Incidental exposure of the heart to ionizing radiation during radiotherapy for breast cancer increases the subsequent rate of ischemic heart disease, according to a study published March 14 in the New England Journal of Medicine.

The increase in heart disease risk is proportional to the mean dose to the heart and begins within a few years of exposure, according to Sarah C. Darby, PhD, of the University of Oxford, U.K., and colleagues.

“The relevance of our findings to a woman receiving radiotherapy for breast cancer today is that they make it possible to estimate her absolute risk of radiation-related ischemic heart disease,” wrote the authors. “This absolute risk can be weighed against the probable absolute reduction in her risk of recurrence or death from breast cancer that would be achieved with radiotherapy.”

Results were based on a population-based, case-control study of major coronary events—myocardial infarction, coronary revascularization or death from ischemic heart disease—in 2,168 breast cancer patients who underwent radiotherapy between 1958 and 2001 in Sweden and Denmark. Of the study population, 963 women experienced major coronary events while the remaining 1,205 served as controls.

The overall average of mean doses to the whole heart, estimated from the women’s radiotherapy charts, was 4.9 Gy, with a range of 0.03 to 27.72 Gy. Rates of major coronary events increased linearly with dose by 7.4 percent per gray, with no apparent threshold, reported Darby and colleagues. Women with pre-existing cardiac risk factors had a greater absolute increase in risk from radiotherapy, though the proportional increase per gray was similar whether or not the women had risk factors at the time of radiotherapy.

Increased risks began within the first five years after radiotherapy and continued for a least 20 years, according to the authors.

The doses of radiation to which the heart is exposed are generally lower today than with early breast cancer radiotherapy treatments, but incidental exposure has not been eliminated. “Current mean doses of radiation to the heart from radiotherapy for breast cancer are typically about 1 or 2 Gy for disease of the right breast. For disease of the left breast, the doses are usually higher but vary widely, and for some women, including those in whom the distance of the heart to the thoracic wall is small and those who require internal mammary irradiation, the mean dose may be around 10 Gy.”

Darby and colleagues concluded that “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.”