F. Roellinghoff

Prompt gamma imaging with a slit camera for real-time range control in proton therapy.

Treatments delivered by proton therapy are affected by uncertainties on the range of the beam within the patient. To reduce these margins and deliver safer treatments, different projects are currently investigating real-time range control by imaging prompt gammas emitted along the proton tracks in the patient. This study reports on the development and test of a prompt gamma camera using a slit collimator to obtain a 1-dimensional projection of the beam path on a scintillator detector. A first prototype slit camera using the HICAM gamma detector, originally developed for low-energy gamma-ray imaging in nuclear medicine and modified for this purpose, was tested successfully up to 230 MeV beam energy. Results now confirm the potential of this concept for real-time range monitoring with millimeter accuracy in pencil beam scanning mode for the whole range of clinical energies. With the experience gained, a new prototype is under study for clinical beam currents. In this work, we present both the profiles obtained at 230 MeV using HICAM and the description of the new gamma camera prototype design.

Design Guidelines for a Double Scattering Compton Camera for Prompt-

In hadrontherapy in order to fully take advantage of the assets of the ion irradiation, the position of the Bragg peak has to be monitored accurately. Here, we investigate a monitoring method relying on the detection in real time of the prompt γ emitted quasi instantaneously during the nuclear fragmentation processes. Our detection system combines a beam hodoscope and a double scattering Compton camera. The prompt-γ emission points are reconstructed by intersecting the ion trajectories given by the hodoscope and the Compton cones reconstructed with the camera. We propose here to study in terms of point spread function and efficiency the theoretical feasibility of the emission points reconstruction with our set-up in the case of a photon point source in air. First we analyze the nature of all the interactions which are likely to produce an energy deposit in the three detectors of the camera. It is underlined that upper energy thresholds in both scatter detectors are required in order to select mainly Compton events (one Compton interaction in each scatter detector and one interaction in the absorber detector). Then, we study the influence of various parameters such as the photon energy and the inter-detector distances on the Compton camera response. These studies are carried out by means of Geant4 simulations. We use a source with a spectrum corresponding to the prompt-γ spectrum emitted during the carbon ion irradiation of a water phantom. In the current configuration, the spatial resolution of the Compton camera is about 6 mm (Full Width at Half Maximum) and the detection efficiency 10-5. Finally, provided the detection efficiency is increased, the clinical applicability of our system is considered.

\gamma

Prompt-gamma profile was measured at WPE-Essen using 160 MeV protons impinging a movable PMMA target. A single collimated detector was used with time-of-flight (TOF) to reduce the background due to neutrons. The target entrance rise and the Bragg peak falloff retrieval precision was determined as a function of incident proton number by a fitting procedure using independent data sets. Assuming improved sensitivity of this camera design by using a greater number of detectors, retrieval precisions of 1 to 2 mm (rms) are expected for a clinical pencil beam. TOF improves the contrast-to-noise ratio and the performance of the method significantly.

Imaging During Ion Beam Therapy: A Monte Carlo Simulation Study

The goal of this study is to tune the design of the absorber detector of a Compton camera for prompt γ-ray imaging during ion beam therapy. The response of the Compton camera to a photon point source with a realistic energy spectrum (corresponding to the prompt γ-ray spectrum emitted during the carbon irradiation of a water phantom) is studied by means of Geant4 simulations. Our Compton camera consists of a stack of 2 mm thick silicon strip detectors as a scatter detector and of a scintillator plate as an absorber detector. Four scintillators are considered: LYSO, NaI, LaBr3 and BGO. LYSO and BGO appear as the most suitable materials, due to their high photo-electric cross-sections, which leads to a high percentage of fully absorbed photons. Depth-of-interaction measurements are shown to have limited influence on the spatial resolution of the camera. In our case, the thickness which gives the best compromise between a high percentage of photons that are fully absorbed and a low parallax error is about 4 cm for the LYSO detector and 4.5 cm for the BGO detector. The influence of the width of the absorber detector on the spatial resolution is not very pronounced as long as it is lower than 30 cm.

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

The HICAM gamma camera is an imaging device recently developed in the framework of a European project, based on Silicon Drift Detectors (SDDs) as photodetectors. Although originally designed for low-energy gamma-ray imaging in nuclear medicine (140 keV of 99mTc), in this work we attempt to use the camera, suitably modified, to image high energy prompt gamma rays (2 to 7 MeV) emitted by a target irradiated by protons. The final objective of our experiment is to assess the feasibility of proton beam range measurements by prompt gamma imaging with a slit camera, and the HICAM camera was chosen for a first prototype. Although a SDD-based camera would not be fast enough for real treatment conditions, the prototype here employed benefited from the camera modularity, compactness, high resolution and low noise. The camera here employed is composed of 25 SDDs of 1 cm2 active area each, arranged in a 5×5 format, already used in clinical and research environments with a high intrinsic spatial resolution (∼1 mm). The SDD matrix has been coupled to a LYSO crystal (1cm thickness), to improve efficiency with high-energy gammas, and has been characterized preliminarily with a 60Co source. Good imaging performances have been obtained in this test. Moreover, results of a first test of the camera to detect prompt gammas emitted with a proton beam impinging on a plastic target are presented in this work.

Design study of the absorber detector of a compton camera for on-line control in ion beam therapy

Treatments delivered by proton therapy are affected by uncertainties on the range of the beam within the patient. To reduce these margins and deliver safer treatments, different projects are currently investigating real-time range control by imaging prompt gammas emitted along the proton tracks in the patient. This study reports on the development and test of a prompt gamma camera using a slit collimator to obtain a 1-dimensional projection of the beam path on a scintillator detector. A first prototype slit camera using the HICAM gamma detector, originally developed for low-energy gamma-ray imaging in nuclear medicine and modified for this purpose, was tested successfully up to 230 MeV beam energy. Results now confirm the potential of this concept for real-time range monitoring with millimeter accuracy in pencil beam scanning mode for the whole range of clinical energies. With the experience gained, a new prototype is under study for clinical beam currents. In this work, we present both the profiles obtained at 230 MeV using HICAM and the description of the new gamma camera prototype design.

Application of the HICAM camera for imaging of prompt gamma rays in measurements of proton beam range

Purpose: Hadrontherapy is an innovative radiotherapy modality that allows for high tumour conformality due to the characteristic Bragg peak in the dose-depth profile. However, the high precision involved also makes the monitoring of dose delivery more critical. Currently, only positron emission tomography is clinically implemented but other techniques may also provide clinically-relevant information, namely the ones relying on prompt-radiation emission. This work addresses the research and development (R&D) of a TOFbased multi-slit collimated camera, one of the possible devices that use prompt-radiation information to monitor dose delivery in particular for proton and carbon ion treatments. This camera involves the detection of prompt gammas emitted during the treatment along the beam axis, for which the correlation with the range of particles in matter has been extensively demonstrated.