Level 2: Mid-level evidence
Incidental exposure of the heart to radiation is a common unintended consequence of radiation therapy for breast cancer. The association between cardiac radiation exposure and the development of ischemic heart disease was investigated in a recent population-based case-control study with women from Swedish and Danish cancer registries.1
A total of 2,168 women younger than age 75 years who had radiation therapy for breast cancer between 1958 and 2001 were analyzed. The cases were 963 women who had a major coronary event (myocardial infarction, revascularization or death from ischemic heart disease) before breast cancer recurrence or diagnosis of a second cancer. The controls were 1,205 women (matched to cases on age and year of diagnosis) who also had radiation therapy, but who did not have a major coronary event before the time to event of the matched case.
The women were assessed for incidental radiation exposure of whole heart and left anterior descending coronary artery based on reviews of their treatment fields and dose plans.
The estimated mean radiation exposure of the heart was 4.9 Gy overall (cases and controls). For each 1-Gy increase in heart exposure, the risk of a major coronary event was increased by 7.4% (95% CI: 2.9%-14.5%) overall. The increase in risk was highest within the first four years following radiation treatment (16.3%, 95% CI: 3%-64.3%) and fell to 8.2% (95% CI: 0.4%-26.6%) at ≥20 years after treatment.
Cases had higher rates of baseline cardiovascular factors than did controls, and women with risk factors had higher absolute event rates than did women without, but the increase in risk per radiation dose was independent of underlying risk.
The treatment period of this study spans many decades, and breast-radiation techniques have evolved to limit the exposure of the heart and left anterior descending coronary artery, although radiation exposures may not be eliminated entirely. Current data suggest that there is no threshold level of radiation at which cardiovascular risk begins to increase. Therefore, these findings may be useful in deciding what treatment for breast cancer is most appropriate, taking into account the woman’s underlying cardiovascular risk profile.
They also may help guide the care of women who were treated before improved radiation methods were in use and may have increased risks comparable to the women in this study.
Alan Ehrlich, MD, is a deputy editor for DynaMed, in Ipswich, Mass., and assistant clinical professor in Family Medicine, University of Massachusetts Medical School in Worcester.
DynaMed is a database that provides evidence-based information on more than 3,000 clinical topics and is updated daily through systematic surveillance covering more than 500 journals.