S1113
Clinical – Upper GI
ESTRO 2026
Toronto, Toronto, Canada. 3 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Canada Purpose/Objective: We report our initial clinical experience with a two- phase radiotherapy approach for esophageal cancer patients. This approach integrates conventional CBCT- based linear accelerator radiotherapy with MR-guided online adaptive radiotherapy on the Unity MR-Linac system. This strategy addresses the challenge of OAR proximity, which often limits full radiation dose delivery and compromises tumor control. Material/Methods: We implemented a two-phase, 25-fraction protocol, consisting of a conventional radiotherapy phase followed by a 5- or 7-fraction adaptive boost on the Unity MR-Linac system. To date, we have accrued six patients to our PET-CT/MR study, and three patients were treated with the boost (two with 5 fractions, one with 7 fractions). To establish safe and effective treatment parameters for this novel boost phase, conservative clinical goals were developed. These goals were derived from analyzing dose accumulation in study patients who had previously received only conventional radiotherapy, implicitly optimizing OAR sparing. To validate plan delivery and account for internal tumor motion, a "dose-of-the-day" (DOTD) methodology was adapted for esophageal cases. This involved using deformable image registration (DIR) within the RayStation planning system to align pre- treatment "adapt-to-shape" (ATS) MR images with "during beam-on" (BON) MR images. Target and OAR contours were propagated from ATS to BON images, allowing for the calculation and recording of any changes in dosimetric metrics. We also evaluated the dosimetric advantage of using online adaptive versus IGRT treatments by evaluating the conventional phase II plan using the MR plan data from the boost fractions. Results: MR image quality proved consistently adequate for accurate GTV delineation and for effective monitoring of patient setup under motion conditions across all Unity online adaptive treatment fractions. The DOTD analysis confirmed that both GTV and OAR clinical goals were consistently met, remaining within 10cGy per fraction of their corresponding clinical goals. GTV volumes from the MR-based boost fractions showed on average a less than 1 cc inter-fraction variation, but a 7.5cc difference with respect to CT. DICE values were 0.86 for MR-to-MR and varied between 0.68–0.79 compared to CT. The dosimetric gain for the heart (main OAR) was between 1-2Gy for 5-7 fractions when compared to the conventional phase-II conventional RT.
Figure1. Example case treated with the combined approach.
Figure2. DOTD plan delivery evaluation of the Unity online plans.
Conclusion: Online adaptive MRI-guided radiotherapy is a feasible and effective treatment modality for esophageal cancer. This approach significantly enhances the sparing of OARs, offering a promising advancement in precision radiation oncology. Keywords: esophagus, MR-guided, MRI-Linac, dose accumulation
Digital Poster Highlight 4302
Target dose deposition and tumor control in proton therapy for distal esophageal cancer Kyra A.L van Keeken 1 , Vlad Badiu 1 , Gloria Vilches- Freixas 1 , Jeroen Buijsen 1 , Bastiaan Ta 1 , Britt Hupkens 1 , Inge Steenbakkers 1 , Valery E.P.P. Lemmens 2 , Richard Canters 1 , Maaike Berbée 1 1 Radiation Oncology, MAASTRO, Maastricht, Netherlands. 2 GROW Research Institute for Oncology and Reproduction, Maastricht University Medical Centre+, Maastricht, Netherlands Purpose/Objective: In esophageal cancer, dose uncertainties during chemoradiotherapy arise from anatomical changes and diaphragm motion (1). This is particularly relevant in proton therapy, where high conformity increases sensitivity to anatomical shifts. Although ITV coverage is monitored, this does not guarantee adequate GTV dose, especially in distal tumors near the diaphragm – prone to underdosage from baseline shifts (2) and diaphragm motion. Moreover, ITV underdosage is sometimes accepted, yet its impact on tumor response remains unclear. This study investigates the relationship between target dose deposition and tumor response.
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