title: Parity, hormones and breast cancer subtypes - results from a large nested case-control study in a national screening program.
abstract: Breast cancer comprises several molecular subtypes with different prognoses and possibly different etiology. Reproductive and hormonal factors are associated with breast cancer overall, and with luminal subtypes, but the associations with other subtypes are unclear. We used data from a national screening program to conduct a large nested case-control study.
  
  
  
  We conducted a nested case-control study on participants in the Norwegian Breast Cancer Screening Program in 2006 - 2014. There was information on estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) for 4748 cases of breast cancer. Breast cancer subtypes were defined as luminal A-like (ER+ PR+ HER2-), luminal B-like (ER+ PR- HER2- or ER+ PR+/PR-HER2+), HER2-positive (ER- PR- HER2+) and triple-negative (ER- PR- HER2-). Conditional logistic regression was used to estimate odds ratios (ORs) of breast cancer associated with age at first birth, number of pregnancies, oral contraceptive use, intrauterine devices and menopausal hormone therapy. Analyses were adjusted for age, body mass index, education, age at menarche, number of pregnancies and menopausal status.
  
  
  
  Number of pregnancies was inversely associated with relative risk of luminal-like breast cancers (p-trend ≤0.02), and although not statistically significant, with HER2-positive (OR = 0.60, 95% CI 0.31-1.19) and triple-negative cancer (OR = 0.70, 95% CI 0.41-1.21). Women who had ≥4 pregnancies were at >40% lower risk of luminal-like and HER2-positive cancers than women who had never been pregnant. However, there was a larger discrepancy between tumor subtypes with menopausal hormone use. Women who used estrogen and progesterone therapy (EPT) had almost threefold increased risk of luminal A-like cancer (OR = 2.92, 95% CI 2.36-3.62) compared to never-users, but were not at elevated risk of HER2-positive (OR = 0.88, 95% CI 0.33-2.30) or triple-negative (OR = 0.92, 95% CI 0.43 - 1.98) subtypes.
  
  
  
  Reproductive factors were to some extent associated with all subtypes; the strongest trends were with luminal-like subtypes. Hormone therapy use was strongly associated with risk of luminal-like breast cancer, and less so with risk of HER2-positive or triple-negative cancer. There are clearly some, but possibly limited, etiologic differences between subtypes, with the greatest contrast between luminal A-like and triple-negative subtypes.
  
  
  
  Not applicable.
authors.affiliation: Cancer Registry of Norway, Oslo, Norway.
authors.forename: Merete
authors.identifier: 0000-0003-2758-0140
authors.initials: M
authors.lastname: Ellingjord-Dale
authors.affiliation: Cancer Registry of Norway, Oslo, Norway.
authors.forename: Linda
authors.initials: L
authors.lastname: Vos
authors.affiliation: Cancer Registry of Norway, Oslo, Norway.
authors.forename: Steinar
authors.initials: S
authors.lastname: Tretli
authors.affiliation: Cancer Registry of Norway, Oslo, Norway.
authors.forename: Solveig
authors.initials: S
authors.lastname: Hofvind
authors.affiliation: Department of Non-Communicable Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, UK.
authors.forename: Isabel
authors.initials: I
authors.lastname: Dos-Santos-Silva
authors.affiliation: Cancer Registry of Norway, Oslo, Norway. Giske.Ursin@kreftregisteret.no.|University of Oslo, Oslo, Norway. Giske.Ursin@kreftregisteret.no.|University of Southern California, Los Angeles, CA, USA. Giske.Ursin@kreftregisteret.no.
authors.forename: Giske
authors.initials: G
authors.lastname: Ursin
mesh_terms: D000368:Aged; D014408:Biomarkers, Tumor; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology* / Q000473:pathology; D016022:Case-Control Studies; D015986:Confounding Factors, Epidemiologic; D005260:Female; D020249:Hormone Replacement Therapy / Q000009:adverse effects; D006728:Hormones / Q000378:metabolism; D006801:Humans; D008403:Mass Screening; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D016017:Odds Ratio; D010298:Parity; D011159:Population Surveillance; D018719:Erb-b2 Receptor Tyrosine Kinases; D011960:Receptors, Estrogen; D011980:Receptors, Progesterone; D012042:Registries; D017584:Reproductive History; D012307:Risk Factors
chemical_list: D014408:Biomarkers, Tumor; D006728:Hormones; D018719:Erb-b2 Receptor Tyrosine Kinases; D011960:Receptors, Estrogen; D011980:Receptors, Progesterone
keywords: Breast cancer subtypes; Hormone therapy; Reproductive factors