Childhood CT scans and cancer risks estimates: an update of the French CT cohort study

International audienceBackground: Computed tomography (CT) has been used increasingly worldwide over the last decades. However, concerns have been raised about potentially radiation-related cancer risks, particularly after exposure to CT in childhood, due to the greater radiation sensitivity of children and to their longer life expectancy allowing to develop radiation associated late health effects. Increased risks of central nervous system (CNS) tumours and leukaemia associated with CT exposure during childhood have been reported in recent epidemiological studies. However, no evidence of significant increased risks was suggested in a previous analysis of almost 60,000 patients of the French CT cohort.Methods: The French CT cohort includes patients born after 1994 who received at least one CT scan before the age of 10 years between 2000 and 2011 and had no cancer diagnosis before the first CT. Examinations and radiological protocols carried out between 2000 and 2011 in the 21 participating hospitals were retrieved to estimate cumulative absorbed doses to the brain and the red bone marrow (RBM). In this work, the cohort was updated in order to extend the follow-up (5 additional years) and to increase the sample size of patients (40,000 new patients with reported vital status). Moreover, the cohort was linked with the National Health Data System (Système national des données de santé, SNDS) to collect CTs performed outside the participating hospitals or after the inclusion period. Hazard ratios (HRs) associated to cumulative organ doses and gender were estimated from Cox models. To rule out the possibility of reverse causation, an exclusion period of 2 years was applied. A latency period was also applied to consider the minimal latency period expected between the exposure and the studied outcome. To address the potential issue of bias by indication, the models were fitted to the restricted sub-cohorts of patients with and without cancer predisposing factors (PFs) separately.Results: The updated French CT cohort includes 103,015 patients followed for 9.3 years in average. 3.1% of patients had PFs. Considering only the CTs performed in the participating hospitals until 2011, mean cumulative doses were 24.5 and 9.3 mGy for the brain and the RBM respectively. Adding the CTs performed outside the participating hospitals or after the inclusion period increased the mean cumulative doses to 27.7 mGy and 10.3 mGy for the brain and the RBM respectively. This study showed statistically significant dose-response relationships for CNS tumours (HR per 10 mGy: 1.05, 95% CI: 1.01 – 1.09) and leukaemia (HR per 10 mGy: 1.17, 95% CI: 1.09 – 1.26) for patients without PFs. No evidence of association was observed for lymphoma for these patients and for the three types of cancer for the patients with PFs. Inclusion of additional CTs registered in the SNDS did not impact the HR estimates.Conclusions: Estimates were compatible with the ones obtained in the previous analysis of the French CT cohort. However, the extended follow-up and the larger sample size of the cohort increased the statistical power, leading to statistically significant dose-response relationships for CNS tumours and leukaemia for patients without PFs