Validation of a diode-based phantom for high temporal and spatial measurements in a 1.5 T MR-linac

Background: For the development and validation of dynamic treatment modalities and processes on the MR-linac, independent measurements should be performed that validate dose delivery and linac behavior at a high temporal resolution. To achieve this, a detector with both high temporal and spatial resolution is necessary. Purpose: This study investigates the suitability of a Delta4 Phantom+ MR (Delta4) detector array for time-resolved dosimetry in the 1.5 T MR-linac and characterizes the Delta4's performance with dynamic dose delivery, such as beam gating and field modulation during radiation. Methods: A Delta4 detector was used, including software for time-resolved dosimetry. First validation experiments were performed and compared to reference measurements. Subsequently, demonstrator measurements were performed to show use cases of the Delta4's time-resolved dose readouts. An example of such an experiment is the determination of the field speed during a sliding window experiment, traveling between 0.7 and 6.8 cm/s in the cranial-caudal direction. Results: Validation experiments of the dose reproducibility and dose rate dependency showed no difference relative to the standard static delivery. The field speed measured by the Delta4 showed an average field speed difference of −0.3% relative to MR-linac log files. The Delta4 was capable of measuring the dose with high accuracy and temporal resolution during dynamic radiation delivery. Conclusion: The Delta4 can be used for time-resolved dosimetry in a 1.5 T MR-linac.