Background: Exposure to medium or high doses of ionizing radiation is a known risk factor for cancer in children. The extent to which low-dose radiation from natural sources contributes to the risk of childhood cancer remains unclear.

Objectives: In a nationwide census-based cohort study, we investigated whether the incidence of childhood cancer was associated with background radiation from terrestrial gamma and cosmic rays.

Methods: Children < 16 years of age in the Swiss National Censuses in 1990 and 2000 were included. The follow-up period lasted until 2008, and incident cancer cases were identified from the Swiss Childhood Cancer Registry. A radiation model was used to predict dose rates from terrestrial and cosmic radiation at locations of residence. Cox regression models were used to assess associations between cancer risk and dose rates and cumulative dose since birth.

Results: Among 2,093,660 children included at census, 1,782 incident cases of cancer were identified including 530 with leukemia, 328 with lymphoma, and 423 with a tumor of the central nervous system (CNS). Hazard ratios for each millisievert increase in cumulative dose of external radiation were 1.03 (95% CI: 1.01, 1.05) for any cancer, 1.04 (95% CI: 1.00, 1.08) for leukemia, 1.01 (95% CI: 0.96, 1.05) for lymphoma, and 1.04 (95% CI: 1.00, 1.08) for CNS tumors. Adjustment for a range of potential confounders had little effect on the results.

Conclusions: Our study suggests that background radiation may contribute to the risk of cancer in children, including leukemia and CNS tumors.

Citation: Spycher BD, Lupatsch JE, Zwahlen M, Röösli M, Niggli F, Grotzer MA, Rischewski J, Egger M, Kuehni CE, for the Swiss Pediatric Oncology Group and the Swiss National Cohort. 2015. Background ionizing radiation and the risk of childhood cancer: a census-based nationwide cohort study. Environ Health Perspect 123:622–628;  http://dx.doi.org/10.1289/ehp.1408548

Address correspondence to B.D. Spycher, Institute of Social and Preventive Medicine (ISPM), University of Bern, Finkenhubelweg 11, CH-3012 Bern, Switzerland. Telephone: 41 31 631 56 97. E-mail: ben.spycher@ispm.unibe.ch

This study was supported by the Swiss Federal Office of Public Health (grants 08.001616, 10.002946, 12.008357), Swiss Cancer League (grant 02224-03-2008), Swiss Cancer Research (grants 3049-08-2012, 3515-08-2014), and Swiss National Science Foundation (grant 33CS30-134273). B.D.S. was supported by a Swiss National Science Foundation fellowship (PZ00P3_147987). The work of the Swiss Childhood Cancer Registry is supported by the Swiss Pediatric Oncology Group ( http://www.spog.ch ), Schweizerische Konferenz der kantonalen Gesundheitsdirektorinnen und –direktoren ( http://www.gdk-cds.ch ), Swiss Cancer Research ( http://www.krebsforschung.ch ), Kinderkrebshilfe Schweiz ( http://www.kinderkrebshilfe.ch ), Ernst-Göhner Stiftung, Stiftung Domarena, and National Institute of Cancer Epidemiology and Registration ( http://www.nicer.ch ).

The members of the Swiss Pediatric Oncology Group Scientific Committee: R.A. Ammann (Bern), R. Angst (Aarau), M. Ansari (Geneva), M. Beck Popovic (Lausanne), E. Bergstraesser (Zurich), P. Brazzola (Bellinzona), J. Greiner (St. Gallen), M. Grotzer (Zurich), H. Hengartner (St. Gallen), T. Kuehne (Basel), K. Leibundgut (Bern), F. Niggli (Zurich), J. Rischewski (Lucerne), and N. von der Weid (Basel). The members of the Swiss National Cohort Study Group: F. Gutzwiller (Chairman of the Executive Board, Zurich), M. Bopp (Zurich), M. Egger (Chairman of the Scientific Board, Bern), A. Spoerri (Bern), M. Zwahlen (Bern), N. Künzli (Basel), F. Paccaud (Lausanne), M. Oris (Geneva).

The authors declare they have no actual or potential competing financial interests.

Received: 11 April 2014 Accepted: 28 January 2015 Advance Publication: 23 February 2015 Final Publication: 1 June 2015