Left Anterior Descending Artery Avoidance in Patients Receiving Breast Irradiation

Abstract

Dose to the left anterior descending artery (LAD) contributes to cardiac toxicity for patients receiving left-sided irradiation for breast cancer. We investigated if prospective contouring and avoidance of the LAD during treatment planning were associated with lower LAD dose. We reviewed dosimetric plans for 323 patients with left-sided breast cancer who received whole breast or chest wall irradiation with or without internal mammary lymph node (IMN) coverage between 1/2014 and 1/2019 at a single institution. All patients received non-contrast CT-based, inverse planned, segmented, 3D-conformal, or volumetric arc treatment. The LAD was contoured prospectively for 155 cases, and techniques, including beam angle modification, collimator leaf adjustment, and planning optimization, were utilized to minimize LAD dose. Dose-volume-histograms from these patients were compared to those of 168 patients for whom the LAD was contoured retrospectively after treatment completion. When needed to account for differences in fractionation, doses were expressed as an EQD2, calculated as D{[D/n+r]/[2+r]}, where D is the composite dose, n is the number of fractions, and r is the alpha/beta ratio, taken as 2 for late effects of heart and coronary vessels. Median values of unadjusted or EQD2 mean and max doses were compared using the Wilcoxon Rank Sum test. Of 250 patients who received WBI without IMN coverage, 161 (64%) received a surgical cavity boost and 202 (81%) received hypofractionated treatment. Compared to cases where the LAD was contoured retrospectively (n=126), prospective LAD contouring (n=124) was associated with lower unadjusted max and mean LAD doses: 8.5 Gy vs. 5.2 Gy (p<0.0001) and 3.6 Gy vs. 2.7 Gy (p<0.0001), respectively. EQD2 max and mean LAD doses were also lower with prospective LAD contouring: 5.1 Gy vs. 2.9 Gy (p<0.001) and 1.9 Gy vs. 1.5 Gy (p<0.0001), respectively. Of 73 patients with IMN coverage, 67 (92%) received a surgical cavity or scar boost, and all received conventionally fractionated treatment. Compared to cases where the LAD was contoured retrospectively (n=42), prospective LAD contouring (n=31) was associated with lower max LAD doses: 20.4 Gy vs. 14.3 Gy (p=0.042). There was a non-significant reduction in mean LAD dose: 6.2 Gy vs. 6 Gy (p=0.33). Reductions in LAD doses were achieved while maintaining IMN coverage (mean V90%Rx>90%). Prospective contouring and avoidance of the LAD were associated with lower max and mean LAD doses in patients receiving WBI. In patients requiring IMN coverage, prospective LAD contouring was associated with lower max but not mean LAD doses. Further reduction in LAD dose may require stricter optimization weighting or compromise in IMN coverage. Correlation between LAD dose and cardiac morbidity is needed to establish the clinical relevance of LAD dosimetry and appropriate dose constraints for this structure.