BACKGROUND: Improved breast cancer (BC) survival and evidence showing beneficial effects of internal mammary chain (IMC) irradiation underscore the importance of studying late cardiovascular effects of BC treatment.METHODS: We assessed cardiovascular disease (CVD) incidence in 14,645 Dutch BC patients aged <62 years, treated during 1970-2009. Analyses included proportional hazards models and general population comparisons.RESULTS: CVD rate-ratio for left-versus-right breast irradiation without IMC was 1.11 (95% CI 0.93-1.32). Compared to right-sided breast irradiation only, IMC irradiation (interquartile range mean heart doses 9-17 Gy) was associated with increases in CVD rate overall, ischaemic heart disease (IHD), heart failure (HF) an...