Influence of sub-nanosecond time of flight resolution for online range verification in proton therapy using the line-cone reconstruction in Compton imaging.

Online ion range monitoring in hadron therapy can be performed via detection of secondary radiation, such as prompt γ-rays, emitted during treatment. The prompt γ emission profile is correlated with the ion depth-dose profile and can be reconstructed via Compton imaging. The line-cone reconstruction, using the intersection between the primary beam trajectory and the cone reconstructed via a Compton camera, requires negligible computation time compared to iterative algorithms. A recent report hypothesised that time of flight (TOF) based discrimination could improve the precision of the γ fall-off position measured via line-cone reconstruction, where TOF comprises both the proton transit time from the phantom entrance until γ emission, and the flight time of the γ-ray to the detector. The aim of this study was to implement such a method and investigate the influence of temporal resolution on the precision of the fall-off position. Monte Carlo simulations of a 160 MeV proton beam incident on a homogeneous PMMA phantom were performed using GATE. The Compton camera consisted of a silicon-based scatterer and CeBr3 scintillator absorber. The temporal resolution of the detection system (absorber + beam trigger) was varied between 0.1 and 1.3 ns RMS and a TOF-based discrimination method applied to eliminate unlikely solution(s) from the line-cone reconstruction. The fall-off position was obtained for varying temporal resolutions and its precision obtained from its shift across 100 independent γ emission profiles compared to a high statistics reference profile. The optimal temporal resolution for the given camera geometry and 108 primary protons was 0.2 ns where a precision of 2.30 ± 0.15 mm (1σ) on the fall-off position was found. This precision is comparable to current state of-the-art Compton imaging using iterative reconstruction methods or 1D imaging with mechanically collimated devices, and satisfies the requirement of being smaller than the clinical safety margins.