Unit:
Κατεύθυνση Ιατρική Φυσική-ΑκτινοφυσικήLibrary of the School of Health Sciences
Author:
Margaroni Vasiliki
Supervisors info:
Παντελής Καραΐσκος, Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ, Επιβλέπων
Παναγιώτης Παπαγιάννης, Αναπληρωτής Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Κωνσταντίνος Λουκάς, Επίκουρος Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Original Title:
Monte Carlo Δοσιμετρικοί Υπολογισμοί σε 1.5Τ MR-Linac: Διορθωτικοί Παράγοντες Απόκρισης Θαλάμων Ιονισμού
Translated title:
Monte Carlo Dosimetric Calculations in 1.5T MR-Linac: Ionization chamber correction factors
Summary:
The development of an integrated system of a linear accelerator and a magnetic resonance imaging device (MR-Linac) provides real-time imaging during the radiation treatment, with high soft tissue contrast for high precision image and enables to track the motion of the tumor and control the irradiation beam to account for this motion. However, the presence of a 1.5T magnetic field raises challenges in MR-Linac dosimetry due to the Lorentz force which influences the trajectories of the secondary electrons. Taking this under consideration, this study investigates the response of a variety of ionization chambers in a solid phantom and therefore the air gap effect that may exist between the ionization chambers and the solid phantom when the measurements are performed in 1.5T magnetic field.
A total of 8 ionization chambers were modelled in the EGSnrc C++ geometry package based on blueprints made available by the corresponding manufactures. The detectors effective points of measurement were positioned at a depth of 5cm on the beam central axis, inside a RW3 solid phantom with a source-to-surface distance of 138.5cm and gantry angle always set at 0°. Phase space files for a 10cm x 10cm field size of an Elekta Unity 7MV flattering filter free MR-Linac were used as a source throughout this study. The 1.5T magnetic field was always perpendicular to the irradiation field and parallel to the treatment couch. The ionization chambers were simulated to have their central electrode either parallel or perpendicular to the magnetic field, but always lateral to the beam axis. Symmetrical and asymmetrical air gaps of 0.1mm up to 2mm were introduced around the detectors. The dose scored in each detector’s sensitive volume was compared against the zero-gap geometry.
The results of this study showed that the effect of symmetrical air gaps was depended on the detectors’ shape and dimensions. For the parallel set-up, the symmetrical air gaps induced differences less than 2%. In the case of asymmetrical air gaps, there was no dependence on the detectors’ shape and dimensions. Differences in the ionization chamber responses above 1.6% were observed for asymmetric air gaps greater than 0.3mm and especially for the asymmetrical air gap with thick of 2mm the chamber responses ranged from 9.45% to 10.94%. However, the impact of the air gaps was systematically reduced when the chamber oriented perpendicularly to the magnetic field, as compared to the parallel set-up.
Consequently, taking the above into account, the interaction between the magnetic field and the secondary charged particles in a small volume between a plastic phantom and the detector could considerable affect the detector response, especially when the latter is oriented perpendicular to the magnetic field.
Main subject category:
Health Sciences
Keywords:
Monte Carlo, EGSnrc, MR-Linac, Dosimetry, Plastic phantom
File:
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MARGARONI_MSc_THESIS.pdf
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