The paper by Darby et al provides extremely important information on quantitative estimates of the effect of radiotherapy on coronary events in women with breast cancer.1 The authors have determined a direct relationship between radiation dose and effects on the heart and provide strong evidence that major coronary events increase by approximately 7% for every increase of 1 Gy in the mean heart dose. And although this rate was similar in women with and without cardiac risk factors, as expected, the overall rate was greater in women with preexisting cardiac risk factors. Surprisingly, the increase in events was evident within the first 5 years after radiotherapy, earlier than previous reports have suggested.
These data further stress the importance of the use of radiotherapy techniques that exclude the heart from the radiation field. Individualized treatment planning techniques that are computed tomography (CT)-based are critical when determining the optimal field arrangement. When necessary, planning and treatment modifications such as use of respiratory gating or active breathing control should be incorporated to displace the heart from the treatment field and/or heart blocks inserted to exclude the heart from the radiation beam. This paper also emphasizes the importance of treating other cardiac risk factors to minimize additive risk.
However, the study has an important limitation that needs to be considered when interpreting its findings. Due to the years of treatment, almost all patients were treated in a pre–CT-based treatment-planning era. Therefore, information from the radiation plans was used to reconstruct treatment fields using a CT scan from a single woman of typical anatomy. Body habitus is highly variable, and perfect alignment between the dose delivered and estimated dose is not possible based on planning using a single case.
This limitation is most critical when estimating dose in the low-dose/scatter region. It is not clear that we know, based on these data, that the linear relationship demonstrated at higher doses does in fact extend to the low-dose region. Such an analysis would be possible only with planning and outcome data from patients treated using CT-based individualized plans. To that end, studies are needed to correlate factors such as mean heart dose and other cardiac metrics with cardiac outcomes in patients treated with three-dimensional plans if we are to fully understand the complex dose/volume relationships.
In the meantime, patients should be informed of the potential for increased cardiac risks following radiotherapy but be reassured that all efforts are being taken to protect the heart from the effects of radiation. Furthermore, oncologists—radiation oncologists or otherwise—should increasingly work with internists and primary care physicians to maximize overall cardiac health for our patients. ■
Dr. Pierce is Professor of Radiation Oncology, University of Michigan School of Medicine, Ann Arbor.
Disclosure: Dr. Pierce reported no potential conflicts of interest.
1. Darby SC, Ewertz M, McGale P, et al: Risk of ischemic heart disease in women after radiotherapy for breast cancer. N Engl J Med 368:987-998, 2013.