In-beam proton range monitoring during proton therapy : a Monte Carlo study on the feasibility of secondary gamma imaging

Abstract: Proton radiation therapy allows for delivering a high dose to a well-confinedregion of interest due to the characteristic proton dose deposition. Due to protonrange straggling, anatomic variations in patients and small patient setup errors,treatment plans needs to account for proton range uncertainties of up to 3.5% invivo.Therefore, it is highly desirable to measure the proton range on-line in orderto minimize margins in the treatment plan. Initially, the feasibility of on-linerange monitoring through prompt gamma imaging and Positron EmissionTomography (PET) at different proton energies is evaluated using GEANT4Application for Tomographic Emission (GATE) Monte Carlo (MC) simulations.In the second phase, the performance of a lead knife-edge slit system for promptgamma imaging was evaluated with MC simulations. Results from simulationsindicate that prompt gamma emission and PET isotope production is correlatedwith proton range, with discrete prompt gamma emission lines from Carbon (4.4MeV) showing good correlation. The evaluated system was able to image thepeak gamma emission location at three different slit positions with promisingprecision ± 1 mm, ± 0.7 mm and ± 1.3 mm, and average shifts of -2 mm, -3 mmand -4 mm, respectively. The proton range was resolved with mean profile shiftsof -12 ± 1 mm, -13 ± 0.7 mm and -14 ± 1.3 mm, following prompt gamma crosssectionbehavior with peak emission- and threshold energies. The results providean indication of the potential of the knife-edge slit system and future work willinclude more extensive MC simulations and experimental measurements at the Skandion clinic to determine its clinical validity.