A recent study by Darby and colleagues showed a significant linear increase in risk for major coronary events according to mean cardiac dose of radiation (7.4% per Gy) in patients receiving radiation therapy for breast cancer between 1958 and 2001. In an analysis reported in a research letter in JAMA Internal Medicine, David J. Brenner, PhD, DSc, of Columbia University Medical Center, and colleagues assessed risk associated with more modern radiation therapy techniques and found that estimated lifetime risks associated with treatment now range from 0.05% to 3.5%.
The study was a prospective evaluation of patient-specific radiation doses in 48 patients with stage 0 to IIA breast cancer treated after 2005 at New York University Department of Radiation Oncology. Excess absolute risks for major coronary events were calculated using the dose-response relationship reported by Darby and colleagues. Risk estimates were calculated for patients with low, medium, and high baseline risk for coronary events, with baseline risk assessed according to the standard Reynolds risk algorithm.
Average Cardiac Dose
For standard supine position radiation therapy, the patient-averaged mean cardiac dose was 1.37 Gy, less than one-third of the average dose reported for 1958 to 2001 by Darby and colleagues. For left-sided treatment, mean doses were 2.17 Gy for the supine position and 1.03 Gy for the prone position. For right-sided treatment, mean doses were 0.62 Gy for the supine position and 0.64 for the prone position.
Lifetime excess risk for patients receiving left-sided treatment was calculated at 0.22%, 0.42%, and 3.52% for patients with low, medium, and high baseline risk, respectively, who received supine position treatment and 0.09%, 0.17%, and 1.31%, respectively, for patients receiving prone position treatment. For patients receiving right-sided treatment, excess risk was 0.05%, 0.10%, and 0.79%, respectively, in patients receiving supine treatment and 0.06%, 0.11%, and 0.84%, respectively, in those receiving prone treatment.
The investigators concluded: “Cardiac doses from breast radiotherapy have generally decreased during recent decades … so typical risks of major cardiac events associated with contemporary radiotherapy are lower than in earlier eras…. Because the effects of radiation exposure on cardiac disease risk seem to be multiplicative, the highest absolute radiation exposure risks correspond to the highest baseline cardiac risk. Consequently, radiotherapy-induced risks of major coronary events are likely to be reduced in these patients by targeting baseline cardiac risk factors … by lifestyle modification, and/or by pharmacological treatment.”
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