Optimization of collimator designs for real-time proton range verification by measuring prompt gamma rays

Abstract

Precise monitoring of the particle range inside the body, preferably in real-time, is a primary interest in quality assurance of particle therapy. Prompt-gamma (PG) detection aims at this task, but such a system is technically challenging to implement. For mechanically collimated PG detection, neutron-induced background radiation originating in the collimator material is a major issue. We study by Monte-Carlo simulations (Geant4), the influence of different collimator designs and their geometric parameters -such as distance to source, size and septa thickness -on the correlation between transmitted PG profiles and the longitudinal depth-dose profile. The impact of background rejection by time-of-flight (TOF) is also addressed. A single-parallel-slit collimator scanned along the beam axis was primarily studied and results show that the background can be efficiently reduced by TOF discrimination. Due to reduced statistics, signal integration from neighbouring pencil-beams was further considered, accounting for typical clinical proton fluences. Furthermore, first direct-comparison results between multi-slat collimation and a knife-edge-slit collimator are presented, obtained both by simulation and analytical calculations of their geometric performances. Advantages and disadvantages of both approaches are highlighted and some design improvements are proposed. Energy thresholding was also addressed, showing that a low-energy threshold of 4 MeV increases the signal-to-background ratio in all simulation results, a factor mostly important if TOF rejection cannot be applied.

CeBr3 scintillators for 4He prompt gamma spectroscopy: Results from a Monte Carlo optimization study

“…However, the contribution of the scattered component of the gamma radiation reaching the CeBr 3 can be reduced with the application of a narrow TOF window about the prompt component. The discrimination is possible due to the fact that the latter presents a well defined time structure, while the former has a broader time distribution . The collimation about the metal implant provides an excellent scenario to test the detection performance of simultaneous high‐ and low‐energy discrete lines.…”

“…The discrimination is possible due to the fact that the latter presents a well defined time structure, while the former has a broader time distribution. 28 The collimation about the metal implant provides an excellent scenario to test the detection performance of simultaneous high-and low-energy discrete lines.…”

A review of the use and potential of the GATE Monte Carlo simulation code for radiation therapy and dosimetry applications

“…Although an extensive validation of GEANT4 is still needed to fully trust results in terms of prompt gamma emission predictions, GEANT4 simulations have also been used to investigate the feasibility of new systems to image promptgamma profiles using Compton camera techniques [135][136][137] or mechanical collimators such as knife-edge slits and multiparallel slits. 138,139 A time-resolved simulation was recently published exploring the potential use of time-of-flight rejection to suppress the background signal created by neutrons in prompt-gamma imaging. 140 To compare pros and cons of PET and gamma prompt imaging, 141 used GEANT4 V9.0 to study four patient cases treated for head and neck, prostate, spine, and abdomen cancers by proton therapy.…”

Real-time proton beam range monitoring by means of prompt-gamma detection with a collimated camera

“…The main challenges are the relatively high energies and count rates as well as the elevated neutron background. Several designs have been proposed to image prompt gamma, amongst them pinhole, knife-edge-slit and parallel-slit collimator designs (see Kim 2009, Testa et al 2010, Lee et al 2012, Smeets et al 2012, Diblen et al 2012, Bom et al 2012, Lopes et al 2012 as well as electronic collimation via Compton cameras (see Richard et al 2011, Peterson proposed by Testa et al (2008) to decrease the neutron-induced background, and enable the observation of prompt gamma for carbon ions. The parallel-slit collimator is a promising and straightforward solution for prompt-gamma imaging that does not have to contend with the complexity of, for example, a Compton imaging system.…”