Clinical implementation and performance evaluation of a motorized MR-compatible OCTAVIUS 4D system for SBRT patient-specific QA on a 1.5 T MR-LINAC.

Abstract

BackgroundMagnetic resonance-guided radiotherapy using 1.5 T MR-LINAC systems introduces specific constraints for patient-specific quality assurance (PSQA), requiring MR-compatible dosimetric devices capable of accurate three-dimensional dose verification. While previous studies have characterized the dosimetric performance of MR-compatible detector arrays, the clinical implementation of rotational three-dimensional QA systems in MR-guided workflows remains insufficiently documented.PurposeThis study investigated the clinical implementation and practical performance of a motorized MR-conditional OCTAVIUS 4D system for stereotactic body radiotherapy (SBRT) PSQA on a 1.5 T MR-LINAC.MethodsThe MR-conditional OCTAVIUS 4D phantom equipped with a 1500 MR ionization chamber array was implemented for PSQA measurements on a Unity 1.5 T MR-LINAC (Elekta, Stockholm, Sweden). A total of 40 clinical step-and-shoot treatment plans were analyzed with an SBRT prescription. The system was evaluated in terms of three-dimensional dose reconstruction accuracy under clinically relevant off-axis condition, angular positioning consistency with the gantry positioning, and integration into clinical workflow. Measured dose distributions were compared with treatment planning system (TPS) calculations using a 3D gamma analysis with a local 2.5%/2.5 mm criterion and a 10% dose threshold. Independent angular measurements were additionally performed using a digital inclinometer.ResultsThe system demonstrated high agreement between measured and calculated dose distributions, with a mean gamma passing rate of 98.3% (local 2.5%/2.5 mm). All plans met institutional acceptance criteria (GPR ≥ 95%). Independent angular measurements demonstrated excellent agreement between gantry angle, phantom angle, and software-reported angle, with deviations below 0.6° and no systematic bias observed. No visible dependence of gamma passing rate on radial off-axis target positioning was observed within the investigated clinical range. The evaluated beam arrangements included irregular angular spacing and clockwise/counterclockwise gantry transitions during delivery, supporting effective synchronization between the OCTAVIUS 4D MR system and the Unity gantry under clinical step-and-shoot conditions.ConclusionsThe motorized MR-conditional OCTAVIUS 4D system demonstrated clinically feasible implementation for rotational three-dimensional PSQA for SBRT on a 1.5 T MR-LINAC. The system provided consistent dose reconstruction accuracy and reliable angular positioning under clinically relevant delivery conditions, supporting its integration into routine MR-guided radiotherapy QA workflows.