Four-Dimensional In Vivo Dosimetry By Dose Reconstruction Using Continuous Epid Images And Phase Sorting Method

In vivo dosimetry is widely used for patient specific dose evaluation and treatment verification in external beam radiation therapy (EBRT). There is a growing interest in using an electronic portal imaging device (EPID) for in vivo dosimetry, by correlating pixel values of measured EPID images to three-dimensional (3D) dose in the patient. In this research, a non-iterative dose reconstruction method has been employed, which utilizes phantom and EPID response functions calculated from pre-acquired Computed Tomography (CT) image and simulated EPID image from Monte Carlo (MC) simulation. The accuracy of reconstructed dose, however, is limited by that of the simulated EPID image. In this study, a previously proposed density scaled model in XVMC code has been improved by physically relevant effective atomic number modeling and employing realistic phase space data. The new model has been tested under various field sizes and phantom thickness, including homogeneous and heterogeneous media. It has been shown that the calculated EPID images from the new EPID model are agreeable to the measured EPID images after field size and phantom thickness factors applied. The improved EPID model in MC code has been used in four-dimensional (4D) dose reconstruction. Considering the patient respiratory motion, breathing phase of acquired EPID images are determined by the phase sorting method for various treatment plans. For each phase, dose reconstruction has been performed using the sorted EPID images and 4D CT image of corresponding phase. The reconstructed 3D doses of each phase have been transformed to a reference phase to calculate the accumulated dose. The reconstructed doses have been shown good agreements to the forward 4D calculation in gamma analysis and dose-volume histogram (DVH) comparison. Based on the results, the new EPID model and the suggested phase sorting method accurately reconstruct the dose to the phantom for the cases shown in this study.Ph.D