Vintró, Luis LeónMousli, Majed Hussain2024-12-242024-12https://hdl.handle.net/20.500.14154/74422Radiotherapy treatment aims to administer a substantial dose of ionising radiation to the tumour volume while limiting exposure to surrounding healthy tissue to reduce potential side effects. Typically, daily fractionated doses of about 2 Gy each, extended over an average of around 20 fractions (typically ranging from 5 to 39 fractions) across diverse tumour sites are employed. This conventional dose fractionation utilises techniques like 3D conformal radiotherapy (3DCRT), intensity-modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), and stereotactic ablative radiotherapy (SABR) to deliver the dose, with average dose rates varying from 6 Gy/min (using conventional flattened beams) to 24 Gy/min (employing flattening filter-free (FFF) beams) to expedite treatment duration and enhance patient comfort. Recently, FLASH radiotherapy, which makes use of ultra-high dose rates (UHDR) of >40 Gy/s to irradiate targets, has been the subject of much research. This approach holds the potential to augment the therapeutic ratio and significantly reduce treatment times, where the therapeutic ratio signifies the balance between tumour control probability (TCP) and normal tissue control probability (NTCP) likelihood. The main aim of this thesis was to investigate the possibility of making use of a suitably modified standard radiotherapy linear accelerator (LINAC) to produce FLASH level electron beams, and to characterise the resulting beams with a range of different detectors, including a novel inorganic scintillator detector. As a first step in this study, a Monte Carlo (MC) model of the Elekta Precise LINAC was developed using the EGSnrc MC code to characterise conventional systems utilising 10, 12, and 15 MeV electrons. The MC model was employed to investigate the effect of making changes to the LINAC towards achieving FLASH-level beams, and to assess detector responses, encompassing dose, dose linearity, and dose rate dependence when subjected to FLASH beam conditions. The model's validation was performed against an advanced Markus ionising chamber (AMIC) and Gafchromic EBT XD film measurements for both conventional and modified LINAC set ups. The results showed very good agreement between simulated and measured data, with depth dose percentages (PDD), as well as the relative distances R90, R50, and Rp giving variations within ±1 mm for both conventional LINAC across the 10, 12, and 15 MeV electron energies and modified LINAC for the 12 MeV electron energy. Bremsstrahlung contamination was found to be 1% and 2% for conventional LINAC and modified LINAC, respectively. The beam profiles showed deviations of ±2% (cross-plane) and (in-plane) for the 12 MeV electron beam under conventional and modified LINAC conditions. In conclusion, the MC model demonstrates robust agreement between measured and calculated data for both conventional and modified LINAC settings, offering potential utility in validating diverse dosimetry detectors for assessing dose, dose rate, and dose per pulse (DPP) dependencies under FLASH irradiation. The second objective was to characterise multiple dosimetric detectors using a modified Elekta Precise linear accelerator (LINAC) to generate an electron FLASH beam for radiotherapy (eFLASH-RT), achieving radiobiological research-level FLASH conditions, and to compare the results with those predicted using Monte Carlo (MC) simulation. The 12 MeV electron beam was adapted into an eFLASH setup by adjusting beam parameters and positioning the carousel at an open port with primary and secondary scattering foils. A microcontroller unit (MCU) circuit was used to control the pulse count. Gafchromic EBT-XD films were used to established PDD, beam profiles and DPP, and these were validated against MC simulations. Both conventional beam radiotherapy (CONV-RT) and eFLASH-RT showed that the simulated and measured data for reference depths R90, R50, and Rp agreed to within ± 1 mm. Gamma analysis, with passing rates >95% (gamma criteria 3%/3mm), showed good PDD and beam profile agreement. DPP changed with SSD (54 to 150 cm) from 0.2 to 0.025 cGy/pulse in CONV-RT to 60 to 8 cGy/pulse in FLASH-RT, with negligible differences (< 2 cGy) between MC and each detector. By taking secondary scattering foils out of the beam path and reducing the SSD, the model predicted that dose rates in excess of 40 Gy/s could be achieved. This was confirmed by using the PTW AMIC detector and EBT XD films. The model was used to analyse parameters like PDD, large-field beam profiles, dose, dose rate, and DPP dependencies. The results of this study demonstrate that the modified LINAC can be used as a tool to assess dosimetry detectors for FLASH radiotherapy, potentially aiding online real-time radiation dose evaluation during therapy and facilitating radiobiological investigations into eFLASH-RT's cellular impact. The final part of the thesis investigated the use of a novel inorganic scintillating detector (ISD) employing Gd2O2S:Tb with a short temporal resolution (2.8 ms), as well as a plastic scintillation material (PSD), for real-time dosimetry in ultra-high dose rate applications. The modified LINAC was used to deliver a 12-MeV electron FLASH beam (eFLASH) at an average dose rate of 85 Gy/s. The response of the scintillation dosimeters (operated using the HYPERSCINT radiation platform) were compared to that of the AMIC detector across various field sizes and electron applicators to assess linearity, repeatability, DPP, and output factors at a 4-cm water-equivalent depth. The plastic (PSD) and inorganic scintillator detector (ISD) measurements were validated against EBT-XD Gafchromic films and Monte Carlo (MC) simulations. Relative IC measurements exhibited linear behaviour, with PSD and ISD responses differing within 1% and 14%, respectively, over a 50 to 7000 cGy range. PSD maintained linearity with increasing pulse repetition frequency (PRF) up to 172 Hz (max value 1.1), while ISD response at 40, 72, and 172 Hz yielded linearity indices of 1.2, 1.35, and 1.24. PSD and ISD showed consistent dose responses under ultrahigh dose rate conditions, agreeing within 4% with EBT-XD film readings. Furthermore, DPP linearity at different SSDs was studied, yielding favourable agreement for 5 cm2 field sizes using EBT-XD film, AMIC, and MC models. The novel PSD and ISD detectors, integrated into the HYPERSCINT platform, demonstrated a robust response to 12 MeV eFLASH beam irradiation, accommodating DPP of up to 0.65 Gy and an ultra-high dose rate of 85 Gy/s. This work will establish a versatile dosimetry platform that will be applicable to various research domains, including in vitro and in vivo radiobiology investigations of ultrahigh dose rate irradiation.144enplastic scintillation materialelectron FLASH beamnovel inorganic scintillating detectorlinear acceleratorMonte Carloradiotherapydepth dose percentagesEBT-XD Gafchromic filmThesis