Risk of early-onset breast cancer among women exposed to thoracic computed tomography in pregnancy or early postpartum

Background: The risk of breast cancer may be higher with direct exposure to ionizing radiation from thoracic computed tomography (CT) during pregnancy or the postpartum. We evaluated the short-term risk of maternal breast cancer after exposure to thoracic CT during these periods. Methods: We completed a retrospective population-based cohort study of all deliveries between 1995 and 2014 using universal healthcare databases in the province of Ontario, Canada. The main exposure was thoracic CT in pregnancy or <= 42 days postpartum. The passive exposure was ventilation-perfusion scintigraphy (VQ) scan in pregnancy or <= 42 days postpartum. Each was compared to pregnancies unexposed to thoracic CT or VQ scan. The primary study outcome was newly diagnosed breast cancer starting 366 days post-index delivery date. Results: A total of 5859 pregnancies were exposed to thoracic CT, 4075 to VQ scan and 1 292 059 to neither. Starting from 1 year after the index delivery, the median duration of follow-up was 5.9, 7.3 and 11.1 years, respectively. A total of 10 129 women were diagnosed with breast cancer, of whom 9039 (89.2%) were aged <= 50 years. There were 27 new cases of breast cancer (7.1 per 10 000 person-years) following thoracic CT vs. 10 080 (7.0 per 10 000 person-years) among the unexposed, an adjusted hazard ratio (HR) of 1.17 (95% confidence interval [CI], 0.80-1.70). Following VQ scan exposure, the incidence rate of breast cancer was 7.0 per 10 000 personyears, an adjusted HR of 1.23 (95% CI 0.81-1.87), compared with the unexposed cohort. Conclusion: Exposure to thoracic CT during pregnancy or the postpartum was not associated with an increased short-term risk of maternal breast cancer. The long-term risk should be studied.