Storage Phosphor Proton Therapy Dosimetry

Proton beams have been increasingly utilized for radiation therapy purposes as they can confer unique radiotherapeutic advantages due to their highly targeted dose deposition behavior within a narrow Bragg peak volume. While this allows for the creation of highly conformal proton treatment plans using advanced treatment planning methods like intensity modulated proton therapy (IMPT), their inherently steep 3D spatial dose gradients make their clinical delivery challenging. As such, accurate and precise dose verification measurements are required to guarantee patient safety prior to actual treatment. Ideally, these measurements should be performed by a reusable, multi-dimensional dosimeter that is both water-equivalent and of a high spatial resolution, but such a dosimeter is currently commercially unavailable. Exacerbating this issue further are the tendencies for (a) proton beams to be more biologically damaging due to their relatively higher linear energy transfer (LET) values, and for (b) many dosimeter materials to under-respond in regions of elevated LETs. As such, the goals of this dissertation research are to investigate the use of various storage phosphor materials as candidate reusable proton dosimeters for (a) water-equivalent quantitative proton dosimetry, (b) simultaneous proton dose and LET measurements and (c) two-dimensional spatial dose measurements to address the current unfulfilled clinical need for a more accurate multidimensional high-resolution proton dosimeter.Chapters 1 to 3 present a brief overview of the relevant physics and experimental methods that were relevant to this dissertation research. Chapter 1 briefly introduces the basic concepts of proton therapy physics and the clinical and physics limitations of current proton dosimeters. Chapter 2 introduces the optoelectronic physics of storage phosphor dosimeters and in particular, the physics of photostimulated luminescence (PSL). Chapter 3 contains a summary of all the experimental techniques that were employed during this dissertation research, which includes dosimeter fabrication, preparation, irradiation, 1D and 2D readout protocols and subsequent signal erasures. Chapters 4-6 present the results directly addressing the main aims of this dissertation research. Chapter 4 presents our feasibility investigations on the use of the low Zeff water-equivalent storage phosphor material KCl:Eu2+ as a candidate proton dosimeter. Under proton irradiation, KCl:Eu2+ had excitation and emission spectral peaks at 560 and 421 nm respectively with full-width-half-maxima (FWHM) values of 86 and 29 nm respectively. KCl:Eu2+ is highly linear from 0 to 8 Gy proton dose with a large dynamic range of up to 60 Gy proton dose. It was also determined to be dose-rate independent from 83 to 500 cGy/min. Its PSL signal was found to stabilize after approximately 12 h post-irradiation for both proton and photon irradiation and it did not exhibit any significant radiation damage of up to 200 Gy of cumulated proton dose history. Most importantly, it was determined to be LET independent, responding identically to the PPC05 ionization chamber within experimental uncertainty. Chapter 5 investigates the feasibility of deliberately using a non-water equivalent storage phosphor material BaFBr0.85I0.15:Eu2+ in conjunction with KCl:Eu2+ for simultaneous proton dose and LET measurements. BaFBr0.85I0.15:Eu2+ was determined to under-respond noticeably in regions of high proton LET and the magnitude of under-response can be adequately modeled with a dose accuracy of 3% and a distance-to-agreement (DTA) of 1 mm using the Bethe-Bloch theory. The excitation and emission peaks of BaFBr0.85I0.15:Eu2+ were at 586 and 400 nm respectively and its sufficient overlap with the corresponding spectra of KCl:Eu2+ made it optically compatible to be used in conjunction with KCl:Eu2+ for simultaneous proton dose and LET measurements. BaFBr0.85I0.15:Eu2+ also had ideal proton dosimetric properties; it was found to be linear up to 10 Gy, is dose-rate independent and impervious to radiation damage of up to 200 Gy cumulated proton dose. For both storage phosphor dosimeters, KCl:Eu2+ and BaFBr0.85I0.15:Eu2+, their PSL and PL lifetimes were determined to be in the order of microseconds, making them feasible to be employed for high-resolution 2D dosimetric measurements without any undesirable pixel bleeding effects. Finally, Chapter 6 presents our work on the development of an optical scanner for 2D readouts of the spatial dose information contained within 2D storage phosphor film dosimeters at a high spatial resolution. We tested our optimized optical scanner with commercial BaFBr0.85I0.15:Eu2+ dosimeter samples and a sub-millimeter spatial resolution was easily achievable. A single proton spot at maximum energy (227.1 MeV) was then delivered to the BaFBr0.85I0.15:Eu2+ dosimeter samples and subsequently readout optically. The BaFBr0.85I0.15:Eu2+ storage phosphors responded linearly from 1 to 50 monitor units (MUs) and their profiles did not saturate up to 150 MU. Our system was also able to detect lateral displacements of ±1 mm in both the crossline and inline directions as well as detect ±0.3 mm beam spread changes that were artificially introduced. Future work will entail the development of a uniform storage phosphor film dosimeter consisting of the water-equivalent material KCl:Eu2+ for accurate proton dose measurements at a high spatial resolution along with the possibility of a high spatial resolution pixel-to-pixel LET mapping in conjunction with BaFBr0.85I0.15:Eu2+ storage phosphors. Some preliminary results pertaining to the erasability, micronization and tape casting of KCl:Eu2+ storage phosphor materials are presented in Chapters 7, 8 and 9 respectively