Dosimetry for the MR-linac

Abstract

The purpose of this thesis is to investigate the inuence of the MR scanner on dosimetry for the radiation modality, and to investigate the possible solutions for the dosimetric measurements discussed in section 1.7. Chapter 2 investigates the feasibility to use a standardized national reference dosimetry protocol for the MR-linac. Firstly, the feasibility of using an ionisation chamber in an MR-linac was assessed by investigating possible inuences of the magnetic field on an NE2571 Farmer type ionisation chamber characteristics: linearity, repeatability, orientation in the magnetic field; and AAPM TG51 correction factor for voltage polarity and ion recombination. Secondly, the inuence of the permanent 1.5 T magnetic field on the NE2571 chamber reading was quantified. Chapter 3 presents the design and performance of a prototype MR-linac compatible scanning water phantom. In order to use a scanning water phantom, the performance of air filled ionisation chambers in the magnetic field must be characterised. The performance of the scanning water phantom will be validated at a clinical set-up in a 0 T magnetic field. Inside the MR-linac set-up, the performance of the MR-linac scanning water phantom is validated using radiographic film. Chapter 4 investigates the performance of the IC PROFILERTM, a multi-axis ionisation chamber array, in a 1.5 T magnetic field. The inuence of the magnetic field on the IC PROFILERTM reproducibility, dose response linearity, pulse rate frequency dependence, power to electronics, panel orientation and ionisation chamber shape are investigated. IC PROFILERTM dose profiles were compared with film dose profiles obtained simultaneously in the MR-linac. Chapter 5 investigates the feasibility of using the STARCHECKTM multi-axis ionisation chamber array panel, in a transverse 1.5 T magnetic field. The method of investigation is similar to that used for the IC PROFILERTM in chapter 4. The investigated characteristics are short term reproducibility, dose response linearity, accuracy of output factor measurements and the inuence of the magnetic field on a purposefully introduced misalignment. As a validation of feasibility, STARCHECKTM measurements were compared with film measurements simultaneously obtained in the MR-linac. Chapter 6 investigates the feasibility of using an MV portal imager in an MRlinac set-up. MV imaging integrated with the MR-linac has the potential to provide an independent position verification tool, a field edge check and a calibration for alignment of the coordinate systems of the MRI and the accelerator. A standard aSi MV detector panel is added to the system and both qualitative and quantitative performance are determined. Chapter 7 examines the performance characteristics of the ArcCHECK-MR QA system in a transverse 1.5 T magnetic field. This ArcCHECK-MR system is used for QA of patient treatment plans. To this end, the short-term reproducibility, dose linearity, dose rate dependence, field size dependence, dose per pulse dependence and inter-diode variation of the ArcCHECK-MR diodes were evaluated on a conventional linac and on the MR-linac. Chapter 8 investigates the inuence of the closed bore MRI scanner structures on several radiation beam characteristics for squared fields of sizes 5.6, 9.8 and 23.8 cm2. The MR-linac set-up will be implemented into a Monte Carlo simulation environment facilitating dose profile simulations in a 1.5 T magnetic field with and without MRI scanner structures. The results of the Monte Carlo simulations will be validated against scanning water phantom measurement results obtained in the MR-linac for the PDD and lateral profiles.