Room: Stars at Night Ballroom 1
Purpose: Multiple Coulomb scattering (MCS) reduces imaging resolution in PBS-based pRG and pCT. We proposed to divide each spot into sub-spots to improve the resolution. In this study, we validate spot decomposition by calculating the MLP from each sub-spot.
Methods: TOPAS Monte-Carlo simulation tool was used for calculation of the MLP in a water phantom (20Ã—20Ã—20 cmÂ³ centered at the isocenter). The initial energy of the proton spot was set to 210 MeV and the source position and spot size (1Ïƒ) was 54.6 cm (from isocenter) and 2.3 mm at the nozzle. All the histories of the protons and the exiting position and energy information were recorded by a phase space detector. Based on the protonsâ€™ distribution at the detector surface, the spot was divided into 19 sub-spots. The mean and standard deviation (SD) of the residual energy for all protons within each sub-spot were calculated. An energy selection filter was simulated to filter out protons outside Â±1 SD of the mean energy. A MLP and its variation were calculated for each sub-spot from all protons fall into the sub-spot, with and without energy selection.
Results: The spot size on the detector is 8.930 mm (1Ïƒ). The sub-spot sizes were 4.518 cm, 3.278 cm, and 5.307 cm for the center, intermedium, and outside sub-spots, respectively. The corresponding mean residual energies are 103.75Â±3.13 Gy, 103.93Â±3.09 Gy, and 103.54Â±3.06 Gy, respectively. The variation of the MLPs for the center sub-spots are [0.28Â±2.26, -0.15Â±2.11] and [0.12Â±2.21, -0.05Â±2.11] for MLPs with and without energy selections, respectively. The calculated MLPs reasonably represent the mean path of the protons reaching each sub-spot.
Conclusion: Our spot decomposition method is validated by the calculated MLPs. Energy selection can further reduce the uncertainty of the MLP. This method is being used in our PBS-based pRG and pCT imaging system.