Radiation Dosimetry in the Presence of Gold Nanoparticles

The use of gold nanoparticles (GNPs) to enhance the dose due to radiation, through increased photoelectric effect interactions, has shown promise in vivo and in vitro. Monte Carlo studies have worked towards quantifying the dose enhancement and the dosimetry surrounding GNPs. This thesis investigates the dosimetry in the presence of GNPs for several scenarios using PENELOPE Monte Carlo simulations. Accurate simulation of GNPs can be challenging due to the large number of particles present in realistic scenarios – up to 10^15 particles/cm^3. Because of this, many Monte Carlo studies have approximated GNPs in tissue as a homogeneous mixture of tissue and gold. However, such models ignore details of energy deposition on nanoscopic scales, including absorption of dose within the GNPs. In this thesis, we first quantified the dosimetric impact of this assumption, finding dose differences up to 31% for the scenarios investigated, and enabling fast, accurate simulation of macroscopic dose enhancement. The eventual clinical application of GNP-enhanced radiation therapy will rely on enhancement at macroscopic scales. We next investigated the general feasibility of using GNP-enhanced arc radiation therapy (GEART) to treat deep-seated tumours using kilovoltage photon beams. Applying the method established above, we quantified the quality of GEART treatments compared to conventional 6 MV treatments for a variety of tumour sizes and depths. We recommended those sites for which further investigation should be undertaken. In vivo and in vitro, GNPs are often coated with polyethylene glycol (PEG), a polymer that enables functionalization and biocompatibility. Monte Carlo studies, however, typically do not model these coatings, which may lead to dosimetric errors. We quantified the dose lost to PEG coatings for a variety of GNP sizes, coating thicknesses, and photon beam energies. Dose losses of up to 7.5% and 34% were seen on microscopic and nanoscopic scales, respectively. Through this work we aim to provide a basis for future studies examining clinical implementation of GNP dose enhancement, adding to the base of knowledge that can be drawn upon if GNP dose enhancement is to be brought into clinical use. Further research is needed to evaluate the radiobiological impact of the effects studied here

Radiation shielding and safety implications following linac conversion to an electron FLASH‐RT unit

Purpose Due to their finite range, electrons are typically ignored when calculating shielding requirements in megavoltage energy linear accelerator vaults. However, the assumption that 16 MeV electrons need not be considered does not hold when operated at FLASH-RT dose rates (~200× clinical dose rate), where dose rate from bremsstrahlung photons is an order of magnitude higher than that from an 18 MV beam for which shielding was designed. We investigate the shielding and radiation protection impact of converting a Varian 21EX linac to FLASH-RT dose rates. Methods We performed a radiation survey in all occupied areas using a Fluke Biomedical Inovision 451P survey meter and a Wide Energy Neutron Detection Instrument (Wendi)-2 FHT 762 neutron detector. The dose rate from activated linac components following a 1.8-min FLASH-RT delivery was also measured. Results When operated at a gantry angle of 180° such as during biology experiments, the 16 MeV FLASH-RT electrons deliver ~10 µSv/h in the controlled areas and 780 µSv/h in the uncontrolled areas, which is above the 20 µSv in any 1-h USNRC limit. However, to exceed 20 µSv, the unit must be operated continuously for 92 s, which corresponds in this bunker and FLASH-RT beam to a 3180 Gy workload at isocenter, which would be unfeasible to deliver within that timeframe due to experimental logistics. While beam steering and dosimetry activities can require workloads of that magnitude, during these activities, the gantry is positioned at 0° and the dose rate in the uncontrolled area becomes undetectable. Likewise, neutron activation of linac components can reach 25 µSv/h near the isocenter following FLASH-RT delivery, but dissipates within minutes, and total doses within an hour are below 20 µSv. Conclusion Bremsstrahlung photons created by a 16 MeV FLASH-RT electron beam resulted in consequential dose rates in controlled and uncontrolled areas, and from activated linac components in the vault. While our linac vault shielding proved sufficient, other investigators would be prudent to confirm the adequacy of their radiation safety program, particularly if operating in vaults designed for 6 MV