Age-related crossover in breast cancer incidence rates between black and white ethnic groups

Abstract

Background: Although breast cancer incidence is higher in black women than in white women among women younger than 40 years, the reverse is true among those aged 40 years or older. This crossover in incidence rates between black and white ethnic groups has been well described, has not been completely understood, and has been viewed as an artifact.

Methods: To quantify this incidence rate crossover, we examined data for 440 653 women with invasive breast cancer from the National Cancer Institute's Surveillance, Epidemiology, and End Results database from January 1, 1975, through December 31, 2004. Data on invasive female breast cancers were stratified by race, age at diagnosis, year of diagnosis, and tumor characteristics. Standard descriptive analyses were supplemented with Poisson regression models, age-period-cohort models, and two-component mixture models. All statistical tests were two-sided.

Results: We observed qualitative (ie, crossing or reversing) interactions between age and race. That is, age-specific incidence rates overall (expressed as number of breast cancers per 100 000 woman-years) were higher among black women (15.5) than among white women (13.1) younger than 40 years (difference = 2.4, 95% confidence interval [CI] = 2.4 to 2.4), and then, age-specific rates crossed with rates higher among white women (281.3) than among black women (239.5) aged 40 years or older (difference = 41.8, 95% CI = 41.7 to 41.9). The black-to-white incidence rate crossover was observed for all tumor characteristics assessed, although the crossover occurred at earlier ages of diagnosis for low-risk tumor characteristics than for high-risk tumor characteristics. The incidence rate crossover between ethnic groups was robust (ie, reliable and reproducible) to adjustment for calendar period and birth cohort effects in age-period-cohort models (P < .001 for difference by race).

Conclusion: Although this ecologic study cannot determine the individual-level factors responsible for the racial crossover in vital rates, it confirms that the age-related crossover in breast cancer incidence rates between black and white ethnic groups is a robust age-specific effect that is independent of period and cohort effects.

Figure 1

Age-standardized (US population in the year 2000) temporal trends and age-specific incidence trends for breast cancer among white and black women in the National Cancer Institute's Surveillance, Epidemiology, and End Results 9 Registries database from 1975 through 2004. A) Overall age-standardized incidence rate trends by race. B) Age-specific incidence rate trends by race (black vs white) and age at diagnosis (age groups ≥70, 50–69, 40–49, and <40 years). Poisson regression models were used to obtain P values to assess age interactions by race within age groups. All statistical tests were two-sided.

Figure 2

Age-specific incidence rates for breast cancer among white and black women in the National Cancer Institute's Surveillance, Epidemiology, and End Results 9 Registries database from 1975 through 2004. A) Age-specific incidence rates. Age-specific incidence rates were higher among black women than among white women younger than 40 years and higher among white women aged 40 years or older. This so-called incidence rate crossover between black and white ethnic groups was near Clemmesen's menopausal hook, which occurs at approximately age 50 years. Poisson regression models were used to obtain P values to assess age interactions by race. All statistical tests were two-sided. B) Age-specific incidence rate curves from the age–period–cohort fitted model that was adjusted for calendar period and birth cohort effects. The incidence rate crossover between black and white ethnic groups was robust (ie, reliable and reproducible) to these adjustments.

Figure 3

Incidence rate crossover between black and white women stratified by tumor characteristics (tumor size, LN status, tumor grade, and ER status) in the National Cancer Institute's Surveillance, Epidemiology, and End Results 9 Registries database from 1975 through 2004. The crossover was observed for all tumor characteristics (as indicated at the top of each panel), although more so for those with low-risk disease (tumor size ≤2.0 cm, axillary LN-negative disease, low-grade tumor, or ER-positive status) than for those with high-risk disease (tumor size >2.0 cm, axillary LN-positive disease, high-grade tumor, or ER-negative status). LN = lymph node; ER = estrogen receptor.

Figure 4

Age distributions at diagnosis by race (black vs white) and time period (1975–1979, 1980–1984, 1985–1989, 1990–1994, 1995–1999, and 2000–2004) in the National Cancer Institute's SEER 9 Registries database from 1975 through 2004. Bimodal age distributions are expressed as percent relative frequency among white and black women; 95% confidence intervals are shown as shaded bands and were calculated by using bootstrap resampling techniques. Of note, 95% confidence intervals were too narrow to be discernable for white women and are only slightly visible for black women, providing further evidence for distinct bimodal breast cancer populations among white and black women. Additionally, bimodal breast cancer populations fluctuated over time among white and black women. SEER = Surveillance, Epidemiology, and End Results.

Figure 5

Percentage of breast cancer patients with an early age at onset by race in the National Cancer Institute's Surveillance, Epidemiology, and End Results 9 Registries database from 1975 through 2004. Error bars = 95% confidence intervals.