Dosimetric impact of interfractional organs at risk variation during high-dose rate interstitial brachytherapy for gynecologic malignancies

Abstract

Dose to the left anterior descending artery (LAD) may be significant in patients receiving leftsided irradiation for breast cancer. We investigated if prospective contouring and avoidance of the LAD during treatment planning were associated with lower LAD dose. Methods and Materials: We reviewed dosimetric plans for 323 patients who received left whole breast or chest wall irradiation with or without internal mammary node (IMLN) coverage between 1/2014 and 1/2019 at a single institution. The LAD was contoured prospectively for 155 cases, and techniques 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. EQD2 was calculated to account for fractionation differences. Results: Compared to cases where the LAD was contoured retrospectively ( n = 126), prospective LAD contouring ( n = 124) was associated with lower unadjusted median max and mean LAD doses for 250 patients receiving whole-breast irradiation (WBI) without IMLN coverage: 8.5 Gy vs 5.2 Gy ( p < 0.0 0 01) and 3.6 Gy vs 2.7 Gy ( p < 0.0 0 01), respectively. EQD2 median max and mean LAD doses were also lower with prospective LAD contouring: 5.2 Gy vs 3.0 Gy ( p < 0.0 0 01) and 1.9 Gy vs 1.5 Gy ( p < 0.0 0 01), respectively. Compared to cases where the LAD was contoured retrospectively ( n = 42), prospective LAD contouring ( n = 31) was associated with lower max LAD doses for 73 patients with IMLN coverage: 20.4 Gy vs 14.3 Gy ( p = 0.042). There was a nonsignificant reduction in median mean LAD dose: 6.2 Gy vs 6.1 Gy ( p = 0.33). LAD doses were reduced while maintaining IMLN coverage (mean V90% Rx > 90%). Conclusions: Prospective contouring and avoidance of the LAD were associated with lower max and mean LAD doses in patients receiving WBI and with lower max LAD doses in patients receiving IMLN treatment. Further reduction in LAD dose may require stricter optimization weighting or compromise in IMLN coverage.