K. E. Romer

First test of the prompt gamma ray timing method with heterogeneous targets at a clinical proton therapy facility.

Ion beam therapy promises enhanced tumour coverage compared to conventional radiotherapy, but particle range uncertainties significantly blunt the achievable precision. Experimental tools for range verification in real-time are not yet available in clinical routine. The prompt gamma ray timing method has been recently proposed as an alternative to collimated imaging systems. The detection times of prompt gamma rays encode essential information about the depth-dose profile thanks to the measurable transit time of ions through matter. In a collaboration between OncoRay, Helmholtz-Zentrum Dresden-Rossendorf and IBA, the first test at a clinical proton accelerator (Westdeutsches Protonentherapiezentrum Essen, Germany) with several detectors and phantoms is performed. The robustness of the method against background and stability of the beam bunch time profile is explored, and the bunch time spread is characterized for different proton energies. For a beam spot with a hundred million protons and a single detector, range differences of 5 mm in defined heterogeneous targets are identified by numerical comparison of the spectrum shape. For higher statistics, range shifts down to 2 mm are detectable. A proton bunch monitor, higher detector throughput and quantitative range retrieval are the upcoming steps towards a clinically applicable prototype. In conclusion, the experimental results highlight the prospects of this straightforward verification method at a clinical pencil beam and settle this novel approach as a promising alternative in the field of in vivo dosimetry.

Tests of a Compton imaging prototype in a monoenergetic 4.44 MeV photon field—a benchmark setup for prompt gamma-ray imaging devices

The finite range of a proton beam in tissue opens new vistas for the delivery of a highly conformal dose distribution in radiotherapy. However, the actual particle range, and therefore the accurate dose deposition, is sensitive to the tissue composition in the proton path. Range uncertainties, resulting from limited knowledge of this tissue composition or positioning errors, are accounted for in the form of safety margins. Thus, the unverified particle range constrains the principle benefit of proton therapy. Detecting prompt γ-rays, a side product of proton-tissue interaction, aims at an on-line and non-invasive monitoring of the particle range, and therefore towards exploiting the potential of proton therapy. Compton imaging of the spatial prompt γ-ray emission is a promising measurement approach. Prompt γ-rays exhibit emission energies of several MeV. Hence, common radioactive sources cannot provide the energy range a prompt γ-ray imaging device must be designed for. In this work a benchmark measurement-setup for the production of a localized, monoenergetic 4.44 MeV γ-ray source is introduced. At the Tandetron accelerator at the HZDR, the proton-capture resonance reaction 15N(p,α γ4.439)12C is utilized. This reaction provides the same nuclear de-excitation (and γ-ray emission) occurrent as an intense prompt γ-ray line in proton therapy. The emission yield is quantitatively described. A two-stage Compton imaging device, dedicated for prompt γ-ray imaging, is tested at the setup exemplarily. Besides successful imaging tests, the detection efficiency of the prototype at 4.44 MeV is derived from the measured data. Combining this efficiency with the emission yield for prompt γ-rays, the number of valid Compton events, induced by γ-rays in the energy region around 4.44 MeV, is estimated for the prototype being implemented in a therapeutic treatment scenario. As a consequence, the detection efficiency turns out to be a key parameter for prompt γ-rays Compton imaging limiting the applicability of the prototype in its current realization.

Scintillator-Based High-Throughput Fast Timing Spectroscopy for Real-Time Range Verification in Particle Therapy

Range verification of particle beams in real time is considered a key for tapping the full potential of radio-oncological particle therapies. The novel technique of prompt gamma-ray timing (PGT), recently proposed and explored in first proof-of-principle experiments, promises range assessment at reasonable expense but challenges detectors, electronics, and data acquisition. Energy-selected time distributions have to be measured at very high throughput rates to obtain the statistics necessary for range verification with single pencil beam spots. Clinically applicable systems should provide a time resolution of about 200 ps, to be obtained with large (about 2” diameter) scintillators, detector loads in the few-Mcps range, and data acquisition rates around 1 Mcps, if possible with compact and inexpensive systems. Such requirements can be met best with CeBr3 scintillators read out with conventional photomultiplier tubes, coupled to commercial but customized electronics featuring high-resolution pulse digitization and fast digital signal processing. The paper deduces design parameters from the constraints given by typical treatment conditions, and presents first results obtained with prototype detectors and electronics developed in accordance with the derived specifications.

Compton Camera and Prompt Gamma Ray Timing: Two Methods for In Vivo Range Assessment in Proton Therapy

Proton beams are promising means for treating tumors. Such charged particles stop at a defined depth, where the ionization density is maximum. As the dose deposit beyond this distal edge is very low, proton therapy minimizes the damage to normal tissue compared to photon therapy. Nevertheless, inherent range uncertainties cast doubts on the irradiation of tumors close to organs at risk and lead to the application of conservative safety margins. This constrains significantly the potential benefits of protons over photons. In this context, several research groups are developing experimental tools for range verification based on the detection of prompt gammas, a nuclear by-product of the proton irradiation. At OncoRay and Helmholtz-Zentrum Dresden-Rossendorf, detector components have been characterized in realistic radiation environments as a step toward a clinical Compton camera. On the one hand, corresponding experimental methods and results obtained during the ENTERVISION training network are reviewed. On the other hand, a novel method based on timing spectroscopy has been proposed as an alternative to collimated imaging systems. The first tests of the timing method at a clinical proton accelerator are summarized, its applicability in a clinical environment for challenging the current safety margins is assessed, and the factors limiting its precision are discussed.

Prompt Gamma Rays Detected With a BGO Block Compton Camera Reveal Range Deviations of Therapeutic Proton Beams

The dose deposition profile of protons is interesting for tumour treatment due to the increased ionization density at the end of their track. However, the inaccurate knowledge of the proton stopping point limits the precision of the therapy. Prompt gamma rays, a by-product of the irradiation, are candidates for an indirect measurement of the particle range. Compton cameras have been proposed for prompt gamma ray imaging, but struggle with high trigger rates and low coincident efficiency. The feasibility in a clinical environment has yet to be proved. At Universitäts Protonen Therapie Dresden, two bismuth germanate (BGO) block detectors arranged face-to-face are deployed for imaging tests with a homogeneous target irradiated by a proton pencil beam. Shifts of the target, increase of its thickness and beam energy variation experiments are conducted. Each measurement lasts about 15 minutes at a low proton beam current. The effect of one centimetre proton range deviations on the backprojected images is analysed. The number of valid Compton events as well as the trigger rate expected in a realistic treatment plan with pencil beam scanning are estimated. The results support the use of a high density material despite its moderate energy resolution, in order to maximize the coincident efficiency. Nevertheless, they discourage the applicability of a two-plane Compton camera in a clinical scenario with usual beam currents.

Linearization of Gamma Energy Spectra in Scintillator-Based Commercial Instruments

This paper presents a novel technique developed for linearizing the energy spectra of radiation detectors in commercial radioisotope identification devices. Based on few spectrum measurements with standard radio-nuclide sources, this method allows generation of individual nonlinear calibration functions at minimum expense in the routine instrument setup. Instead of fitting peak positions, the measured raw data are compared with simulated spectrum templates, and local gain factors providing the best correspondence are taken as reference points for the calibration function. This approach avoids the problem of fitting multiple peaks with intensity ratios influenced by absorbing layers and assures an accuracy of 1% in the energy range of 30 keV to 3 MeV.

CaF2(Eu): an ``old'' scintillator revisited

Homeland security applications demand high performance Compton-camera systems, with high detector efficiency, good nuclide identification and able to operate in-field conditions. A low-Z scintillator has been proposed and studied as a promising candidate for use in the scattering plane of a scintillator-based Compton camera: CaF2(Eu). All the relevant properties for the application of this scintillator in a mobile Compton camera system have been addressed: the energy resolution and the non-linearity at room temperature and in the temperature range of −20°C to +55°C, the photoelectron yield and the relative light yield in the relevant temperature range. A new method of inferring the relative light output of scintillators as a function of temperature has been proposed.

Fast timing with BGO (and other scintillators) on digital silicon photomultipliers for Prompt Gamma Imaging

Particle therapy is supposed to be an advanced treatment modality compared to conventional radiotherapy because of the well-defined range of the ions. Prompt gamma rays, produced in nuclear reactions between ion and nuclei, can be utilized for real-time range verification to exploit the full potential of particle therapy. Several devices have been investigated in the field of Prompt Gamma Imaging (PGI), like Slit and Compton Cameras. The latter need very high detection efficiency as well as good time and energy resolution, requiring a versatile scintillation detector. In Positron Emission Tomography (PET), LSO and LYSO are known for their good time resolution, while the lower cost alternative BGO shows worse performance. In PGI however, where gamma rays have energies up to 10 MeV, the light output of a scintillator is up to 20 times larger compared to PET. This reduces the statistical contribution of the time resolution, which is the dominant part in case of BGO. Thus, BGO could be a reasonable alternative to LSO/LYSO for applications in PGI. Hence, experiments at the ELBE accelerator at HZDR (Germany) were performed using digital silicon photomultiplier (dSiPM) from Philips with monolithic BGO and LYSO crystals, and for completeness with GAGG, CeBr3, CsI, CaF2, and GSO. The time resolution of BGO compared to the other scintillators will be presented for a wide range of trigger- and validation levels as well as validation lengths of the dSiPM. Timing resolutions below 220 ps are obtained for BGO, while LYSO and CeBr3 achieve about 170 ps.

CaF 2 (Eu): An “Old” scintillator revisited

Homeland security applications demand high performance Compton-camera systems, with high detector efficiency, good nuclide identification and able to operate in-field conditions. A low-Z scintillator has been proposed and studied as a promising candidate for use in the scattering plane of a scintillator-based Compton camera: CaF 2 (Eu). All the relevant properties for the application of this scintillator in a mobile Compton camera system have been addressed: the energy resolution and the non-linearity at room temperature and in the temperature range of −20°C to +55°C, the photoelectron yield and the relative light yield in the relevant temperature range. A new method of inferring the relative light output of scintillators as a function of temperature has been proposed.

Prompt gamma imaging of a pencil beam with a high efficiency compton camera at a clinical proton therapy facility

Protons are excellent particles for tumour treatment due to the increased ionization density close to their stopping point. In practice, the uncertainty on the particle range compromises the achievable accuracy. Compton cameras imaging prompt gamma rays, a by-product of the irradiation, have been proposed for indirect range verification years since. At Universitats Protonen Therapie Dresden, two BGO block detectors (from PET scanners) arranged as Compton camera are deployed for imaging tests with high energy prompt gamma rays produced in PMMA by a proton pencil beam. Target shifts, thickness increase and beam energy variation experiments are conducted. Each measurement lasts about 15 minutes at a low proton beam current. The effect of one centimetre proton range deviations on the backprojected images is analysed. In conclusion, the experimental results highlight the potential application of Compton cameras for high energy prompt gamma ray imaging of pencil beams, as a real-time and in vivo range verification method in proton therapy.