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This thesis concerns the Unity MRI-linac (and its prototypes). The overall aim was to ensure that the MRI-linac was safe and effective for clinical use, and could be introduced into the clinic. To this end it was necessary to identify and solve issues related to reference dosimetry in a magnetic field, acceptance testing of a new system in a new physical configuration, and beam characterisation in a magnetic field. Results and analysis are provided. A major contribution was the development of a reference dosimetry framework that allows dosimetry to be performed at an MRI-Linac by the local clinical physicist using standard equipment, and where all correction factors can be independently verified. Additional contributions included evidence of the consistency of ion chamber magnetic field correction factors; evidence of the suitability of a microDiamond small-field detector; a linac acceptance procedure that can be performed by local clinical physicists, and; the first beam characterisation data suitable for modeling the radiation beam in the treatment planning system, such that the system could then be used to plan and deliver radiation treatments.

This thesis concerns the Unity MRI-linac (and its prototypes). The overall aim was to ensure that the MRI-linac was safe and effective for clinical use, and could be introduced into the clinic. To this end it was necessary to identify and solve issues related to reference dosimetry in a magnetic field, acceptance testing of a new system in a new physical configuration, and beam characterisation in a magnetic field. Results and analysis are provided. A major contribution was the development of a reference dosimetry framework that allows dosimetry to be performed at an MRI-Linac by the local clinical physicist using standard equipment, and where all correction factors can be independently verified. Additional contributions included evidence of the consistency of ion chamber magnetic field correction factors; evidence of the suitability of a microDiamond small-field detector; a linac acceptance procedure that can be performed by local clinical physicists, and; the first beam characterisation data suitable for modeling the radiation beam in the treatment planning system, such that the system could then be used to plan and deliver radiation treatments.

Purpose The PTW microDiamond has an enhanced spatial resolution when operated in an edge‐on orientation but is not typically utilized in this orientation due to the specifications of the IAEA TRS‐483 code of practice for small field dosimetry. In this work the suitability of an edge‐on orientation and advantages over the recommended face‐on orientation will be presented. Methods The PTW microDiamond in both orientations was compared on a Varian TrueBeam linac for: machine output factor (OF), percentage depth dose (PDD), and beam profile measurements from 10 × 10 cm2 to a 0.5 × 0.5 cm2 field size for 6X and 6FFF beam energies in a water tank. A quantification of the stem effect was performed in edge‐on orientation along with tissue to phantom ratio (TPR) measurements. An extensive angular dependence study for the two orientations was also undertaken within two custom PMMA plastic cylindrical phantoms. Results The OF of the PTW microDiamond in both orientations agrees within 1% down to the 2 × 2 cm2 field size. The edge‐on orientation overresponds in the build‐up region but provides improved penumbra and has a maximum observed stem effect of 1%. In the edge‐on orientation there is an angular independent response with a maximum of 2% variation down to a 2 × 2 cm2 field. The PTW microDiamond in edge‐on orientation for TPR measurements agreed to the CC01 ionization chamber within 1% for all field sizes. Conclusions The microDiamond was shown to be suitable for small field dosimetry when operated in edge‐on orientation. When edge‐on, a significantly reduced angular dependence is observed with no significant stem effect, making it a more versatile QA instrument for rotational delivery techniques.

A Monte Carlo (MC) method was used to determine small field output correction factors for several active detectors (Exradin A16, Exradin A26, PTW microLion, PTW microDiamond, Exradin W1 and IBA RAZOR) for an Elekta Axesse linac equipped with circular cones. MC model of the linac was built with the GamBet software, using the Penelope code system. The dose-to-water simulation for each cone, ranging from 5 to 30 mm of diameter size, was used to calculate field factors and the results were validated together with Gafchromic EBT3 film. Output factors (OFs) were measured with the active detectors and correction factors were determined using the MC results. The MC simulations agreed with films within 1.2%. OFs measured with Exradin W1 scintillator were in agreement within 0.8% with MC simulations. The Exradin A16 and A26 microchambers under-responded for small fields relative to the MC (-13.1% and -4.6%, respectively). PTW microLion, IBA RAZOR and PTW microDiamond overestimated the output factor for the smallest field (+3.9%, +5.4 and +7.1%, respectively). The present study pointed out that it is crucial to apply the appropriate correction factors in order to provide accurate measurements in small beams geometry. The results showed that the Exradin W1 can be used for very small field dosimetry without correction factors, which shall be contrariwise employed for other detectors.