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Reference: N Engl J Med 2013 Mar 14;368(11):987, (level 2 [mid-level] evidence)
Incidental exposure of the heart to radiation is a common unintended consequence of radiation therapy for breast cancer. A recent population-based case-control study with women from Swedish and Danish cancer registries investigated the association between cardiac radiation exposure and the development of ischemic heart disease.
A total of 2,168 women < 75 years old who had radiation therapy for breast cancer from 1958-2001 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 4 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 controls, and women with risk factors had higher absolute event rates than 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. The 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 approach for breast cancer is most appropriate, taking into account the woman’s underlying cardiovascular risk profile. They may 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.
For more information, see the Radiation therapy for breast cancer topic in DynaMed.