Mediators of Racial Disparities in Heart Dose Among Whole Breast Radiotherapy Patients

Abstract

Background: Racial disparities in survival of patients with cancer motivate research to quantify treatment disparities and evaluate multilevel determinants. Previous research has not evaluated cardiac radiation dose in large cohorts of breast cancer patients by race nor examined potential causes or implications of dose disparities.

Methods: We used a statewide consortium database to consecutively sample 8750 women who received whole breast radiotherapy between 2012 and 2018. We generated laterality- and fractionation-specific models of mean heart dose. We generated patient- and facility-level models to estimate race-specific cardiac doses. We incorporated our data into models to estimate disparities in ischemic cardiac event development and death. All statistical tests were 2-sided.

Results: Black and Asian race independently predicted higher mean heart dose for most laterality-fractionation groups, with disparities of up to 0.42 Gy for Black women and 0.32 Gy for Asian women (left-sided disease and conventional fractionation: 2.13 Gy for Black women vs 1.71 Gy for White women, P < .001, 2-sided; left-sided disease and accelerated fractionation: 1.59 Gy for Asian women vs 1.27 Gy for White women, P = .002). Patient clustering within facilities explained 22%-30% of the variability in heart dose. The cardiac dose disparities translated to estimated excesses of up to 2.6 cardiac events and 1.3 deaths per 1000 Black women and 0.7 cardiac events and 0.3 deaths per 1000 Asian women vs White women.

Conclusions: Depending on laterality and fractionation, Asian women and Black women experience higher cardiac doses than White women. This may translate into excess radiation-associated ischemic cardiac events and deaths. Solutions include addressing inequities in baseline cardiac risk factors and facility-level availability and use of radiation technologies.

Figure 1.

Mean heart dose by race for women with left-sided disease undergoing conventionally fractioned (A) and accelerated (B) whole breast radiotherapy. Six separate models control for modifiable individual- and facility-level mediators of cardiac dose. For conventionally fractionated whole breast radiotherapy, doses for Black women are elevated compared with those of Asian women and White women. This disparity decreases as radiotherapy (RT) technique and clustering for facilities are controlled for. For accelerated whole breast radiotherapy, doses for Asian and Black women are elevated compared with those of White women. Clustering within facilities accounts for a substantial proportion of the disparity among Black women but only a modest proportion for Asian women. Error bars represent 95% confidence intervals. Tx = treatment.

Figure 2.

Cumulative risk of death or development of at least 1 of radiation-associated ischemic events for women who received conventionally fractionated or accelerated whole breast irradiation at age 60 years for left-sided disease. This figure shows race-stratified estimates for the number of women experiencing at least 1 radiation-associated ischemic cardiac event or death from an ischemic cardiac event by age 80 years as a result of radiation received at age 60 years. Mean cardiac doses were derived from Model 1, given that these doses most closely reflect the present-day experience. Absolute risks were calculated using the prevalence of 0 vs 1 or more cardiac risk factors.