BACKGROUND: Previous reports suggest that radiotherapy for breast cancer (BC) can cause ischemic heart disease, with the radiation-related risk increasing linearly with mean whole heart dose (MWHD). This study aimed to validate these findings in younger BC patients and to investigate additional risk factors for radiation-related myocardial infarction (MI). METHODS: A nested case-control study was conducted within a cohort of BC survivors treated during 1970-2009. Cases were 183 patients with MI as their first heart disease after BC. One control per case was selected, matched on age and BC diagnosis date. Information on treatment and cardiovascular risk factors was abstracted from medical- and radiation-charts. Cardiac doses were estimated for each woman by reconstructing her regimen using modern 3D computed tomography (CT) planning on a typical patient CT-scan. RESULTS: Median age at BC of cases and controls was 50.2 years (interquartile range (IQR): 45.7-54.7). Median time to MI was 13.6 years (IQR: 9.9-18.1). Median MWHD was 8.9 Gray (Gy) (range: 0.3-35.2 Gy). MI rate increased linearly with increasing MWHD (excess rate ratio (ERR) per Gy: 6.4%, 95% confidence interval (95% CI): 1.3%-16.0%). Patients receiving ≥20 Gy MWHD had a 3.4-fold (95% CI: 1.5-7.6) higher MI rate than unirradiated patients. ERRs were higher for younger women, with borderline significance (ERR<45years: 24.2%/Gy, ERR≥50years: 2.5%/Gy, pinteraction=0.054). Whole heart dose-volume parameters did not modify the dose-response relationship significantly. CONCLUSION: MI rate after radiation for BC increases linearly with MWHD. Reductions in MWHD are expected to contribute to better cardiovascular health of BC survivors.
Int J Radiat Oncol Biol Phys