To determine factors that may modify the association between hormone therapy (HT) and breast cancer risk.
Prospective cohort study (the Melbourne Collaborative Cohort Study) of 24,479 women aged 40 to 69 years. History of HT use was collected at baseline and 4 years later by questionnaire. By June 2002, 336 cases of breast cancer were diagnosed among 13,444 women postmenopausal at baseline. Association of breast cancer risk with history of HT use was analyzed using proportional hazards models.
The hazard ratio (HR) for recent HT use (current or stopped within the last year) was elevated (HR 1.51; 95% CI, 1.16-1.98) but was not significantly increased for past HT users (HR 1.19; 95% CI, 0.86-1.64). Recent HT use was associated with better differentiated tumors but was not more likely to be associated with estrogen receptor-positive / progesterone receptor-positive tumors. There was little evidence of interactions between recent HT use and body mass index, alcohol intake, parity, and smoking, although the HR for recent HT use in categories of alcohol consumption was greatest in women consuming the most alcohol (HR 2.37; 95% CI, 1.45-3.88 for those consuming ≥ 10 g/d versus HR 1.33; 95% CI, 0.85-2.08 for nondrinkers, P interaction = 0.32).
The risk of breast cancer for recent users of HT in this Australian population is increased by approximately 50%. Our results suggest that any potential modifying effect of the association between HT and breast cancer risk by factors such as alcohol intake and body mass index is likely to be modest.
This prospective Australian cohort study of nearly 25,000 women found that although the risk of breast cancer for recent users of HT was elevated by approximately 50%, any potential modifying effect of the association by factors such as alcohol intake and body mass index was likely to be modest.
From the 1Centre for Genetic Epidemiology, University of Melbourne, Melbourne Victoria, Australia; 2Department of Human Services, Melbourne, Victoria, Australia; 3Cancer Epidemiology Centre, The Cancer Council Victoria, Melbourne, Victoria, Australia; and 4School of Population Health, University of Melbourne Victoria, Australia.
Received March 9, 2005; revised and accepted June 27, 2005.
The Cancer Council Victoria and VicHealth funded cohort recruitment. This study was funded by grants from the National Health and Medical Research Council (126402; 209057; 170215; 251533), the National Breast Cancer Foundation and was further supported by infrastructure provided by The Cancer Council Victoria. Dorota Gertig is funded by a Victorian Health Promotion Foundation Senior Research Fellowship.
Address correspondence to: Dr. Dorota Gertig, Centre for Genetic Epidemiology, University of Melbourne, 723 Swanston St., Carlton, Melbourne Victoria, Australia. E-mail: firstname.lastname@example.org.