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

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Physics in Medicine & Biology
Authors:
Jayde Livingstone at European Synchrotron Radiation Facility
Jayde Livingstone
Denis Dauvergne at Laboratoire de Physique Subatomique et Cosmologie, Grenoble, France
Denis Dauvergne
  • Laboratoire de Physique Subatomique et Cosmologie, Grenoble, France
Ane Etxebeste at Institut National des Sciences Appliquées de Lyon
Ane Etxebeste
Mattia Fontana at DE.TEC.TOR.
Mattia Fontana
  • DE.TEC.TOR.
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Abstract and Figures

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.
The geometry used in the GATE simulations. A Compton camera, comprised of seven plane silicon detectors and a CeBr3 scintillator, was positioned so as to detect prompt γ-rays generated in a polymethyl methacrylate (PMMA) phantom by an incident proton beam.
The geometry used in the GATE simulations. A Compton camera, comprised of seven plane silicon detectors and a CeBr3 scintillator, was positioned so as to detect prompt γ-rays generated in a polymethyl methacrylate (PMMA) phantom by an incident proton beam.
… 
(a) The proton and γ tracks corresponding to the solution r 1 = (x 1, y 1, z 1) with TOFest1 and (b), the proton and γ tracks corresponding to the solution r 2 = (x 2, y 2, z 2) with TOFest2. In both cases, the star represents the solution, or the reconstructed emission point of the γ-ray, the red line represents the track of the proton and the green line represents the track of the γ-ray.
(a) The proton and γ tracks corresponding to the solution r 1 = (x 1, y 1, z 1) with TOFest1 and (b), the proton and γ tracks corresponding to the solution r 2 = (x 2, y 2, z 2) with TOFest2. In both cases, the star represents the solution, or the reconstructed emission point of the γ-ray, the red line represents the track of the proton and the green line represents the track of the γ-ray.
… 
2D histograms representing the proton time of flight, TOF p , as a function of the emission point of the correlated γ photon. Figure (a) is based on the real photon emission point recorded in the phase space file and figures (b)–(f) are based on the reconstructed emission points for temporal resolutions of 0, 100, 200, 500 and 1000 ps respectively. The emission points are the beam axis coordinates of the two solutions (no selection has been made). The solid black curve is the 4th order polynomial fit to (a).
2D histograms representing the proton time of flight, TOF p , as a function of the emission point of the correlated γ photon. Figure (a) is based on the real photon emission point recorded in the phase space file and figures (b)–(f) are based on the reconstructed emission points for temporal resolutions of 0, 100, 200, 500 and 1000 ps respectively. The emission points are the beam axis coordinates of the two solutions (no selection has been made). The solid black curve is the 4th order polynomial fit to (a).
… 
Line-cone reconstructed γ-emission profiles for a 160 MeV proton beam (10⁷ incident protons for the γ emission profiles in black, 2 × 10¹⁰ for all others) in single proton counting mode with (a) the Compton camera centred on the centre (x = 0 mm) of the PMMA phantom and (b) the Compton camera centred close to the expected Bragg peak position (x = 50 mm). The shaded region represents the length of the PMMA phantom in the direction of beam travel. Each curve has been normalised to its maximum value.
Line-cone reconstructed γ-emission profiles for a 160 MeV proton beam (10⁷ incident protons for the γ emission profiles in black, 2 × 10¹⁰ for all others) in single proton counting mode with (a) the Compton camera centred on the centre (x = 0 mm) of the PMMA phantom and (b) the Compton camera centred close to the expected Bragg peak position (x = 50 mm). The shaded region represents the length of the PMMA phantom in the direction of beam travel. Each curve has been normalised to its maximum value.
… 
Area under the profiles displayed in figure 4 without normalisation as a function of temporal resolution with (a) the Compton camera centred on the centre of the PMMA phantom and (b) the Compton camera centred close to the expected Bragg peak position. The corresponding profiles were obtained for a 160 MeV proton beam (2 × 10¹⁰ incident protons) in single proton counting mode.
+3
Area under the profiles displayed in figure 4 without normalisation as a function of temporal resolution with (a) the Compton camera centred on the centre of the PMMA phantom and (b) the Compton camera centred close to the expected Bragg peak position. The corresponding profiles were obtained for a 160 MeV proton beam (2 × 10¹⁰ incident protons) in single proton counting mode.
… 
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Phys. Med. Biol. 66 (2021)125012 https://doi.org/10.1088/1361-6560/ac03cb
PAPER
Influence of sub-nanosecond time of flight resolution for online
range verification in proton therapy using the line-cone
reconstruction in Compton imaging
Jayde Livingstone
1,2
, Denis Dauvergne
2,∗
, Ane Etxebeste
3
, Mattia Fontana
1
,
Marie-Laure Gallin-Martel
2
, Brent Huisman
3
, Jean Michel Létang
3
, Sara Marcatili
2
,
David Sarrut
3
and Étienne Testa
2
1
Univ Lyon, Université Claude Bernard Lyon 1, CNRS/IN2P3, Institut de Physique des 2 Infinis, F-69622 Villeurbanne, France
2
Université Grenoble Alpes, CNRS/IN2P3, Laboratoire de Physique Subatomique et de Cosmologie, F-38026 Grenoble, France
3
University of Lyon, INSA-Lyon, Université Claude Bernard Lyon 1, UJM-Saint Etienne, CNRS, Inserm, CREATIS UMR 5220, U1206,
F-69373 Lyon, France
∗
Author to whom any correspondence should be addressed.
E-mail: denis.dauvergne@lpsc.in2p3.fr
Keywords: hadron therapy, proton therapy, Compton imaging, Compton camera, time of flight, online verification, prompt-gamma
Supplementary material for this article is available online
Abstract
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 γemission 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 γfall-off position (FOP)
measured via line-cone reconstruction, where TOF comprises both the proton transit time from the
phantom entrance until γemission, 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
FOP. 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 CeBr
3
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 FOP was obtained for varying temporal resolutions and its
precision obtained from its shift across 100 independent γemission profiles compared to a high statistics
reference profile. The optimal temporal resolution for the given camera geometry and 10
8
primary protons
was 0.2 ns where a precision of 2.30 ±0.15 mm (1σ)on the FOP 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.
1. Introduction
Hadron therapy involves the use of light positively charged ions in the treatment of malignant tumours. The
depth-dose profile of ions, characterised by a relatively low entrance dose, well defined range and highly localised
dose deposition at the end of the particle track, makes ions an interesting choice in radiotherapy. In theory, the
sharp distal fall-off following the Bragg peak could be used to define the edge of the treatment field where an
organ at risk lies in close proximity to the tumour, sparing the organ at risk as all of the particles are stopped
inside the tumour. However, due to uncertainties either in the calculation of the ion stopping power at the
RECEIVED
14 December 2020
REVISED
10 May 2021
ACCEPTED FOR PUBLICATION
21 May 2021
PUBLISHED
14 June 2021
© 2021 Institute of Physics and Engineering in Medicine
... The example simulated PG profile in figure 12(a) resembles shape-wise the profiles presented in other works (see e.g. Pinto et al 2016, Krimmer et al 2018, Livingstone et al 2021, though the main peak is enhanced due to weighting the entries by their energy deposits. Additionally, it shows a small peak at a distance of 15 mm to 18 mm to the Bragg peak. ...
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