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Accelerated clinical prompt gamma simulations for proton therapy

Abstract : After an introduction to particle therapy and prompt gamma detection, this doctoral dissertation comprises two main contributions: the development of a fast prompt gammas (PGs) simulation method and its application in a study of change detectability in clinical treatments. The variance reduction method (named vpgTLE) is a two-stage track length estimation method developed to estimate the PG yield in voxelized volumes. As primary particles are propagated throughout the patient CT, the PG yields are computed as function of the current energy of the primary, the material in the voxel and the step length. The second stage uses this intermediate image as a source to generate and propagate the number of PGs throughout the rest of the scene geometry, e.g. into a detection device. For both a geometrical heterogeneous phantom and a complete patient CT treatment plan with respect to analog MC, at a convergence level of 2% relative uncertainty on the PG yield per voxel in the 90% yield region, a gain of around 10³ was achieved. The method agrees with reference analog MC simulations to within 10⁻⁴ per voxel, with negligible bias. The second major study conducted in this PhD program was on PG FOP estimation in clinical simulations. The number of protons (spot weight) required for a consistent FOP estimate was investigated for the first time for two optimized PG cameras, a multi-parallel slit (MPS) and a knife edge design (KES). Three spots were selected for an in depth study, and at the prescribed spot weights were found to produce results of insufficient precision, rendering usable clinical output on the spot level unlikely. When the spot weight is artificially increased to 10⁹ primaries, the precision on the FOP reaches millimetric precision. On the FOP shift the MPS camera provides between 0.71 - 1.02 mm (1σ) precision for the three spots at 10⁹ protons; the KES between 2.10 - 2.66 mm. Grouping iso-energy layers was employed in passive delivery PG detection for one of the PG camera prototypes. In iso-depth grouping, enabled by active delivery, spots with similar distal dose fall-offs are grouped so as to provide well-defined fall-offs as an attempt to sidestep range mixing. It is shown that grouping spots does not necessarily negatively affect the precision compared to the artificially increased spot, which means some form of spot grouping can enable clinical use of these PG cameras. With all spots or spot groups the MPS has a better signal compared to the KES, thanks to a larger detection efficiency and a lower background level due to time of flight selection.
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Submitted on : Tuesday, April 9, 2019 - 2:31:55 PM
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  • HAL Id : tel-02094109, version 1


Brent Huisman. Accelerated clinical prompt gamma simulations for proton therapy. Medical Imaging. Université de Lyon, 2017. English. ⟨NNT : 2017LYSEI039⟩. ⟨tel-02094109⟩



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