title: Risk factors for breast cancer for women aged 40 to 49 years: a systematic review and meta-analysis.
abstract: BACKGROUND
  Identifying risk factors for breast cancer specific to women in their 40s could inform screening decisions.
  
  
  OBJECTIVE
  To determine what factors increase risk for breast cancer in women aged 40 to 49 years and the magnitude of risk for each factor.
  
  
  METHODS
  MEDLINE (January 1996 to the second week of November 2011), Cochrane Central Register of Controlled Trials and Cochrane Database of Systematic Reviews (fourth quarter of 2011), Scopus, reference lists of published studies, and the Breast Cancer Surveillance Consortium.
  
  
  METHODS
  English-language studies and systematic reviews of risk factors for breast cancer in women aged 40 to 49 years. Additional inclusion criteria were applied for each risk factor.
  
  
  METHODS
  Data on participants, study design, analysis, follow-up, and outcomes were abstracted. Study quality was rated by using established criteria, and only studies rated as good or fair were included. Results were summarized by using meta-analysis when sufficient studies were available or from the best evidence based on study quality, size, and applicability when meta-analysis was not possible. Data from the Breast Cancer Surveillance Consortium were analyzed with proportional hazards models by using partly conditional Cox regression. Reference groups for comparisons were set at U.S. population means.
  
  
  RESULTS
  Sixty-six studies provided data for estimates. Extremely dense breasts on mammography or first-degree relatives with breast cancer were associated with at least a 2-fold increase in risk for breast cancer. Prior breast biopsy, second-degree relatives with breast cancer, or heterogeneously dense breasts were associated with a 1.5- to 2.0-fold increased risk; current use of oral contraceptives, nulliparity, and age 30 years or older at first birth were associated with a 1.0- to 1.5-fold increased risk.
  
  
  CONCLUSIONS
  Studies varied by measures, reference groups, and adjustment for confounders, which could bias combined estimates. Effects of multiple risk factors were not considered.
  
  
  CONCLUSIONS
  Extremely dense breasts and first-degree relatives with breast cancer were each associated with at least a 2-fold increase in risk for breast cancer in women aged 40 to 49 years. Identification of these risk factors may be useful for personalized mammography screening.
  
  
  BACKGROUND
  National Cancer Institute.
authors.affiliation: Oregon Evidence-based Practice Center, Oregon Health & Science University, Portland, 97239-3098, USA. nelsonh@ohsu.edu
authors.forename: Heidi D
authors.initials: HD
authors.lastname: Nelson
authors.forename: Bernadette
authors.initials: B
authors.lastname: Zakher
authors.forename: Amy
authors.initials: A
authors.lastname: Cantor
authors.forename: Rongwei
authors.initials: R
authors.lastname: Fu
authors.forename: Jessica
authors.initials: J
authors.lastname: Griffin
authors.forename: Ellen S
authors.initials: ES
authors.lastname: O'Meara
authors.forename: Diana S M
authors.initials: DS
authors.lastname: Buist
authors.forename: Karla
authors.initials: K
authors.lastname: Kerlikowske
authors.forename: Nicolien T
authors.initials: NT
authors.lastname: van Ravesteyn
authors.forename: Amy
authors.initials: A
authors.lastname: Trentham-Dietz
authors.forename: Jeanne S
authors.initials: JS
authors.lastname: Mandelblatt
authors.forename: Diana L
authors.initials: DL
authors.lastname: Miglioretti
mesh_terms: D000328:Adult; D001706:Biopsy; D001940:Breast / Q000033:anatomy & histology; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000235:genetics; D055088:Early Detection of Cancer; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008327:Mammography; D008403:Mass Screening / Q000379:methods; D008875:Middle Aged; D016016:Proportional Hazards Models; D017584:Reproductive History; D018570:Risk Assessment; D012307:Risk Factors; D014481:United States / Q000453:epidemiology