Date of Award
12-25-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Proton therapy is the most advanced radiation treatment modality. Proton treatment is delivered via pencil beam scanning (PBS) technique, in which a pencil beam scans the target spot-by-spot and layer-by-layer. An effective quality assurance (QA) device is needed to verify the quality of treatment dose delivery. Current clinical practice employs films or a two-dimensional (2D) detector to measure the dose at one or a few arbitrarily selected depths, which is insensitive to delivery error and time-consuming. A 3D dosimeter that can measure the 3D dose distribution is needed to improve the effectiveness and efficiency of proton QA. Furthermore, preclinical studies have shown the potential of radiation therapy (RT) at ultra-high dose rate (UHDR) or FLASH-RT in better sparing healthy tissues while maintaining the tumor-killing effect compared to RT at conventional dose rates (CDR). Proton currently is the only modality that can deliver FLASH-RT to deeply seated targets in the human body. Safe translation of proton FLASH-RT into the clinic requires ultra-fast 3D dosimeters capable of measuring dose and dose distribution at high spatiotemporal resolution. Proton treatment planning calculates radiation dose on the stopping power images converted from the CT x-ray attenuation measurements. Such a process involves about 3.5$\%$ range uncertainties compromising the precision of proton treatment delivery. An imaging device that can directly measure the stopping power information in vivo is needed to further improve the precision of proton treatment. Through this study, a novel multi-layer strip ionization chamber (MLSIC) device is developed to address all the unmet needs in proton therapy. The multi-layer strip ionization chamber (MLSIC) comprises a total of 66 layers of strip ion chamber (IC) plates arranged, alternatively, in the x and y direction. A total of 768-channel IC signals are integrated and sampled at a speed of 6 kfps. A dose reconstruction algorithm is developed to reconstruct 3D dose distribution for each spot at all depths by considering a double-Gaussian Cauchy-Lorentz model. The 3D dose distribution of each beam is obtained by summing all spots. And 3D dose rate distributions were generated using the 3D spatial and temporal information. The performance of our MLSIC is evaluated for various clinical pencil beam scanning (PBS) plans on conventional dose rate and FLASH dose rate. Various proton treatment fields in CDR and UHDR were measured and validated. The MLSIC device can also be used as the imaging device for proton radiography and proton CT. We further measured and reconstructed the proton radiography of various phantoms. On the other hand, pCT and CT require accurate projection and back-projection models for both iterative reconstruction and deep learning reconstruction algorithms. Traditional projection and back-projection methods compute the intersection length, the overlap length of the projected image voxel and the detector pixel boundary on a common plane or the approximation of the voxel footprint on the detector plane. This study describes an efficient and accurate algorithm to estimate the ray-voxel intersection volume, which can improve the accuracy of the projection and back-projection model for both iterative reconstruction and deep learning reconstruction algorithms. In summary, a novel ultra-fast 3D dosimeter is developed through this thesis study. The MLSIC is the first device that can measure and reconstruct 3D dose and dose rate information of proton beams. It also is the first device that can obtain high-resolution proton radiography by scanning a single high-energy proton beam.
Language
English (en)
Chair
Tiezhi Zhang