The estimated lifetime risk of major coronary events for women with breast cancer treated with radiation therapy in contemporary practice is 0.3 percent in a typical scenario, according to an analysis published Oct. 28 in JAMA Internal Medicine. But the risk can climb as high as 3.5 percent.

David J. Brenner, PhD, DSc, of the Center for Radiological Research at Columbia University Medical Center in New York City, and colleagues were motivated by a study published earlier this year that looked at cardiovascular outcomes in women treated with radiation therapy for breast cancer between 1958 and 2001. That study found the increase in heart disease risk was proportional to the mean dose to the heart, with an overall average mean dose to the whole heart of 4.9 Gy.

Brenner et al wanted to examine risk in a contemporary setting. To do so, they derived the mean cardiac dose for 48 women with early stage breast cancer who were treated after 2005 at their center. They used the same dose-response relationship and definition for radiation-induced coronary events as in the earlier study and applied that to three baseline risk levels: low, medium and high. They calculated cardiac risk out to 20 years.

Standard supine-position radiation therapy had a mean cardiac dose of 1.37 Gy, which was a third of the dose reported for 1958 through 2001. Mean cardiac dose for left-sided treatment in the supine position was half that reported for 1958 through 2001.

Women in the high-risk category who had left-sided treatment in the supine position carried the highest estimated lifetime risk for major coronary events, at 3.52 percent. Women who had low cardiac risk at baseline and received right-sided radiation therapy had the lowest risk, at less than 0.1 percent.  

Brenner et al placed the range of risk in contemporary practice at 0.05 percent to 3.5 percent, “with a typical value of 0.3 percent for a typical scenario.”

They suggested that the risk from radiation exposure could be lowered by applying lifestyle modification interventions or pharmaceutical treatments to risk factors such as hypertension or high cholesterol in patients deemed high risk at baseline.

In an accompanying letter, the authors of the analysis that used 1958 through 2001 data wrote that results for contemporary practice should be reassuring for most patients, whose absolute risk from their treatment “is likely small compared to the likely absolute benefit of radiotherapy.”

Carolyn Taylor, DPhil, and Sarah C. Darby, PhD, of the University of Oxford in the U.K., continued that their dose-response findings also can be used to identify the fraction of women whose risk might exceed the benefits of radiation therapy. They recommended further research to look at the relationship between radiation therapy and other types of heart disease as well as cardiac risk to women who underwent both radiation and chemotherapy.