Improving radiation therapy planning by CT metal artefact suppression and electron spectrum reconstruction
This thesis comprises of 3 major parts of my graduate research work, including development of two novel CT metal artefact suppression methods, investigation of the dosimetric effect of CT metal artefact suppression on commercial radiation therapy treatment planning systems, and application of a variational method with adaptive Tichonov regularization to reconstruct electron phase space distribution required for Monte Carlo radiation dose calculation and treatment planning. All the work aims at improving radiation therapy dose calculation and treatment planning. The major advantage of the subtraction technique is that it can successfully suppress the high-density artefacts in bone abundant body regions such as the pelvis. The smoothing-plus-scaling method utilizes two important features of the CT image with metal artefacts: (a) metal and bone pixels are not severely affected by the high density artefacts and (b) the high density artefacts can be located in specific projection channels in the profile domain, although they are spread out in the image domain. It is found that the target could be severely underdosed with significantly altered DVH if the artefact-contaminated image data are used without any artefact suppression procedure for all beam types. Dose perturbation for photon and electron beams is due to both density and scattering effects while for proton beam the cumulative electron density along the beam line plays the major role in determining how accurate the dose calculation is. A water phantom study showed that the CT numbers after image processing are of high fidelity compared to the CT numbers without any metal objects. A variational method with adaptive regularization technique is used to reconstruct electron spectra from depth dose curves for the 6 MeV, 9 MeV and 18 MeV electron beams of a Varian Clinac 2100C accelerator. This new adaptive regularization technique proves to be a very simple, effective and accurate approach. Results using this variational method with adaptive regularization almost perfectly reconstruct electron spectra from depth dose distributions. This technique is promising in reconstructing the full electron initial phase space with angular distribution and the reconstructed phase space can be used for commissioning a Monte Carlo dose calculation and treatment planning system.
Sandison, Purdue University.
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