Integrating static and dynamic modulation: An evaluation of RapidArc Dynamic for left-sided chest wall irradiation.

Abstract

e12511 Background: Left-sided breast cancer radiotherapy poses a significant challenge in balancing target coverage with the sparing of critical cardiac substructures. RapidArc Dynamic (RAD) is a novel hybrid technique that integrates the continuous arc delivery of volumetric modulated arc therapy (VMAT) with the fixed-beam modulation of intensity modulated radiotherapy (IMRT) through strategic gantry pauses and dynamic collimator rotation. This study aims to evaluate the dosimetric performance of RAD, benchmarking it against IMRT and VMAT, with a specific focus on cardiac substructure sparing and normal tissue complication probability (NTCP). Methods: Twenty patients with left-sided breast cancer treated with post-mastectomy adjuvant radiotherapy were retrospectively enrolled. For each patient, RAD (2 partial arcs with 6 gantry pauses), IMRT (9 fields), and VMAT (5 partial arcs) plans were generated with a prescription of 50 Gy in 25 fractions. Dosimetric metrics, plan complexity, and conformity index were analyzed, and NTCP values were calculated using the Lyman-Kutcher-Burman (LKB) model for multiple endpoints. Results: RAD demonstrated significantly lower (P < 0.05) doses to the heart, left lung, and spinal cord compared to both IMRT and VMAT. Specifically, RAD achieved superior sparing of the left anterior descending coronary artery (LAD) and left ventricle (LV), reducing mean and maximum doses significantly compared to competing modalities. While VMAT resulted in a higher low-dose bath to the contralateral breast, RAD successfully mitigated this issue, delivering significantly lower contralateral doses. Furthermore, RAD yielded significantly lower predicted NTCPs for cardiac perfusion defects, LA major adverse cardiac events, and pulmonary pneumonitis compared to IMRT and VMAT. Conclusions: RAD offers a feasible and effective alternative for left-sided chest radiotherapy, providing an optimal balance between high dose conformity and superior ipsilateral organ sparing. By significantly reducing doses to critical cardiac substructures and minimizing predicted complications, RAD may reduce the risk of late toxicity in breast cancer survivors.