title: Cigarette smoking and mammographic density in the Danish Diet, Cancer and Health cohort.
abstract: OBJECTIVE
  Smoking before first childbirth increases breast cancer risk, but the biological mechanism remains unknown and may involve mammographic density (MD), one of the strongest biomarkers of breast cancer risk. We aimed to examine whether active smoking and passive smoking were associated with MD.
  
  
  METHODS
  For the 5,356 women (4,489 postmenopausal) from the Danish Diet, Cancer and Health cohort (1993-1997) who attended mammographic screening in Copenhagen (1993-2001), we used MD (mixed/dense or fatty) assessed at the first screening after cohort entry. Active smoking (status, duration, and intensity) and passive smoking were assessed at cohort baseline (1993-1997) via questionnaire, together with other breast cancer risk factors. Logistic regression was used to estimate associations (odds ratios, 95 % confidence intervals) between smoking and MD, adjusting for confounders.
  
  
  RESULTS
  Two thousand and twenty-six (56.5 %) women had mixed/dense MD, 2,214 (41.4 %) were current, and 1,175 (21.9 %) former smokers. Current smokers had significantly lower odds (0.86, 0.75-0.99) of having mixed/dense MD compared to never smokers, while former smoking was not associated with MD. Inverse association between smoking and MD was strongest in women who initiated smoking before age of 16 years (0.79, 0.64-0.96), smoked ≥15 cigarettes/day (0.83, 0.71-0.98), smoked ≥5 pack-years (0.62, 0.43-0.89), smoked >30 years (0.86, 0.75-0.99), and smoked ≥11 years before first childbirth (0.70, 0.51-0.96). Association between smoking and MD diminished after smoking cessation, with increased odds of having mixed/dense breasts in women who quit smoking >20 years ago as compared to current smokers (1.37, 1.01-1.67). There was no association between passive smoking and MD.
  
  
  CONCLUSIONS
  We found an inverse association between active smoking and MD.
authors.affiliation: Department of Public Health, Center for Epidemiology and Screening, University of Copenhagen, Øster Farimagsgade 5, 1014, Copenhagen K, Denmark. kkja@sund.ku.dk.|Department of Technology, Faculty of Health and Technology, Metropolitan University College, Copenhagen N, Denmark. kkja@sund.ku.dk.
authors.forename: Katja Kemp
authors.initials: KK
authors.lastname: Jacobsen
authors.affiliation: Department of Public Health, Center for Epidemiology and Screening, University of Copenhagen, Øster Farimagsgade 5, 1014, Copenhagen K, Denmark.
authors.forename: Elsebeth
authors.initials: E
authors.lastname: Lynge
authors.affiliation: Diagnostic Imaging Centre, Copenhagen University Hospital, Rigshospitalet, Copenhagen K, Denmark.
authors.forename: Ilse
authors.initials: I
authors.lastname: Vejborg
authors.affiliation: Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen K, Denmark.
authors.forename: Anne
authors.initials: A
authors.lastname: Tjønneland
authors.affiliation: Department of Public Health, Center for Epidemiology and Screening, University of Copenhagen, Øster Farimagsgade 5, 1014, Copenhagen K, Denmark.
authors.forename: My
authors.initials: M
authors.lastname: von Euler-Chelpin
authors.affiliation: Department of Public Health, Center for Epidemiology and Screening, University of Copenhagen, Øster Farimagsgade 5, 1014, Copenhagen K, Denmark.
authors.forename: Zorana J
authors.initials: ZJ
authors.lastname: Andersen
mesh_terms: D000367:Age Factors; D000071060:Breast Density; D001943:Breast Neoplasms / Q000453:epidemiology*; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D005260:Female; D006801:Humans; D016015:Logistic Models; D042361:Mammary Glands, Human / Q000002:abnormalities*; D008327:Mammography; D008875:Middle Aged; D016017:Odds Ratio; D017584:Reproductive History; D012307:Risk Factors; D012907:Smoking / Q000453:epidemiology*; D011795:Surveys and Questionnaires; D014028:Tobacco Smoke Pollution / Q000706:statistics & numerical data*
chemical_list: D014028:Tobacco Smoke Pollution
keywords: Breast cancer risk factor; Breast density; Cigarette smoking; Mammographic density; Secondhand smoke