Could recent decreases in breast cancer incidence really be due to lower HRT use? Trends in attributable risk for modifiable breast cancer risk factors in Canadian women.

OBJECTIVES Recent downward trends in breast cancer incidence have been attributed to declining use of hormone replacement therapy (HRT). To determine whether this is a credible conclusion, this study calculated population attributable risk (PAR) for HRT and other modifiable breast cancer risk factors. METHODS PAR calculation needs both the prevalence of a risk factor, and the relative risk (RR) for breast cancer incidence for that risk factor. Prevalences were calculated for Canadian women, aged 50-69, participating in the National Population Health Survey, 1994-2006. RR were derived from published research: 1.4 for HRT use, 1.4 for excessive alcohol use, 1.15 for physical inactivity, 1.25 for smoking, 1.4 for BMI over 30 kg/m2. Trends for PAR were calculated for the risk factors separately, as well as combined. Age-adjusted breast cancer incidence rates were calculated for Canadian women aged 50-69 for the years 1994-2004. RESULTS Between 1998 and 2004, PAR for HRT decreased by 50%. PAR for other risk factors showed only small changes, and the combined PAR decreased by 18.6%. Age-adjusted breast cancer incidence for women aged 50-69 peaked in 2000 at 330.0/100,000, then dropped by 17.2% by 2004. CONCLUSION Patterns of PAR for HRT use in Canada are consistent with the noticeable decrease in breast cancer incidence observed for women of the same age group. Combining PAR for all risk factors indicated that changes in HRT use overpowered any trends of other risk factors. The combined PAR suggest that alterations in lifestyle could have considerable impact on breast cancer incidence.