High-dose ionizing radiation is an established risk factor for childhood central nervous system tumors (CNST) but the role of low doses remains debated. In particular, there are few studies of natural background radiation (NBR, gamma radiation and radon) and childhood CNST, and their results are inconclusive. This study aimed to investigate the association between NBR exposure and childhood CNST in France, over the period 2000-2012, based on data from the French national registry of childhood cancers. 5,471 childhood CNST cases, and their municipality of residence at diagnosis, were recorded, and municipality NBR exposures were estimated by cockriging models, using NBR measurements and additional geographic data. The standardized incidence ratios (SIR) per unit variation of exposure were estimated with Poisson log-linear regression models. NBR exposures were considered at the time of diagnosis, and cumulatively from birth to diagnosis. In an exploratory analysis, the cumulative brain dose due to NBR was used. Overall, there was no significant association between NBR exposure and childhood CNST, but an association was suggested for pilocytic astrocytomas with NBR levels at diagnosis: SIR = 1.12 (1.00,1.25) per 50 nSv/h for gamma radiation, and SIR = 1.15 (1.01,1.32) per 100 Bq/m3 for radon). Upward trends for this subtype were also suggested with the cumulative exposures to gamma radiation and radon separately. The results for the total brain dose were similar to those for the cumulative exposure to gamma radiation. Adjustment for socio-demographic factors did not change the findings. Conclusions: Our study was based on high quality incidence data, large numbers of CNST cases, and validated models of assessment of NBR exposures. However, we could not disentangle the possible role of each type of radiation in the association with pilocytic astrocytomas, even though gamma radiation may be more implicated than radon, given its physical properties.

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