Proton therapy range verification method via delayed γ-ray spectroscopy of a molybdenum tumour marker

Abstract

In this work, a new method of range verification for proton therapy (PT) is experimentally demonstrated for the first time. If a metal marker is implanted near the tumour site, its response to proton activation will result in the emission of characteristic γ rays. The relative intensity of γ rays originating from competing fusion-evaporation reaction channels provides a unique signature of the average proton energy at the marker, and by extension the beam’s range, in vivo and in real time. The clinical feasibility of this method was investigated at the PT facility at TRIUMF with a proof-of-principle experiment which irradiated a naturally-abundant molybdenum foil at various proton beam energies. Delayed characteristic γ rays were measured with two Compton-shielded LaBr3 scintillators. The technique was successfully demonstrated by relating the relative intensity of two γ-ray peaks to the energy of the beam at the Mo target, opening the door to future clinical applications where the range of the beam can be verified in real time.

GEANT4 simulation of a range verification method using delayed γ spectroscopy of a 92Mo marker

“…In addition, the emission rate of the characteristic γ ray of interest from 91m Mo was added artificially into GEANT4 since the simulation physics did not accurately portray the population and decay of the isomeric state. A comparison with recent experimental data (Burbadge et al 2020) suggests that, when a 91 Mo nucleus is detected as a reaction product, there is a 10% probability that a γ ray is created with an energy of 652.9 keV, whose emission we randomize with an exponentially distributed time stamp over a half life of 64.6 s. The probability for the population of the 652.9 keV isomer is consistent with the results of the aforementioned TALYS calculation, which shows that, at 22.75 MeV, 20.7% of the total (p,pn) reactions result in the production of the isomeric state, 91m Mo. Combining this information with the γ intensities found on NNDC (Baglin 2013), we calculate a maximum emission rate of 10.01(21)% of the 652.9(7) keV γ ray in a (p,pn) reaction.…”

“…The depth of the marker within the box was selected to be comparable to that of a typical breast tumour (Wang et al 2014). The dimensions of the box and marker region were selected to correspond to experimental targets (Burbadge et al 2020). For this experiment, the thickness of the marker region was selected to minimize proton energy loss in the marker, while still providing a strong response relative to the γ background from the PMMA box.…”

GEANT4 simulation of a new range verification method using delayed gamma spectroscopy of a Mo-92 marker

“…the G4RadioactiveDecayPhysics constructor was added in order to accurately simulate the decay of long-lived reaction products, and the G4HadronPhysicsQGSP BIC AllHP constructor was included for the implementation of TENDL cross sections in the simulation. The TENDL 1.3.2 library for proton-capture cross sections (Koning & Rochman 2012) was used as it corresponded more closely to experimental results than the default cross sections (Burbadge et al 2019). In addition, the emission rate of the characteristic γ ray of interest from 91m Mo was added artificially.…”

“…When a 91 Mo nucleus is detected as a reaction product, there is a 10% probability that a γ ray entry is created with an energy of 652.9(1) keV and an exponentially distributed randomized time stamp with a half life of 64.6 seconds. The probability of such a γ ray being created was set to 10% in order to reproduce the relative population of 653 keV γ rays to 773 keV γ in a similar experimental setup (Burbadge et al 2019). This value is consistent with the results of the aforementioned TALYS calculation, which shows that, at 22.75 MeV, 20.7% of the total (p,pn) reactions result in the production of the isomeric state, 91m Mo.…”

Improved sub-milimeter range-verification method for proton therapy using a composite hadron tumour marker (HTM)

“…In this work, we present the results of a follow-up experiment to Burbadge et al (2021), in which a novel method of sub-milimetre RV for PT was investigated experimentally for the first time following the simulation results published in Kasanda et al (2020). In Kasanda et al (2020), the method for RV using a hadron tumour marker (HTM) was established, using 92 Mo as a marker material.…”