Prompt Gamma Imaging Research Articles

Distributions of secondary particles in proton and carbon-ion therapy: a comparison between GATE/Geant4 and FLUKA Monte Carlo codes

Monte Carlo simulations play a crucial role for in-vivo treatment monitoring based on PET and prompt gamma imaging in proton and carbon-ion therapies. The accuracy of the nuclear fragmentation models implemented in these codes might affect the quality of the treatment verification. In this paper, we investigate the nuclear models implemented in GATE/Geant4 and FLUKA by comparing the angular and energy distributions of secondary particles exiting a homogeneous target of PMMA. Comparison results were restricted to fragmentation of 16O and 12C. Despite the very simple target and set-up, substantial discrepancies were observed between the two codes. For instance, the number of high energy (>1 MeV) prompt gammas exiting the target was about twice as large with GATE/Geant4 than with FLUKA both for proton and carbon ion beams. Such differences were not observed for the predicted annihilation photon production yields, for which ratios of 1.09 and 1.20 were obtained between GATE and FLUKA for the proton beam and the carbon ion beam, respectively. For neutrons and protons, discrepancies from 14% (exiting protons–carbon ion beam) to 57% (exiting neutrons–proton beam) have been identified in production yields as well as in the energy spectra for neutrons.

SP-0300: Prompt gamma imaging for range assessment in proton and ion therapy

SP-0300: Prompt gamma imaging for range assessment in proton and ion therapy

Time-of-flight neutron rejection to improve prompt gamma imaging for proton range verification: a simulation study

Therapeutic proton and heavier ion beams generate prompt gamma photons that may escape from the patient. In principle, this allows for real-time, in situ monitoring of the treatment delivery, in particular, the hadron range within the patient, by imaging the emitted prompt gamma rays. Unfortunately, the neutrons simultaneously created with the prompt photons create a background that may obscure the prompt gamma signal. To enhance the accuracy of proton dose verification by prompt gamma imaging, we therefore propose a time-of-flight (TOF) technique to reject this neutron background, involving a shifting time window to account for the propagation of the protons through the patient. Time-resolved Monte Carlo simulations of the generation and transport of prompt gamma photons and neutrons upon irradiation of a PMMA phantom with 100, 150 and 200 MeV protons were performed using Geant4 (version 9.2.p02) and MCNPX (version 2.7.D). The influence of angular collimation and TOF selection on the prompt gamma and neutron longitudinal profiles is studied. Furthermore, the implications of the proton beam microstructure (characterized by the proton bunch width and repetition period) are investigated. The application of a shifting TOF window having a width of ΔTOFz = 1.0 ns appears to reduce the neutron background by more than 99%. Subsequent application of an energy threshold does not appear to sharpen the distal falloff of the prompt gamma profile but reduces the tail that is observed beyond the proton range. Investigations of the influence of the beam time structure show that TOF rejection of the neutron background is expected to be effective for typical therapeutic proton cyclotrons.

MO‐G‐BRA‐06: Three‐Stage Compton Camera Image Resolution Losses Due Detector Effects

Purpose: Three‐stage Compton camera (CC) imaging systems may provide an effective means of detecting and imaging the prompt gamma (PG) radiation emitted from tissue during proton therapy. For these systems to be clinically viable, the resolution of the CC images should be better than the treatment planning margins, ∼3 mm. Detector inaccuracies in the measured gamma interaction positions and energy depositions and Doppler broadening limit the image resolution. The purpose of our study was to determine the inherent limitations of the image resolution due to these detector effects Methods: Using the Geant4 Monte Carlo software program, a three‐stage CC detecting radiation from both gamma point sources and prompt gamma radiation emitted during proton therapy was simulated. To simulate the effects of the detector energy and spatial resolutions an independent, identically distributed random uncertainty term was applied to the MC calculated energy depositions and interaction positions. Doppler broadening was modeled using Geant4. The effects were then measured using the distance of closest approach and the point of closest approach. Results: The major effect of Doppler broadening was to increase the size of the tails of the spatial distributions. Its effect on the full width at half maximum of the point spread function was less than 2 mm. The lateral and energy resolutions of the detector stages moderately enhanced the effects of Doppler broadening, but for the tested values of detector depth resolution, no additional effect was evident. The detector effects changed the depth of the distal falloff for PG emitted during proton therapy by < 1 mm. Conclusions: The spatial and energy resolutions of three‐stage CCs are sufficient for PG imaging during proton therapy. However, because the detector effects render many of the detected gammas unusable for reconstruction, the overall efficiency of the CC is reduced. This work was supported by the National Institutes of Health through award number R21CA137362 from the National Cancer Institute.

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, requiring medical physicists to add safety margins on the penetration depth of the beam. 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 feasibility, development and test of a new concept of prompt gamma camera using a slit collimator to obtain a one-dimensional projection of the beam path on a scintillation detector. This concept was optimized, using the Monte Carlo code MCNPX version 2.5.0, to select high energy photons correlated with the beam range and detect them with both high statistics and sufficient spatial resolution. To validate the Monte Carlo model, spectrometry measurements of secondary particles emitted by a PMMA target during proton irradiation at 160 MeV were realized. An excellent agreement with the simulations was observed when using subtraction methods to isolate the gammas in direct incidence. A first prototype slit camera using the HiCam gamma detector was consequently prepared and tested successfully at 100 and 160 MeV beam energies. Results confirmed the potential of this concept for real-time range monitoring with millimetre accuracy in pencil beam scanning mode for typical clinical conditions. If we neglect electronic dead times and rejection of detected events, the current solution with its collimator at 15 cm from the beam axis can achieve a 1–2 mm standard deviation on range estimation in a homogeneous PMMA target for numbers of protons that correspond to doses in water at the Bragg peak as low as 15 cGy at 100 MeV and 25 cGy at 160 MeV assuming pencil beams with a Gaussian profile of 5 mm sigma at target entrance.

Front-end multi-channel PMT-associated readout chip for hodoscope application

For developing a prompt gamma imaging system, we have designed a 16-channel readout chip in a BiCMOS process to be associated with multi-anode photomultipliers (MaPMTs). Each channel has one current input and two separated outputs. The input has very low impedance to minimize electrical crosstalk and effects of capacitances. The two outputs serve to respectively detect signal event and quantify signal charge. The channel architecture is a current-mode one, employing a current conveyor to drive both a buffered current comparator and a charge-sensitive amplifier (CSA). The current conveyor is built with super-common-base (SCB) transistor structures to obtain input impedance in the order of a few ohms. Circuit design with the use of bipolar transistor components also improves frequency and noise performances. The chip has been tested and the evaluated characteristics meet the system requirements.

TH‐C‐BRB‐06: Comparison of Prompt Gamma and PET Imaging for Range Verification in Proton Therapy

Purpose: To compare the clinical adaptability of prompt gamma (PG) imaging and positron emission tomography (PET) as in‐vivo treatment verification tools in proton radiation therapy. Methods: In order to study the proton induced photon emission process in a patient during proton therapy, Monte Carlo simulations were performed using the GEANT4 toolkit. A thorough investigation of several different tumor sites such as head and neck, prostate, spine and abdomen, irradiated by passive scattered and pencil fields was performed. The calculated dose, PG rays and PET isotopes with decay times between 2 and 20 minutes were scored on the CT grid. The acquisition time delay and biological washout were implemented in the PET distribution post simulation. The correlation between dose and PG/PET was analyzed along lines parallel to the beam direction spaced about 2 mm apart from each other. Results: For all subjects, the PG method showed approximately 10 times higher gamma production rate than the PET method before, and 60 to 80 times higher production after including the washout and time corrections. This rate was directly proportional to the tissue density. For passive fields, the correlation between dose and PG/PET was strongly dependent on the location of the line profile within the field. This dependence can be related to the tissue heterogeneity and the position of the distal falloff within the patient. For pencil beams, the distances between both PG/PET and dose falloffs were consistent and therefore well correlated. This was mainly due to the smaller beam size and better‐defined distal falloffs. Conclusions: PG imaging was found more appropriate than PET for pencil beam scanning due to in‐situ capabilities and reduced limitations and uncertainties. However, current detector technology does not allow full utilization of this method, while PET range verification has already been studied clinically.

TH‐C‐BRB‐03: Reconstructing Images of Prompt Gammas Emitted during Proton Radiotherapy

Purpose: Recent studies have shown that prompt gammas emitted during proton therapy are strongly correlated to the distribution of dose and proton beam range in the patient. This has led to an increasing interest in the measurement of prompt gammas as a means of providing in‐vivo beam range verification. The purpose of this work was to investigate the feasibility of reconstructing images of prompt gamma emission from tissues irradiated during proton radiotherapy. Methods: For this study we developed an iterative image reconstruction program based on the Stochastic Origin Ensembles (SOE) technique that used data calculated using a Monte Carlo (MC) model of prompt gamma measurement by a 3‐stage Compton camera during proton irradiation. We studied the ability of the SOE technique to reconstruct 2D and 3D images that accurately reproduced the prompt gamma distribution in irradiated tissue phantoms as a function of image pixel size, number of measured prompt gammas, and number of iterations performed by the reconstruction program. Results: Using data from our MC model, we were able to reconstruct images of the prompt gamma emission profile in tissue phantoms irradiated with a clinical proton beams (pencil beam, broad beam, and spread out Bragg peak). The beam range in the reconstructed images agreed with the prompt emission range to within 1–2 mm for reconstructions with 1 mm pixel size, 10,000 iterations, and using 1.5 million measured prompt gammas. Conclusions: We conclude that it is possible to reconstruct both 2D and 3D images of prompt gamma ray emission during proton radiotherapy. Although, the reconstruction algorithm used in this study is not optimal and much more development is needed, we believe the image resolution (1–2 mm) achieved in this study shows the feasibility of prompt gamma imaging (PGI) as clinical technique for verifying in‐vivo dose range for proton radiotherapy.This work was supported by award number R21CA137362 from the National Cancer Institute.

SU‐E‐T‐349: Evaluating a Stochastic Reconstruction Algorithm for Compton Camera Based Prompt Gamma Imaging during Proton Therapy

Purpose: Prompt gamma photons emitted during proton radiotherapy may provide a method to measure dose delivery and range. The utility of this method depends on how well the gammas can be detected and reconstructed. The purpose of this study is to demonstrate the feasibility of the stochastic origin ensemble (SOE) algorithm for reconstructing images of the origins of prompt gammas emitted during proton therapy and to introduce a metric for quantifying the accuracy of this image. Methods: Using Geant4 we simulated a Compton camera detecting prompt gamma emission from proton irradiated tissue phantoms. The information recorded by the simulated detector is used as the input to the SOE reconstruction algorithm, which iteratively determines the likely gamma emission origin. We introduce a metric to quantify the accuracy of the reconstruction and use the metric to provide a quantitative analysis of the SOE reconstructions. Results: We show both qualitatively and quantitatively that the SOE algorithm can reconstruct images which accurately predict the origins of prompt gammas emitted during proton therapy. We also show that the distal falloff of gamma production based on the reconstructed image agrees with the distal falloff in Monte Carlo calculated gamma origin distribution to within 1 mm for both the 50% and 20% regions. Conclusions: The SOE algorithm was able to reconstruct usable images of prompt gamma emission during proton irradiation. We believe these results justify further investigation of this technique for in‐vivo prompt gamma imaging.This work was supported by Award Number R21CA137362 from the National Cancer Institute.

Monte Carlo patient study on the comparison of prompt gamma and PET imaging for range verification in proton therapy

The purpose of this work was to compare the clinical adaptation of prompt gamma (PG) imaging and positron emission tomography (PET) as independent tools for non-invasive proton beam range verification and treatment validation. The PG range correlation and its differences with PET have been modeled for the first time in a highly heterogeneous tissue environment, using different field sizes and configurations. Four patients with different tumor locations (head and neck, prostate, spine and abdomen) were chosen to compare the site-specific behaviors of the PG and PET images, using both passive scattered and pencil beam fields. Accurate reconstruction of dose, PG and PET distributions was achieved by using the planning computed tomography (CT) image in a validated GEANT4-based Monte Carlo code capable of modeling the treatment nozzle and patient anatomy in detail. The physical and biological washout phenomenon and decay half-lives for PET activity for the most abundant isotopes such as 11C, 15O, 13N, 30P and 38K were taken into account in the data analysis. The attenuation of the gamma signal after traversing the patient geometry and respective detection efficiencies were estimated for both methods to ensure proper comparison. The projected dose, PG and PET profiles along many lines in the beam direction were analyzed to investigate the correlation consistency across the beam width. For all subjects, the PG method showed on average approximately 10 times higher gamma production rates than the PET method before, and 60 to 80 times higher production after including the washout correction and acquisition time delay. This rate strongly depended on tissue density and elemental composition. For broad passive scattered fields, it was demonstrated that large differences exist between PG and PET signal falloff positions and the correlation with the dose distribution for different lines in the beam direction. These variations also depended on the treatment site and the particular subject. Thus, similar to PET, direct range verification with PG in passive scattering is not easily viable. However, upon development of an optimized 3D PG detector, indirect range verification by comparing measured and simulated PG distributions (currently being explored for the PET method) would be more beneficial because it can avoid the inherent biological challenges of the PET imaging. The improved correlation of PG and PET with dose when using pencil beams was evident. PG imaging was found to be potentially advantageous especially for small tumors in the presence of high tissue heterogeneities. Including the effects of detector acceptance and efficiency may hold PET superior in terms of the amplitude of the detected signal (depending on the future development of PG detection technology), but the ability to perform online measurements and avoid signal disintegration (due to washout) with PG are important factors that can outweigh the benefits of higher detection sensitivity.

SU‐GG‐J‐160: Monte Carlo Optimization of a Three‐Stage Compton Camera for Prompt Gamma Imaging and Spectroscopy: Materials and Geometry

Purpose: Prompt gamma photons emitted from biological tissues under proton irradiation carry dosimetric and spectroscopic information that can provide treatment verification and indicate biological response. We optimize the materials and geometry of a three‐stage Compton camera for prompt gamma detection and to calculate the efficiency of such a detector. Method and Materials: A three‐stage Compton camera was modeled in Geant4 as a series of parallel square detector stages. The assembly was placed 15 cm from an isotropic point source of mono‐energetic gamma‐rays. The primary energies considered were 2.33, 4.44, and 6.13 MeV, which are characteristic emission energies of Nitrogen, Carbon, and Oxygen. Certain detector arrangements were characterized over the energy range from 0–1 MeV in 0.25‐MeV increments and from 1–10 MeV in 0.5‐MeV increments. The scintillating materials evaluated in this study include germanium, BGO (Bi4Ge3O12), NaI, Xenon, and LaBr3. For each material, the thickness of each stage was optimized to produce the maximum number of relevant interactions. Results: For the primary and secondary stages, each material exhibits peak production of single Compton‐scatter events at thicknesses ranging from 2 cm for BGO to 4 cm for Xenon. Transportation efficiency, or the fraction of interacting photons that enter the next stage, decreases linearly with thickness. Peak efficiencies for the primary and secondary stages vary from 15–25% per stage, depending on energy and material. In the final stage, all interactions are used, making detection efficiency an increasing function of thickness. At 10 cm thickness, BGO detection efficiency approaches 80%, while germanium and LaBr3 exhibit 70% efficiency. Conclusion: The most important contributors to detection efficiency are thickness and material. The most efficient detector configurations are BGO‐Germanium‐BGO, with a peak efficiency of 2.25×10−3 at 2 MeV, and Germanium‐Germanium‐BGO, which is the most efficient detector below 1.5 MeV and above 4 MeV.

Experiments on the possible use of the 2.31 MeV gamma ray of 71 second 14O for activation analysis of oxygen

Secondary radiation emission induced by nuclear reactions is correlated to the path of ions in matter. Therefore, such penetrating radiation can be used for in vivo control of hadrontherapy treatments, for which the primary beam is absorbed inside the patient. Among secondary radiations, prompt-gamma rays were proposed for real-time verification of ion range. Such a verification is a desired condition to reduce uncertainties in treatment planning. For more than a decade, efforts have been undertaken worldwide to promote prompt-gamma-based devices to be used in clinical conditions. Dedicated cameras are necessary to overcome the challenges of a broad- and high-energy distribution, a large background, high instantaneous count rates, and compatibility constraints with patient irradiation. Several types of prompt-gamma imaging devices have been proposed, that are either physically-collimated or electronically collimated (Compton cameras). Clinical tests are now undergoing. Meanwhile, other methods than direct prompt-gamma imaging were proposed, that are based on specific counting using either time-of-flight or photon energy measurements. In the present article, we make a review and discuss the state of the art for all techniques using prompt-gamma detection to improve the quality assurance in hadrontherapy.