title: Social inequalities and smoking-associated breast cancer - Results from a prospective cohort study.
abstract: OBJECTIVE
  The association between smoking and breast cancer has been found in most recent, large cohort studies. We wanted to investigate how smoking-associated breast cancer varies by level of education, a well-established measure of socioeconomic status.
  
  
  METHODS
  We included 302,865 women with 7490 breast cancer cases. Participants were assigned to low, moderate or high level of education and analyzed by smoking status (ever/never), and stratified by birth cohorts (≤1950>). We used Cox proportional hazard to estimate hazard ratios (HRs) and confidence intervals (CIs), adjusting for age, number of children, age at first childbirth, BMI, age at enrollment and physical activity.
  
  
  RESULTS
  Women born ≤1950 with low and moderate levels of education had a 40% increase in smoking-associated breast cancer risk (HR=1.40, 95% CI 1.25-1.57 and HR=1.14, 95% CI 1.05-1.24, respectively). Women in the same age group with high level of education did not have an increase in risk. No increased breast cancer risk was found among women born after 1950 for any level of education, when analyzed by smoking status. Longer duration of smoking before first childbirth was consistently associated with increasing risk of breast cancer in all three categories of education (all p for trends<0.01).
  
  
  CONCLUSIONS
  Smoking for several years before first childbirth increases the risk of breast cancer, regardless of educational level.
authors.affiliation: Department of Community Medicine, Faculty of Health Sciences, UiT The Arctic University of Norway, N-9037 Tromsø, Norway.
authors.forename: Eivind
authors.initials: E
authors.lastname: Bjerkaas
authors.affiliation: Department of Community Medicine, Faculty of Health Sciences, UiT The Arctic University of Norway, N-9037 Tromsø, Norway.
authors.forename: Ranjan
authors.initials: R
authors.lastname: Parajuli
authors.affiliation: Division of Epidemiology, Department of Pharmacoepidemiology, Norwegian Institute of Public Health, Oslo, Norway; Department of Global Public Health and Primary Care, University of Bergen, Bergen, Norway.
authors.forename: Anders
authors.initials: A
authors.lastname: Engeland
authors.affiliation: University of Hawaii Cancer Center, Honolulu, HI, USA.
authors.forename: Gertraud
authors.initials: G
authors.lastname: Maskarinec
authors.affiliation: Department of Community Medicine, Faculty of Health Sciences, UiT The Arctic University of Norway, N-9037 Tromsø, Norway; Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden; Department of Genetic Epidemiology, Folkhälsan Research Center, Samfundet Folkhälsan, Helsinki, Finland; Department of Research, Cancer Registry of Norway, Oslo, Norway.
authors.forename: Elisabete
authors.initials: E
authors.lastname: Weiderpass
authors.affiliation: Department of Community Medicine, Faculty of Health Sciences, UiT The Arctic University of Norway, N-9037 Tromsø, Norway.
authors.forename: Inger Torhild
authors.initials: IT
authors.lastname: Gram
mesh_terms: D000328:Adult; D000367:Age Factors; D001943:Breast Neoplasms / Q000209:etiology*; D004522:Educational Status; D005260:Female; D054624:Health Status Disparities; D006801:Humans; D008875:Middle Aged; D009043:Motor Activity; D009664:Norway / Q000453:epidemiology; D010298:Parity; D016016:Proportional Hazards Models; D011446:Prospective Studies; D012907:Smoking / Q000009:adverse effects*; D012959:Socioeconomic Factors
keywords: Breast cancer; CONOR; Education; Incidence; Norway; Smoking