title: Cancer in siblings of children with cancer in the Nordic countries: a population-based cohort study.
abstract: BACKGROUND
  In some rare inherited disorders such as Li-Fraumeni syndrome, relatives of children with cancer are at increased risk of cancer. We aimed to assess relations between childhood cancer and sibling risk, and evaluate the influence of recessive conditions in cancer causation.
  
  
  METHODS
  We did a population-based cohort study in the Nordic countries of 42277 siblings of 25605 children with cancer. Children with cancer were identified from records in the five Nordic cancer registries, and their siblings from nationwide population registries. Cancers in siblings were documented through record linkage with cancer registries and compared with national incidence rates. We also assessed cancer incidence in parents to identify familial cancer syndromes.
  
  
  RESULTS
  284.2 cancers were expected in siblings, whereas 353 were diagnosed (standardised incidence ratio 1.24 95% CI 1.12-1.38). Risk ratios for siblings were highest in the first decade of life (2.59, 1.89-3.46). We excluded 56 families with genetic syndromes linked to cancer, which reduced this ratio from 1.7 to 1.0 (0.7-1.3) for siblings younger than 20 years, and from 1.3 to 1.0 (0.8-1.3) for those aged 20-29 years. We found no new patterns of familial cancer that indicated inherited susceptibility, or evidence that recessive conditions might contribute to cancers not explained by syndromes. 40% of cancers in siblings that occurred before age 20 years could be attributed to known genetic factors, whereas 60% remained unexplained.
  
  
  CONCLUSIONS
  Apart from rare cancer syndromes, paediatric cancer is not an indicator of increased cancer risk in siblings.
authors.affiliation: Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark. jeanette@cancer.dk
authors.forename: J F
authors.initials: JF
authors.lastname: Winther
authors.forename: R
authors.initials: R
authors.lastname: Sankila
authors.forename: J D
authors.initials: JD
authors.lastname: Boice
authors.forename: H
authors.initials: H
authors.lastname: Tulinius
authors.forename: A
authors.initials: A
authors.lastname: Bautz
authors.forename: L
authors.initials: L
authors.lastname: Barlow
authors.forename: E
authors.initials: E
authors.lastname: Glattre
authors.forename: F
authors.initials: F
authors.lastname: Langmark
authors.forename: T R
authors.initials: TR
authors.lastname: Möller
authors.forename: J J
authors.initials: JJ
authors.lastname: Mulvihill
authors.forename: G H
authors.initials: GH
authors.lastname: Olafsdottir
authors.forename: A
authors.initials: A
authors.lastname: Ritvanen
authors.forename: J H
authors.initials: JH
authors.lastname: Olsen
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D008297:Male; D009369:Neoplasms / Q000453:epidemiology* / Q000235:genetics; D009679:Nuclear Family; D011159:Population Surveillance; D012042:Registries; D012306:Risk; D012537:Scandinavian and Nordic Countries / Q000453:epidemiology