title: Aetiologic heterogeneity in pediatric Hodgkin lymphoma? Evidence from the Nordic countries, 1978-2010.
abstract: BACKGROUND
  In epidemiological studies, Hodgkin lymphomas (HL) in children younger than 15 years and HL in adolescents and younger adults age 15-35 years has traditionally been studied separately, under the assumption that HL at age 0-14 constitute a homogeneous entity. However, the continued validity of this research practice in affluent settings may be questioned. Specifically, the boundary at age 15 years may not be epidemiologically justified, and therefore also questionable clinically. We therefore updated and further characterised recent HL incidence patterns among Nordic children.
  
  
  METHODS
  We obtained HL incidence data in children aged 0-14 years for the period 1978-2010 from the five nationwide Nordic cancer registries. The data were analysed by log-linear and/or a mixture of Poisson regression models.
  
  
  RESULTS
  The analyses showed statistically significantly decreasing and increasing HL incidence rates in children younger and older than eight years, respectively during the study period. Statistical modelling suggested that cases in children age 0-6 years constituted a disease entity of its own, whereas cases in older children were more likely to belong to the younger adult HL entity.
  
  
  CONCLUSIONS
  Diverging incidence trends and statistical modelling suggest that HL in children age 0-14 years cannot be assumed to constitute a homogeneous disease entity in affluent settings.
authors.affiliation: a Department of Epidemiology Research , Statens Serum Institut , Copenhagen S , Denmark.|b Department of Paediatrics and Adolescent Medicine , Rigshospitalet , Copenhagen Ø, Denmark.
authors.forename: Lisa Lyngsie
authors.initials: LL
authors.lastname: Hjalgrim
authors.affiliation: a Department of Epidemiology Research , Statens Serum Institut , Copenhagen S , Denmark.
authors.forename: Klaus
authors.initials: K
authors.lastname: Rostgaard
authors.affiliation: c Department of Documentation and Quality , Danish Cancer Society , Copenhagen Ø, Denmark.
authors.forename: Gerda
authors.initials: G
authors.lastname: Engholm
authors.affiliation: d Finnish Cancer Registry, Institute for Statistical and Epidemiological Cancer Research , Helsinki , Finland.|e School of Health Sciences, University of Tampere , Tampere , Finland.
authors.forename: Eero
authors.initials: E
authors.lastname: Pukkala
authors.affiliation: f Cancer Registry of Norway , Majorstuen , Norway.
authors.forename: Tom Børge
authors.initials: TB
authors.lastname: Johannesen
authors.affiliation: g Icelandic Cancer Registry, Icelandic Cancer Society , Reykjavik , Iceland.
authors.forename: Elínborg
authors.initials: E
authors.lastname: Ólafsdóttir
authors.affiliation: a Department of Epidemiology Research , Statens Serum Institut , Copenhagen S , Denmark.
authors.forename: Henrik
authors.initials: H
authors.lastname: Hjalgrim
mesh_terms: D000293:Adolescent; D017677:Age Distribution; D002648:Child; D002675:Child, Preschool; D005260:Female; D006689:Hodgkin Disease / Q000453:epidemiology* / Q000209:etiology; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D012042:Registries / Q000706:statistics & numerical data; D012537:Scandinavian and Nordic Countries / Q000453:epidemiology