Past use of hormone therapy and breast cancer risk.

Abstract

1502 Background: Although current use of hormone therapy (HT) is clearly associated with breast cancer risk, most studies have not seen an increased risk among past users of HT. However, the effects of longer term past use of HT have not been well quantified. Methods: We examined the relationship between past HT use and invasive breast cancer risk within the Nurses’ Health Study, a prospective cohort of 121,700 registered nurses aged 30-55 in 1976 who update information on cancer risk factors and outcomes through biennial questionnaires. For this analysis, the follow-up period was 1980 through 2006 and only included person-time for postmenopausal women. Status of HT use (never, past, or current), type of HT (oral unopposed estrogen (E alone) or combination oral estrogen + progesterone (E+P)), and duration of HT use were assessed every 2 years and defined prospectively. Proportional hazards models controlled for age, body mass index, parity, age at first birth, family history of breast cancer, benign breast disease, alcohol consumption, type of menopause, and ages at menarche and menopause. Results: During 1,458,952 person-years of follow-up among postmenopausal women, invasive breast cancer was diagnosed among 1653 women who had never used HRT, 365 past users of E+P and 532 past users of E alone. Regardless of duration of use, past use of E alone was not associated with breast cancer risk. Although past use of E+P for less than 10 years was not associated with breast cancer risk (RR (95% CI) 0.91 (0.77-1.06) for < 5 years and 1.06 (0.88-1.27) for 5-9.9 years), past use of E+P for greater than 10 years was associated with an increased risk of breast cancer (RR 1.37 (1.06-2.77) for 10-14.9 years and 1.46 (0.97-2.21) for 15-19.9 years, p for trend=0.004). Results were similar regardless of time since last use, but stronger for hormone-receptor positive tumors. Conclusions: Although shorter-term past use of E+P was not associated with breast cancer risk, past use of E+P for greater than 10 years was associated with persistenly increased breast cancer risk even after HT discontinuation.