Variation in Left Anterior Descending Coronary Artery Accumulated Dose Estimates during Breast Cancer Radiotherapy

Abstract

Left anterior descending (LAD) coronary artery RT dose has been associated with the risk of coronary ischemic events in patients with breast cancer treated with radiotherapy (RT). However, consensus dose constraints commonly utilize mean heart dose, which has been shown to be an inadequate surrogate for LAD dose. Given the LAD is adjacent to the steep dose gradients of the left breast/chest wall, we hypothesize that variations in patient positioning or depth of breath hold may contribute to significant deviations in daily LAD dose exposure compared to predicted. Our objective was to investigate variations in accumulated LAD dose in patients with left-sided breast cancer treated with RT. Retrospective analysis of 10 consecutive patients with left-sided breast cancer treated between 2019-2022 with volumetric modulated arc therapy (VMAT) in the supine position. RT was delivered using daily cone beam computed tomography (CBCT) image guidance with deep-inspiratory breath hold technique and an optical surface monitoring system. Daily CBCT scans were individually registered to each planning CT based on daily positional changes. The LAD was manually segmented and transformed to each CBCT. Daily fractional dose was calculated (mean, volume receiving 15 Gy [V15 Gy], V30 Gy, and max) and summed to produce an accumulated dose which was compared to the predicted dose. Significant deviations in accumulated dose were defined as at least ±15% from predicted. The RT targets included breast/chest wall only (n = 1), supraclavicular nodes (n = 8), and/or internal mammary chain (n = 5). All plans were prescribed to 50 Gy in 25 fractions. The median predicted mean heart dose was 5.1 Gy. Overall, there were no significant differences between the median predicted vs. accumulated LAD doses: mean 10.4 vs. 10.2 Gy, V15 Gy 21% vs. 25%, V30 Gy 0% vs. 1%, max 24.5 vs 25.7 Gy (all p>.05). However, there was a subset of patients (n = 5, 50%) with significant deviations in accumulated vs. predicted dose (at least ±15%). For LAD mean, n = 2 had higher accumulated vs. predicted doses (16.6 vs. 11.6 Gy; 14.6 vs. 12.8 Gy), while n = 3 had lower (21.8 vs. 26.5 Gy; 2.7 vs. 4.2 Gy; 13.9 vs. 16.5 Gy). For LAD V15 Gy, n = 3 had higher accumulated vs. predicted doses (43 vs. 35%; 51% vs. 25%; 14% vs. 9%), while n = 2 had lower (49% vs. 61%; 37% vs. 56%). For LAD V30 Gy, n = 4 had higher accumulated vs. predicted doses (12% vs. 5%; 5% vs. 0%; 4% vs. 0%; 4% vs. 2%), while n = 1 had lower (32% vs. 44%). Daily setup differences, including the extent of inspiratory breath hold, may contribute to deviations in accumulated LAD dose (more than 15% of predicted) in approximately half of patients and were more pronounced in V15 and V30 Gy metrics. The potential for significant LAD dose uncertainty in a clinically meaningful subset of patients should be recognized and warrants further analysis in an expanded cohort.