gy). The breast cancer incidence was analyzed by record linkage with the Swedish Cancer Register for the period 1958-1986. Seventy-five breast cancers were found [standardized incidence ratio = 1.24; 95% confidence interval (CI) 0.98-1.54] after a mean absorbed dose of 1.5 Gy in the breasts with cancer. The analyses showed a significant dose-response relationship with a linear model estimate for the excess relative risk (ERR) of 0.38 at 1 Gy (95% CI 0.09-0.85). This relationship was not modified significantly by age at exposure or by dose to the ovaries. The ERR increased significantly with time after exposure and for ≥50 years after exposure the ERR at 1 Gy was 2.25 (95% CI 0.59-5.62). The fitted excess absolute risk (EAR) was 22.9 per 104 breast-year gray. The breast absorbed dose and time after exposure were important risk determinants for breast cancer excess risk. Forty to 50 years of follow-up was necessary for the excess risk to be expressed. The study confirms previous findings that the breast anlage of female infants is sensitive to ionizing radiation.

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