Image guided radiation therapy applications for head and neck, prostate, and breast cancers using 3D ultrasound imaging and Monte Carlo dose calculations

In radiation therapy an uncertainty in the delivered dose always exists because anatomic changes are unpredictable and patient specific. Image guided radiation therapy (IGRT) relies on imaging in the treatment room to monitor the tumour and surrounding tissue to ensure their prescribed position in the radiation beam. The goal of this thesis was to determine the dosimetric impact on the misaligned radiation therapy target for three cancer sites due to common setup errors; organ motion, tumour tissue deformation, changes in body habitus, and treatment planning errors. For this purpose, a novel 3D ultrasound system (Restitu, Resonant Medical, Inc.) was used to acquire a reference image of the target in the computed tomography simulation room at the time of treatment planning, to acquire daily images in the treatment room at the time of treatment delivery, and to compare the daily images to the reference image. The measured differences in position and volume between daily and reference geometries were incorporated into Monte Carlo (MC) dose calculations. The EGSnrc (National Research Council, Canada) family of codes was used to model Varian linear accelerators and patient specific beam parameters, as well as to estimate the dose to the target and organs at risk under several different scenarios.;In collaboration with the manufacturer, the ultrasound system was adapted from transabdominal imaging to neck imaging. Two case studies of nasopharyngeal cancer are discussed. The deformation of disease-positive cervical lymph nodes was monitored throughout treatment. Node volumes shrunk to 17% of the initial volume, moved up 1.3 cm, and received up to a 12% lower dose than that prescribed. It is shown that difficulties in imaging soft tissue in the neck region are circumvented with ultrasound imaging, and after dosimetric verification it is argued that adaptive replanning may be more beneficial than patient realignment when intensity modulated radiation therapy techniques are used.;Some of the largest dose delivery errors were found in external electron beam treatments for breast cancer patients who underwent breast conserving surgery. Inaccuracies in conventional treatment planning resulted in substantial target dose discrepancies of up to 88%. When patient setup errors, interfraction tumour bed motion, and tissue remodeling were considered, inadequate target coverage was exacerbated.;This thesis quantifies the dose discrepancy between that prescribed and that delivered. I delve into detail for common IGRT treatment sites, and illuminate problems that have not received much attention for less common IGRT treatment sites.;After validating the necessity of MC dose calculations in the pelvic region, the impact of interfraction prostate motion, and subsequent patient realignment under the treatment beams, on the delivered dose was investigated. For 32 patients it is demonstrated that using 3D conformal radiation therapy techniques and a 7 mm margin, the prescribed dose to the prostate, rectum, and bladder is recovered within 0.5% of that planned when patient setup is corrected for prostate motion, despite the beams interacting with a new external surface and internal tissue boundaries.