title: Breast cancer risk in relation to the interval between menopause and starting hormone therapy.
abstract: BACKGROUND
  Although breast cancer risk is greater in users of estrogen-progestin than estrogen-only formulations of menopausal hormonal therapy, reports on their effects have been somewhat inconsistent. We investigated whether the timing of these therapies affected breast cancer incidence.
  
  
  METHODS
  A total of 1,129,025 postmenopausal UK women provided prospective information on hormonal therapy use and other factors relevant for breast cancer risk. We used Cox regression to estimate adjusted relative risks (RRs) of breast cancer in hormonal therapy users vs never users and calculated standardized incidence rates. All statistical tests were two-sided.
  
  
  RESULTS
  During 4.05 million woman-years of follow-up, 15,759 incident breast cancers occurred, with 7107 in current users of hormonal therapy. Breast cancer incidence was increased in current users of hormonal therapy, returning to that of never users a few years after use had ceased. The relative risks for breast cancer in current users were greater if hormonal therapy was begun before or soon after menopause than after a longer gap (P(heterogeneity) < .001, for both estrogen-only and estrogen-progestin formulations). Among current users of estrogen-only formulations, there was little or no increase in risk if use began 5 years or more after menopause (RR = 1.05, 95% confidence interval [CI] = 0.89 to 1.24), but risk was statistically significantly increased if use began before or less than 5 years after menopause (RR = 1.43, 95% CI = 1.35 to 1.51). A similar pattern was observed among current users of estrogen-progestin formulations (RR = 1.53, 95% CI = 1.38 to 1.70, and RR = 2.04, 95% CI = 1.95 to 2.14, respectively). At 50-59 years of age, annual standardized incidence rates for breast cancer were 0.30% (95% CI = 0.29% to 0.31%) among never users of hormone therapy and 0.43% (95% CI = 0.42% to 0.45%) and 0.61% (95% CI = 0.59% to 0.64%), respectively, among current users of estrogen-only and estrogen-progestin formulations who began use less than 5 years after menopause.
  
  
  CONCLUSIONS
  There was substantial heterogeneity in breast cancer risk among current users of hormonal therapy. Risks were greater among users of estrogen-progestin than estrogen-only formulations and if hormonal therapy started at around the time of menopause than later.
authors.affiliation: University of Oxford, Oxford, OX3 7LF, UK. pa.valerie.beral@ceu.ox.ac.uk
authors.forename: Valerie
authors.initials: V
authors.lastname: Beral
authors.forename: Gillian
authors.initials: G
authors.lastname: Reeves
authors.forename: Diana
authors.initials: D
authors.lastname: Bull
authors.forename: Jane
authors.initials: J
authors.lastname: Green
mesh_terms: D000328:Adult; D000368:Aged; D001943:Breast Neoplasms / Q000139:chemically induced / Q000453:epidemiology* / Q000209:etiology*; D015914:Estrogen Replacement Therapy / Q000009:adverse effects* / Q000379:methods*; D004967:Estrogens / Q000008:administration & dosage / Q000009:adverse effects; D005260:Female; D006801:Humans; D015994:Incidence; D008593:Menopause; D008875:Middle Aged; D009765:Obesity / Q000150:complications*; D045167:Progesterone Congeners / Q000008:administration & dosage / Q000009:adverse effects; D016016:Proportional Hazards Models; D011446:Prospective Studies; D016032:Randomized Controlled Trials as Topic; D012107:Research Design; D018570:Risk Assessment; D012307:Risk Factors; D013997:Time Factors; D006113:United Kingdom / Q000453:epidemiology
chemical_list: D004967:Estrogens; D045167:Progesterone Congeners