Proportion of U.S. Trends in Breast Cancer Incidence Attributable to Long-term Changes in Risk Factor Distributions

Abstract

Background: U.S. breast cancer incidence has been changing, as have distributions of risk factors, including body mass index (BMI), age at menarche, age at first live birth, and number of live births.Methods: Using data for U.S. women from large nationally representative surveys, we estimated risk factor distributions from 1980 to 2008. To estimate ecologic associations with breast cancer incidence, we fitted Poisson models to age- and calendar year-specific incidence data from the NCI's Surveillance, Epidemiology and End Results registries from 1980 to 2011. We then assessed the proportion of incidence attributable to specific risk factors by comparing incidence from models that only included age and calendar period as predictors with models that additionally included age- and cohort-specific categorized mean risk factors. Analyses were stratified by age and race.Results: Ecologic associations usually agreed with previous findings from analytic epidemiology. From 1980 to 2011, compared with the risk factor reference level, increased BMI was associated with 7.6% decreased incidence in women ages 40 to 44 and 2.6% increased incidence for women ages 55 to 59. Fewer births were associated with 22.2% and 3.99% increased incidence in women ages 40 to 44 and 55 to 59 years, respectively. Changes in age at menarche and age at first live birth in parous women did not significantly impact population incidence from 1980 to 2011.Conclusions: Changes in BMI and number of births since 1980 significantly impacted U.S. breast cancer incidence.Impact: Quantifying long-term impact of risk factor trends on incidence is important to understand the future breast cancer burden and inform prevention efforts. Cancer Epidemiol Biomarkers Prev; 27(10); 1214-22. ©2018 AACR.

Figure 1.

Distribution of mean BMI, mean age at menarche, mean number of births, mean age at first live among parous women from 1980 to 2008 for various age groups, reweighted to the population of all U.S. women. Figure 1 shows the distribution of breast cancer risk factors in all U.S. women over time in 4 age groups. Panels A) through D) correspond to the distribution of BMI, age at menarche, number of live births and age at first live birth, respectively.

Figure 2.

Distribution of mean BMI, mean at menarche, mean number of births, mean age at first live birth among parous women from 1980 to 2008 for various age groups for black U.S. women. Figure 2 shows the distribution of breast cancer risk factors in black U.S. women over time in 4 age groups. Panels A) through D) correspond to the distribution of BMI, age at menarche, number of live births and age at first live birth, respectively.

Figure 3.

Plot against calendar time of unadjusted breast cancer log₁₀-incidence and adjusted log₁₀-incidence for women at the reference level of risk factors for a 40-44 year-old woman in the U.S. The reference categories are <25.3 for BMI, (13.1, 13.6] for age at menarche, (2.9,3.61] for number of births and [17.7,21.6] for age at first live birth. Figure 3 contains observed breast cancer incidence on the log-10 scale in 40-44 year old women and breast cancer incidence that would have been observed when risk factors are held fixed at a low-risk reference category. Panels A) through D) correspond to adjustment for BMI, age at menarche, number of live births and age at first live birth, respectively.

Figure 4.

Plot against calendar time of unadjusted breast cancer log₁₀-incidence and adjusted log₁₀-incidence for women at the reference level of risk factors for a 60-64 year-old woman in the U.S. The reference categories are <25.3 for BMI, (13.1, 13.6] for age at menarche, (2.9,3.61] for number of births and (21.6,22.1] for age at first live birth. Figure 4 contains observed breast cancer incidence on the log-10 scale in 60-64 year old women and breast cancer incidence that would have been observed when risk factors are held fixed at a low-risk reference category. Panels A) through D) correspond to adjustment for BMI, age at menarche, number of live births and age at first live birth, respectively.