A population-based study of cardiovascular mortality following early-stage breast cancer

Importance — There is increasing interest in the effect of cardiovascular disease on cancer survivors. However, there are limited contemporary population-based data on the risk of cardiovascular death after early-stage breast cancer.

Objective — To describe the incidence of cardiovascular death in a contemporary population of women with early-stage breast cancer while accounting for competing risks.

Design, Setting, and Participants — A population-based cohort study was conducted among 98 999 women diagnosed with early-stage breast cancer between April 1, 1998, and March 31, 2012. Patients were followed up until death or were censored on December 31, 2013. Baseline characteristics were determined from administrative databases and the Ontario Cancer registry. Vital statistics data were used to determine the cause of death. Cumulative incidence functions were used to estimate the incidence of cause-specific mortality. We studied the association between baseline characteristics and rates of cardiovascular death using cause-specific hazard functions. The analyses accounted for competing risks of noncardiovascular death. Statistical analysis was performed from July 16, 2015, to August 4, 2016.

Exposures — Early-stage breast cancer, age, cardiovascular disease, hypertension, and diabetes.

Main Outcomes and Measures —Cause of death, which was classified as breast cancer, cardiovascular disease, other cancers, or other noncancer causes.

Results — Of the 98 999 women (median age, 60 years [interquartile range, 50-71 years]) in the study, 21 123 (21.3%) died during follow-up. The median time to death was 4.2 years (IQR, 2.2-7.1 years). Breast cancer was the most common cause of death (10 550 deaths [49.9%]); 3444 deaths [16.3%] were from cardiovascular causes. Cardiovascular death was infrequent in women younger than 66 years without prior cardiovascular disease, diabetes, or hypertension. Among women 66 years or older, the risks of breast cancer death and cardiovascular death at 10 years were 11.9% (95% CI, 11.6%-12.3%) and 7.6% (95% CI, 7.3%-7.9%), respectively. Among patients with prior cardiovascular disease, the risk of death from breast cancer and cardiovascular disease were equivalent for the first 5 years, after which death from cardiovascular causes was more frequent (10-year cumulative incidence, 14.6% [95% CI, 13.7%-15.4%] for breast cancer vs 16.9% [95% CI, 16.0%-17.8%] for cardiovascular disease). For women 66 years or older who survived 5 years or more after diagnosis of breast cancer, cardiovascular disease exceeded breast cancer as the leading cause of death at 10 years after diagnosis, when the cumulative incidence of each was 5%.

Conclusions and Relevance— Cardiovascular death is an important competing risk for older women with early-stage breast cancer. This finding mandates adequate attention to cardiovascular preventive therapy after diagnosis of breast cancer.