Experimental Comparison of Knife-Edge and Multi-Parallel Slit Collimators for Prompt Gamma Imaging of Proton Pencil Beams

Abstract

More and more camera concepts are being investigated to try and seize the opportunity of instantaneous range verification of proton therapy treatments offered by prompt gammas emitted along the proton tracks. Focusing on one-dimensional imaging with a passive collimator, the present study experimentally compared in combination with the first, clinically compatible, dedicated camera device the performances of instances of the two main options: a knife-edge slit (KES) and a multi-parallel slit (MPS) design. These two options were experimentally assessed in this specific context as they were previously demonstrated through analytical and numerical studies to allow similar performances in terms of Bragg peak retrieval precision and spatial resolution in a general context. Both collimators were prototyped according to the conclusions of Monte Carlo optimization studies under constraints of equal weight (40 mm tungsten alloy equivalent thickness) and of the specificities of the camera device under consideration (in particular 4 mm segmentation along beam axis and no time-of-flight discrimination, both of which less favorable to the MPS performance than to the KES one). Acquisitions of proton pencil beams of 100, 160, and 230 MeV in a PMMA target revealed that, in order to reach a given level of statistical precision on Bragg peak depth retrieval, the KES collimator requires only half the dose the present MPS collimator needs, making the KES collimator a preferred option for a compact camera device aimed at imaging only the Bragg peak position. On the other hand, the present MPS collimator proves more effective at retrieving the entrance of the beam in the target in the context of an extended camera device aimed at imaging the whole proton track within the patient.

Prompt-gamma monitoring in hadrontherapy: A review

“…Attempts to compare several detection systems have been published. Smeets et al performed experiments with 100, 160 and 230 MeV protons, in order to compare the performances of knife-edge and multislit collimated cameras, with the criterion of the same mass of collimator and the same volume of detector [149]. This comparison is slightly biased by the fact that the Knife-edge camera geometry was optimally designed [89], whereas the multi-collimated one was not optimized.…”

Range Verification by Means of Prompt-Gamma Detection in Particle Therapy 1

“…A generalization of a slit-camera concept is a multi-slit camera, which presents the theoretical advantage of a non-restricted field of view. Smeets et al compared experimentally (using beam energies of 100, 160 and 230 MeV) two collimated setups: one with their optimized knife-edge shaped collimator (KES), and the other with a multi-slit collimator (MPS) of the same weight [35], the slits parallel to each other. Each collimator was combined with the same detector setup, described in the first paragraph of this section.…”

“…A similar collimated setup was built and tested experimentally by Park et al [37]. The geometry of both the collimator (coarser pattern) and the detector (finer granulation) was somewhat different than in case of [35]. The detector was made of CsI(Tl) and read out by photodiodes, as more radiation-hard than SiPMs.…”

Simulation of proton range monitoring in an anthropomorphic phantom using multi-slat collimators and time-of-flight detection of prompt-gamma quanta

“…Although the results are encouraging, the aforementioned works are proof-of-principle studies focused mostly on comparing collimated PG profiles to pristine Bragg peak distributions in homogeneous phantoms and/or phantoms with simple artificial inserts of different materials. Smeets et al [34] have considered a multi-slat camera operated without the shifting time-of-flight (TOF) method, which results in (n,γ) reactions that disturb the optimum profile distribution. In addition, they compared two cameras (knifeedge slit vs multi-slat) with a weight limit being imposed on both, which may not allow optimal multi-slat camera operation.…”