title: Number of siblings and risk of Hodgkin's lymphoma.
abstract: BACKGROUND
  Epidemiologic evidence indicates that risk of Hodgkin's lymphoma (HL) in young adults is associated with correlates of delayed exposure to infection during childhood. In contrast, HL among children and older adults may be associated with earlier childhood infection. This study examines the associations of HL risk with having older or younger siblings.
  
  
  METHODS
  We conducted a case-control study in Sweden comparing 2,140 HL patients identified from the Swedish Cancer Register with 10,024 controls identified from national population registers. The Swedish Multi-Generation Register was used to link individuals to their parents and siblings.
  
  
  RESULTS
  Among young adults ages 15 to 39 years, the odds ratios (OR) associated with having one, two, and three or more older siblings, compared with none, were 0.96 [95% confidence interval (CI), 0.82-1.13], 0.88 (95% CI, 0.72-1.09), and 0.72 (95% CI, 0.55-0.93), respectively (P value for trend = 0.01). In contrast, number of older siblings was not associated with HL risk among children or older adults. Number of younger or total siblings, mother's age at birth, and father's occupation were not associated with HL at any age. The decreased risk of young-adult HL did not vary appreciably by age difference or sex of older siblings.
  
  
  CONCLUSIONS
  Risk of HL was lower among young adults with multiple older but not younger siblings. Having older siblings is associated with earlier exposure to common childhood pathogens. Pediatric and older-adult HL were not associated with number of siblings, suggesting a different pathogenesis of disease in these age groups.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institute, Stockholm, Sweden. ellen.chang@meb.ki.se
authors.forename: Ellen T
authors.initials: ET
authors.lastname: Chang
authors.forename: Scott M
authors.initials: SM
authors.lastname: Montgomery
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Richiardi
authors.forename: Anna
authors.initials: A
authors.lastname: Ehlin
authors.forename: Anders
authors.initials: A
authors.lastname: Ekbom
authors.forename: Mats
authors.initials: M
authors.lastname: Lambe
mesh_terms: D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D001722:Birth Order; D016022:Case-Control Studies; D005191:Family Characteristics; D005260:Female; D006689:Hodgkin Disease / Q000453:epidemiology*; D006801:Humans; D015994:Incidence; D016015:Logistic Models; D008297:Male; D012042:Registries; D012307:Risk Factors; D012737:Sex Factors; D035781:Siblings; D013548:Sweden / Q000453:epidemiology