Time-resolved imaging of prompt-gamma rays for proton range verification using a knife-edge slit camera based on digital photon counters

Abstract

Proton range monitoring may facilitate online adaptive proton therapy and improve treatment outcomes. Imaging of proton-induced prompt gamma (PG) rays using a knife-edge slit collimator is currently under investigation as a potential tool for real-time proton range monitoring. A major challenge in collimated PG imaging is the suppression of neutron-induced background counts. In this work, we present an initial performance test of two knife-edge slit camera prototypes based on arrays of digital photon counters (DPCs). PG profiles emitted from a PMMA target upon irradiation with a 160 MeV proton pencil beams (about 6.5 × 10(9) protons delivered in total) were measured using detector modules equipped with four DPC arrays coupled to BGO or LYSO : Ce crystal matrices. The knife-edge slit collimator and detector module were placed at 15 cm and 30 cm from the beam axis, respectively, in all cases. The use of LYSO : Ce enabled time-of-flight (TOF) rejection of background events, by synchronizing the DPC readout electronics with the 106 MHz radiofrequency signal of the cyclotron. The signal-to-background (S/B) ratio of 1.6 obtained with a 1.5 ns TOF window and a 3 MeV-7 MeV energy window was about 3 times higher than that obtained with the same detector module without TOF discrimination and 2 times higher than the S/B ratio obtained with the BGO module. Even 1 mm shifts of the Bragg peak position translated into clear and consistent shifts of the PG profile if TOF discrimination was applied, for a total number of protons as low as about 6.5 × 10(8) and a detector surface of 6.6 cm × 6.6 cm.

In vivo range verification in particle therapy

“…The slit camera consists of an array of gamma detectors aligned behind a tungsten or lead collimator containing a single knife edge slit 78 , as shown in Figure 5. The knife-edge slit is aligned perpendicular to the beam central axis, and the PGs passing through the slit in the collimator are measured to produce a one-dimensional profile of the PG emission along the beam path 79–81 . Different slit camera verification systems employ different background suppression methods, such as closed-collimator background subtraction 82 or energy and time-of-flight detection systems 83,84 to help improve the cameras ability to localize the distal fall-off to the PG profile.…”

Correlations between the shifts in prompt gamma emission profiles and the changes in daily target coverage during simulated pencil beam scanning proton therapy

“…The sensitivity of proton therapy accuracy to small daily anatomical variations, as well as the corresponding effects on the dose distribution, have been previously described (Paganetti 2012). The detection of secondary radiation exiting the patient has been proposed as a potential tool for day-to-day dose monitoring, so as to ensure patient safety and an effective treatment (Parodi et al 2005, Min et al 2006, Cambraia Lopes et al 2015, 2016, Krimmer et al 2018, Parodi and Polf 2018.…”

Metallic Nanoparticles: A Useful Prompt Gamma Emitter for Range Monitoring in Proton Therapy?

“…Although the reported increase in γ-ray emission might improve PGI, it must be observed in an energy range between 3 and 6 MeV, corresponding to the signal-to-noise ratio that provides the highest correlation to the end of proton track [41]. The simulations did not suggest a higher PG yield for any of the three metals compared to water in the useful energy range (cfr.…”

“…No increase was reported for BiNP. Since the addition of a temporal discrimination in the signal allows for a reduction in energy threshold to a range between 1 and 2 MeV, a potential benefit of NP for range verification, especially AlNP, might be observed in PGT clinical scenarios [41]. For SiNP, the higher production of low-energy γ-rays is mainly due to the 28 Si(p,p') 28 Si* and 28 Si(p,x) 27 Al* nuclear reactions that emit 1.779 and 1.014 MeV γ-rays, respectively.…”