title: Background ionizing radiation and the risk of childhood cancer: a census-based nationwide cohort study.
abstract: 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.
  
  
  OBJECTIVE
  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.
authors.affiliation: Institute of Social and Preventive Medicine (ISPM), University of Bern, Bern, Switzerland.
authors.forename: Ben D
authors.initials: BD
authors.lastname: Spycher
authors.forename: Judith E
authors.initials: JE
authors.lastname: Lupatsch
authors.forename: Marcel
authors.initials: M
authors.lastname: Zwahlen
authors.forename: Martin
authors.initials: M
authors.lastname: Röösli
authors.forename: Felix
authors.initials: F
authors.lastname: Niggli
authors.forename: Michael A
authors.initials: MA
authors.lastname: Grotzer
authors.forename: Johannes
authors.initials: J
authors.lastname: Rischewski
authors.forename: Matthias
authors.initials: M
authors.lastname: Egger
authors.forename: Claudia E
authors.initials: CE
authors.lastname: Kuehni
mesh_terms: D000293:Adolescent; D001417:Background Radiation / Q000009:adverse effects*; D019352:Censuses; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D009381:Neoplasms, Radiation-Induced / Q000453:epidemiology* / Q000209:etiology; D000069079:Radiation Exposure / Q000009:adverse effects*; D018570:Risk Assessment; D013557:Switzerland / Q000453:epidemiology