(HealthDay News) — For women who receive radiotherapy for breast cancer, incidental exposure of the heart is associated with an increased risk of major coronary events, according to a study published in the March 14 issue of the New England Journal of Medicine.

Sarah C. Darby, Ph.D., from the University of Oxford in the United Kingdom, and colleagues examined the effect of incidental exposure of the heart to ionizing radiation and the subsequent risk of ischemic heart disease in a cohort of 2,168 women in Sweden and Denmark who underwent radiotherapy for breast cancer between 1958 and 2001. Participants included 963 women who experienced major coronary events and 1,205 controls.

The researchers found that the average of the mean doses to the whole heart was 4.9 Gy overall. As the mean dose to the heart increased there was a linear increase in the rates of major coronary events, by 7.4% per gray, with no evidence of a threshold. The increase was seen within the first five years following radiotherapy and persisted into the third decade after radiotherapy. The increase was similar for women with and without cardiac risk factors at the time of radiotherapy.

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“In conclusion, we found that incidental exposure of the heart to radiotherapy for breast cancer increased the rate of major coronary events by 7.4% per gray, with no apparent threshold,” the authors write. “Therefore, clinicians may wish to consider cardiac dose and cardiac risk factors as well as tumor control when making decisions about the use of radiotherapy for breast cancer.”

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