Purpose: Regional nodal irradiation, including radiation therapy (RT) to the internal mammary node (IMN) region, improves oncologic outcomes in patients with node-positive breast cancer. Concern remains, however, given the proximity of the IMNs to the heart and the association between cardiac RT exposure and toxicity. The objective of the study was to evaluate rates of ischemic cardiac events (ICEs) and associated risk with treatment of the IMN region.
Methods and materials: The cardiac outcomes of 2126 patients treated with adjuvant breast RT or breast and nodal RT from 1984 to 2007 at a single institution were reviewed. The primary endpoint was an ICE following RT initiation. The association between IMN RT and ICEs was assessed using Cox proportional hazards models. Treatment with both IMN RT and 3-dimensional (3D) conformal radiation therapy (CRT) began in 1997; therefore, subset analyses of patients with only 3D CRT were performed to minimize bias associated with improved treatment technique.
Results: The median follow-up period was 9.3 years. An ICE occurred in 87 patients (4.1%). No increased 10-year rate of ICEs was observed with IMN RT compared with no IMN RT in the total cohort (3.2% [95% confidence interval (CI), 2.4%-4.3%] vs 3.4% [95% CI, 1.5%-7.5%]; hazard ratio [HR], 0.88; P=.73). Similarly, no statistically significant difference was noted in the 3D CRT-planned, left-sided disease subset (5.1% [95% CI, 1.8%-14.1%] vs 4.0% [95% CI, 2.0%-8.0%]; HR, 1.18, P=.76). On multivariate analysis, adjusting for cardiac risk factor imbalances, no significantly increased hazard was noted with IMN RT (HR, 1.84; P=.28) in the 3D CRT-planned, left-sided disease subset.
Conclusions: No statistically significant association between IMN RT and ICEs was demonstrated in a review of patients treated at a single institution from 1984 to 2007. Given the long natural history and low overall rate of ICEs, continued follow-up of this study, as well as additional studies in the 3D CRT era, is warranted to confirm these results. Minimizing cardiac exposure, when treating a limited IMN field, is critical to limit excess risk of ICEs.
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