title: Physical activity in relation to risk of hematologic cancers: a systematic review and meta-analysis.
abstract: BACKGROUND
  Despite the existence of numerous biologic pathways potentially linking increased physical activity to decreased risk of hematologic cancers, the associations between physical activity and subtype-specific hematologic cancers have not been comprehensively quantified.
  
  
  METHODS
  We conducted a systematic review and meta-analysis of physical activity in relation to subtype-specific hematologic cancers. We summarized the data from 23 eligible studies (15 cohort and eight case-control studies) and estimated summary relative risks (RR) and 95% confidence intervals (CI) using random-effects models.
  
  
  RESULTS
  When comparing high versus low physical activity levels, the RR for non-Hodgkin lymphoma was 0.91 (95% CI, 0.82-1.00), for Hodgkin lymphoma it was 0.86 (95% CI, 0.58-1.26), for leukemia it was 0.97 (95% CI, 0.84-1.13), and for multiple myeloma it was 0.86 (95% CI, 0.68-1.09). When focusing on subtypes of non-Hodgkin lymphoma, the RR for diffuse large B-cell lymphoma was 0.95 (95% CI, 0.80-1.14) and for follicular lymphoma it was 1.01 (95% CI, 0.83-1.22). In an exploratory analysis combining all hematologic cancers, high versus low physical activity levels yielded a statistically significant RR of 0.93 (95% CI, 0.88-0.99).
  
  
  CONCLUSIONS
  Physical activity showed statistically nonsignificant associations with risks of non-Hodgkin lymphoma, Hodgkin lymphoma, multiple myeloma, and leukemia. These findings may not represent a true lack of associations given the variation in high versus low physical activity definitions, the quality of physical activity assessments, and the variability in hematologic cancer classification schemes in individual studies.
  
  
  CONCLUSIONS
  Physical activity is unrelated to risks of subtype-specific hematologic cancers.
authors.affiliation: Authors' Affiliation: Department of Epidemiology and Preventive Medicine, University of Regensburg, Regensburg, Germany.
authors.forename: Carmen
authors.initials: C
authors.lastname: Jochem
authors.forename: Michael F
authors.initials: MF
authors.lastname: Leitzmann
authors.forename: Marlen
authors.initials: M
authors.lastname: Keimling
authors.forename: Daniela
authors.initials: D
authors.lastname: Schmid
authors.forename: Gundula
authors.initials: G
authors.lastname: Behrens
mesh_terms: D016022:Case-Control Studies; D015444:Exercise / Q000502:physiology*; D019337:Hematologic Neoplasms / Q000209:etiology* / Q000517:prevention & control*; D006801:Humans; D011379:Prognosis