ESTRO 2025 - Abstract Book

S3128

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Purpose/Objective: Both magnetic resonance-guided photon therapy (using an MR-LINAC) and proton therapy (MRgPT) are nowadays used to treat patients with hepatocellular carcinoma (HCC) [1]. We evaluated dosimetric differences between MR LINAC and MRgPT using a horizontal beamline in thirteen HCC patients, investigating plans without adaptation and adapted plans considering anatomical changes [2]. Material/Methods: Planning-CTs and T2-weighted MRI scans of patients treated on the Unity MR-LINAC (Elekta, Sweden) with the Adapt-to-Shape workflow were included. Using RayStation 2023B-R (RaySearch, Sweden), adaptive step-and-shoot IMRT plans were calculated on all daily MRI scans applying density overrides extracted from the planning-CT for a prescribed dose of 48Gy (D pres,avg,PTV =6x8Gy) to the planning target volume (PTV) [3]. Adaptive intensity-modulated proton therapy plans (robustness settings 3mm,3%) to the clinical target volume (CTV) were created, using two beams from 90° with a couch rotation of ±10° to a total dose of 48Gy(RBE). Additionally, the MR-LINAC and MRgPT plans were recalculated without adaptation on all daily MRI scans. The treatment plans were compared in terms of dose volume histograms parameters for the gross tumor volume (GTV) and CTV (Dx%) and organs at risk (OAR, VxGy and D 700cc,UninvolvedLiver with UninvolvedLiver=Liver-GTV). Results: The planned D 95%,GTV was above 95% for 12/13 MRgPT patients and all MR-LINAC patients. For non-adapted plans, this decreased to 10/13 and 8/13 for MRgPT and MR-LINAC, respectively, while plan adaptation restored the planned D 95%,GTV in all patients. The median D 98%,CTV in the non-adapted plans decreased by 4.7%[range:-0.9;24.1%] for MRgPT and 1.1%[-0.4;25.1%] for MR-LINAC, with the planned D 98%,CTV 47.0Gy(RBE)[37.4;47.5Gy(RBE)] and 45.8Gy[42.6;46.2Gy], respectively. With MRgPT, the planned D 700cc,UninvolvedLiver was 5.7Gy[1;13.6Gy] lower compared to MR-LINAC (Figure 1). For the non-adapted plans, the clinical goal of the colon (V 25Gy,Colon <20cc) was exceeded for 3 and 4 patients for MR-LINAC and MRgPT, respectively. For all 3 patients, the V 32Gy,Heart was larger than 15cc for both modalities. In the adapted plans of both modalities, the clinical goals of the OARs were met in all patients.

Conclusion: The majority of HCC patients needed plan adaptation to fulfill the target volume coverage in MRgPT and MR-LINAC. Moreover, the dose to the uninvolved liver was reduced with adaptive MRgPT compared to MR-LINAC, which correlates with overall survival. Therefore, HCC may be an interesting indication for hypofractionated adaptive MRgPT.

Keywords: MRgPT, MR-LINAC, HCC