Purpose: Existing air-filled ionization chambers are considerably affected by the Lorentz force in a high-field MR-Linac. Thus, this study evaluated a novel plastic scintillation detector (PSD) which uses solid and water equivalent material to minimize the Lorentz force during measurements. The novel PSD was used for different aspects of the dosimetric characteristics in the presence of a 1.5T magnetic field on Elekta Unity (Elekta AB, Stockholm, Sweden) MR-Linac.
Methods: A PSD (Blue Physics LLC, Tampa, FL) was placed in a 3D water phantom to measure a 7MV-FFF photon beam in MR-Linac. Beam profiles, percent depth dose (PDD), and output factors were measured and they were compared to (1)MR-Linac commissioning data measured using a microDiamond and a Semiflex 3D and (2)plan dose data calculated using a Monte-Carlo algorithm on Monaco treatment planning system (TPS). A test plan was created and delivered to an IMT MAX-HD phantom during an MRI scan. The performance of the PSD was evaluated in repeatability, doserate dependence, and dose linearity.
Results: The PSD showed a repeatability with a coefficient of deviation of 0.34% and 0.17% for 2x2cm2 and 10x10cm2 field, respectively. The doserate dependence was within 2.4%. The dose linearity showed less than 1.6% deviations. Compared to the microDiamond detector, the PSD profiles well matched within 2%/2mm criteria and the PDD showed local differences up to 5% at 30cm depth for 2x2cm2, 5x5cm2, and 10x10cm2. The output factor difference was 0.9±0.8% and 0.6±0.6% from the ionization chamber and TPS data, respectively. The phantom point dose measurement showed 3.13±3.15% difference from the TPS calculation.
Conclusion: Overall performance of the novel PSD demonstrated clinically acceptable agreement with ionization chamber, microDiamond, and Monte-Carlo data. Furthermore, the accuracy of measurement during the active MR-imaging was verified. Further investigations will be needed to understand the difference for the PDD measurement at a deeper depth.
Optical Dosimetry, Radiation Dosimetry, MR
TH- Radiation Dose Measurement Devices: optical/photoacoustic/Cerenkov dosimetry