ITV-Negation and Dose Fall-Off Features Result in OAR Dosimetric Superiority Favoring MR-Guided Radiotherapy (MRgRT), and Intensity Modulated Proton Therapy (IMPT) Over Volumetric Modulated Arc Therapy (VMAT) for Treatment of Distal Esophageal Cancer.

Purpose/Objective(s) Radiation therapy (RT) for distal esophageal cancer (EC) may deliver significant unintended dose to organs at risk (OARs). Advanced RT modalities and motion management strategies could substantially reduce OAR dose, potentially resulting in clinically meaningful impact on clinical outcomes. The aim of this study is to investigate dosimetric plan quality of intensity modulated proton therapy (IMPT) and volumetric modulated arc therapy (VMAT), both in free breathing (FB), compared to real-time monitoring breath hold (BH) MR-guided radiotherapy (MRgRT) for distal EC. Materials/Methods Nine EC patients originally treated in mid-inspiration BH on a 0.35T MR-guided LINAC using step-and-shoot IMRT were retrospectively planned in FB using IMPT and VMAT. FB-IMPT and FB-VMAT plans incorporated an internal target volume (ITV) whereas BH-MRgRT plans did not. The prescribed dose was standardized at 50.4 Gy in 28 fractions for all 3 techniques. Both MRgRT and VMAT plans used planning target volumes (PTV) margins for optimization employing Monte Carlo and Acuros dose algorithms, respectively. All IMPT plans used single field optimization (SFO) technique and Monte Carlo dose algorithm (Robust optimization - 5% range and 5mm setup uncertainties) to ITVs. A comparison of plan quality was performed using RTOG-defined metrics for target coverage (TC), high dose conformity (PITV), homogeneity index (HI), and dose fall-off metrics: R50% and D2cm. Additionally, dose metrics to the OARs, (lungs, heart and liver) were evaluated. Results The conformity, target coverage and homogeneity indices among FB-IMPT, BH-MRgRT and FB-VMAT plans were similar; FB-IMPT plans generally had superior dose fall-off (Table 1, top). Achieved lung, liver and heart doses are displayed in Table 1 (bottom). Conclusion Both FB-IMPT and BH-MRgRT achieved a roughly 2-fold or greater reduction in nearly all assessed heart, lung, and liver dose metrics compared to FB-VMAT. Reducing, or ideally eliminating, respiratory motion should be strongly considered for distal EC management regardless of radiation modality or technique.