3D Dosimetry using optical tomography and electronic portal images

Abstract

The primary goal of this thesis is to develop techniques to quantify the radiation dose distributions used in radiotherapy for cancer treatment. It aims at developing a physically profound calculation model for the transit dosimetry by a detailed characterization of the radiation interaction with tissues and the fluence measurements recorded in electronic portal imaging device (EPID). Radiotherapy has undergone great advances with developments such as Intensity Modulated Radiotherapy (IMRT), Volumetric modulated Arc therapy (VMAT), Radiosurgery, CyberKnife, and advances in Brachytherapy. These newer methods help in precisely administering radiation dose to patients as decided by the treatment planning system (TPS), a computer system that takes input from patient CT, medical physicist, oncologist, medical dosimetrist and physician. The radio-therapy treatment system depends on 3D dosimetry for pre-treatment quality assurance. The polymer gel dosimeters are used for estimating the 3D dose distribution using a treatment plan decided by the radiation treatment plan (RTS) before the patient undergoes radiation exposure. Gel phantoms are prepared using monomers to be tissue equivalent radiologically. The optical computed tomography has been used to scan the gel dosimeters. It was observed that upon irradiation, the monomers in gel get polymerized. Calibration measurements with varying levels of radiation exposure show that, optical density and refractive index increase with radiation dose. The optical density increased from (0.01 to 0.06) mm1 and the refractive index increased from (1.34 to 1.37) for gel irradiated from (0.5 to 25) Gy dose. The SEM imaging of calibration gels show that the particle size increases from 20nm to 400nm on radiation exposure. The exposure of radiation to tissue causes an increase in refractive index, thereby bending the light traversing through the tissue, resulting in deterioration in image quality. The solution for this is to immerse the dosimeter in a refractive index matching liquid. However, an exact match is seldom achieved. The refraction of light passing through a dose region results in artefacts in the reconstructed images. These refraction errors are dependent on the scanning geometry and collection optics. The refraction arises primarily due to (1) the refractive index mismatch between the surrounding medium and the dosimeter which results in distortions of dose regions and (2) the refractive index changes caused by radiation dose in the dosimeter itself that result in streaking, and quantitative errors. In order to account for these effects and correct the distortions we used ray path modelling of light traversed through the dosimeter. Exact path length of the ray in a discretized grid was obtained by using ray tracing methods. Rayline errors perturb the system when rays confront a radiation induced RI gradient region. This is more signi ficant in 3D as the ray get deviated and does not reach the detector plane. We extended this study to 3D, used a prototype cone-beam scanning system to collect the projection images. We developed a fully 3D image reconstruction algorithm, algebraic reconstruction technique-refraction correction (ART-rc) that corrects for the refractive index mismatches present in a gel dosimeter scanner not only at the boundary, but also for any rayline refraction due to multiple dose regions inside the dosimeter. In this study, simulation and experimental studies have been carried out to reconstruct a 3D dose volume using 2D CCD measurements taken for various views. Radiation dose absorbed at a tissue voxel can be calculated from kernels which incorporate the effects of all the interactions with matter using Monte Carlo based techniques. We studied pencil beam and point kernel based methods. Radiological

depth calculation using ray tracing technique was used for path length calculations in a inhomogeneous phantom/patient volume. This is integrated with collapsed-cone convolution superposition algorithm to arrive at the complete dose-distribution. Dose reconstruction results using Monte Carlo and collapsed cone methods are presented. The EPID image was corrected with scatter factor measurements. The corrections improved the dose quanti cation from 88.9% to 96.5%. The resulting dose Monitor Unit (MU) values matches well with that from TPS computation. Per eld EPID uence, calculated from segment wise portal images acquired using step and shoot technique of the IMRT prostate eld is validated. The main ndings of this study are: 1 We have demonstrated that gel dosimeters can be used to verify dose pro les delivered using Co-60 telecobalt machines, linear accelerators, IMRT, VMAT and Brachytherapy. 2 Refraction e ects deteriorate dose readout and induce errors in quantifying dose. These can be overcome by using ray tracing method that calculate exact pathlength accounting for refraction. 3 Boundary mismatch can be overcome by using exact matching liquid, but interior refractive index changes induced by radiation can be accounted for using our ray modelling scheme. 4 The Monte Carlo modelling of polarized light propagation in a multi-layered turbid medium is extended to include multiparticle distribution of scatterers and also with embedded absorbing/ scattering inhomogeneities. 5 Fluence measurements acquired using EPID along with appropriate scatter factor corrections were found to match with those calculated by treatment planning system (TPS). In conjunction with collapsed cone convolution/superposition method, it can be used to compute 3D dose distributions.