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Lung Patient Treatment Plan Optimization in the Presence of Magnetic Fields Open Access


Other title
Inverse Planning
Type of item
Degree grantor
University of Alberta
Author or creator
Tamagi, Daniel A M
Supervisor and department
Field, Colin (Medical Physics)
Warkentin, Brad (Medical Physics)
Examining committee member and department
Fallone, Gino (Medical Physics)
Larocque, Matthew (Medical Physics)
Rathee, Satyapal (Medical Physics)
Department of Oncology
Medical Physics
Date accepted
Graduation date
2016-06:Fall 2016
Master of Science
Degree level
The integration of clinical linear accelerators (linacs) with magnetic resonance imaging (MRI) systems represent the next generation of image guidance technologies in radiotherapy. Linac-MR systems have several potential advantages over the current X-ray based state-of-the-art image guidance systems, including superior soft tissue contrast and real-time tumour tracking. However the main magnetic field of the MRI system can have significant effects on dose deposition in the patient, particularly in low density regions such as lung. In this work, we investigated the impact of magnetic fields and tissue heterogeneity on the inverse-planning process for a Linac-MR system. Two Linac-MR configurations were studied, where the main magnetic field of the MRI system was either parallel or perpendicular to the linac beam central axis. These two magnetic field orientations affect the patient dose distribution differently. A 10 beam stereotactic body radiation therapy treatment plan for a lung patient was simplified to 3 beams for this study. We used a research version of the RayStation treatment planning system, which implemented discrete superposition of user supplied Monte Carlo simulated beamlet dose kernels for dose calculation. Dose distributions were compared for three cases: 1) the magnetic field was ignored during treatment planning; 2) the magnetic field was modeled during the forward calculation stage only; and 3) the magnetic field was modeled throughout the planning process. Comparison of these cases helped to determine the importance of modeling the main magnetic field of the MRI at the optimization and forward calculation stages, respectively. The results showed that the magnetic field effects for the parallel Linac-MR configuration were minimal, and that the magnetic field could be likely ignored during inverse-planning without a significant degradation of plan quality. The magnetic field effects for the perpendicular iii configuration were much more significant, and resulted in large dose inhomogeneity in the target volume when the magnetic field was ignored at either the optimization or forward calculation stage of the inverse-planning process.
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