Prompt Gamma Profiles Research Articles

An experimental validation of a filtering approach for prompt gamma prediction in a research proton treatment planning system

Objective. Prompt gamma (PG) radiation generated from nuclear reactions between protons and tissue nuclei can be employed for range verification in proton therapy. A typical clinical workflow for PG range verification compares the detected PG profile with a predicted one. Recently, a novel analytical PG prediction algorithm based on the so-called filtering formalism has been proposed and implemented in a research version of RayStation (RaySearch Laboratories AB), which is a widely adopted treatment planning system. This work validates the performance of the filtering PG prediction approach. Approach. The said algorithm is validated against experimental data and benchmarked with another well-established PG prediction algorithm implemented in a MATLAB-based software REGGUI. Furthermore, a new workflow based on several PG profile quality criteria and analytical methods is proposed for data selection. The workflow also calculates sensitivity and specificity information, which can help practitioners to decide on irradiation course interruption during treatment and monitor spot selection at the treatment planning stage. With the proposed workflow, the comparison can be performed on a limited number of selected high-quality irradiation spots without neighbouring-spot aggregation. Main results. The mean shifts between the experimental data and the predicted PG detection (PGD) profiles (ΔPGD) by the two algorithms are estimated to be 1.5±2.1 mm and −0.6±2.2 mm for the filtering and REGGUI prediction methods, respectively. The ΔPGD difference between two algorithms is observed to be consistent with the beam model difference within uncertainty. However, the filtering approach requires a much shorter computation time compared to the REGGUI approach. Significance. The novel filtering approach is successfully validated against experimental data and another widely used PG prediction algorithm. The workflow designed in this work selects spots with high-quality PGD shift calculation results, and performs sensitivity and specificity analyses to assist clinical decisions.

Development of focusing multi-knife-edge camera for proton beam range verification: A Monte Carlo study

Proton range uncertainties limit the ability to make full clinical use of the sharp distal falloff of the proton beam. To overcome this limitation, prompt gamma (PG) imaging system with mechanical collimation can be used in non-invasive proton beam range verification. In this study, we developed a focusing multi-knife-edge collimator to improve the counting efficiency and accuracy in detecting proton beam range. Unlike the common design of a single-knife-edge slit (SKE) collimator, the multi-knife-edge slit (MKE) collimator consists of three slits and focuses on the target detection area. To compare the performances of these two collimator designs, Monte Carlo simulation was unitized to model the cameras for measuring the PGs during proton irradiation. The simulation was performed using the GATE v9.1. The setup included a pencil beam of 100 MeV, 160 MeV, and 230 MV protons incident upon a cylindrical PMMA phantom with three different proton numbers of 107, 108, and 109. The detection PG profiles were fitted with a 3-line-segment fitting method to estimate the range of proton beams. Both cameras were aimed at the expected range depth for the corresponding proton beam energies. Experiments for range shifts were conducted by shifting the phantom between −20 mm and 20 mm along the beam axis. Results demonstrate that the MKE camera exhibits approximately twice the efficiency of the SKE camera in detecting PGs. In range shift retrieval, with the delivery of 108 protons, the MKE camera achieves submillimeter accuracy (except for incident protons at 230 MeV), while the SKE camera achieves accuracy within 2 mm. In conclusion, this study assesses the feasibility of the MKE camera and illustrates its potential application for range shift detection, contributing to precise range monitoring in proton therapy.

Analytical modeling and Monte Carlo simulations of multi-parallel slit and knife-edge slit prompt gamma cameras

Purpose. Present and validate an analytical model (AM) to calculate efficiency and spatial resolution of multi-parallel slit (MPS) and knife-edge slit (KES) cameras in the context of prompt gamma (PG) imaging in proton therapy, as well as perform a fair comparison between two prototypes of these cameras with their design specifications. Materials and methods. Monte Carlo (MC) simulations with perfect (ideal) conditions were performed to validate the proposed AM, as well as simulations in realistic conditions for the comparison of both prototypes. The spatial resolution obtained from simulations was derived from reconstructed PG profiles. The falloff retrieval precision (FRP) was quantified based on the variability of PG profiles from 50 different realizations. Results. The AM shows that KES and MPS designs fulfilling ‘MPS-KES similar conditions’ should have very close actual performances if the KES slit width corresponds to the half of the MPS slit width. Reconstructed PG profiles from simulated data with both cameras were used to compute the efficiency and spatial resolutions to compare against the model predictions. The FRP of both cameras was calculated with realistic detection conditions for beams with 107, 108 and 109 incident protons. A good agreement was found between the values predicted by the AM and those obtained from MC simulations (relative deviations of the order of 5%). Conclusion. The MPS camera outperforms the KES camera with their design specifications in realistic conditions and both systems can reach millimetric precision in the determination of the falloff position with 108 or more initial protons.

A high sensitivity Cherenkov detector for prompt gamma timing and time imaging

We recently proposed a new approach for the real-time monitoring of particle therapy treatments with the goal of achieving high sensitivities on the particle range measurement already at limited counting statistics. This method extends the Prompt Gamma (PG) timing technique to obtain the PG vertex distribution from the exclusive measurement of particle Time-Of-Flight (TOF). It was previously shown, through Monte Carlo simulation, that an original data reconstruction algorithm (Prompt Gamma Time Imaging) allows to combine the response of multiple detectors placed around the target. The sensitivity of this technique depends on both the system time resolution and the beam intensity. At reduced intensities (Single Proton Regime—SPR), a millimetric proton range sensitivity can be achieved, provided the overall PG plus proton TOF can be measured with a 235 ps (FWHM) time resolution. At nominal beam intensities, a sensitivity of a few mm can still be obtained by increasing the number of incident protons included in the monitoring procedure. In this work we focus on the experimental feasibility of PGTI in SPR through the development of a multi-channel, Cherenkov-based PG detector with a targeted time resolution of 235 ps (FWHM): the TOF Imaging ARrAy (TIARA). Since PG emission is a rare phenomenon, TIARA design is led by the concomitant optimisation of its detection efficiency and Signal to Noise Ratio (SNR). The PG module that we developed is composed of a small PbF_{2} crystal coupled to a silicon photoMultiplier to provide the time stamp of the PG. This module is currently read in time coincidence with a diamond-based beam monitor placed upstream the target/patient to measure the proton time of arrival. TIARA will be eventually composed of 30 identical modules uniformly arranged around the target. The absence of a collimation system and the use of Cherenkov radiators are both crucial to increase the detection efficiency and the SNR, respectively. A first prototype of the TIARA block detector was tested with 63 MeV protons delivered from a cyclotron: a time resolution of 276 ps (FWHM) was obtained, resulting in a proton range sensitivity of 4 mm at 2sigma with the acquisition of only 600 PGs. A second prototype was also evaluated with 148 MeV protons delivered from a synchro-cyclotron obtaining a time resolution below 167 ps (FWHM) for the gamma detector. Moreover, using two identical PG modules, it was shown that a uniform sensitivity on the PG profiles would be achievable by combining the response of gamma detectors uniformly distributed around the target. This work provides the experimental proof-of-concept for the development of a high sensitivity detector that can be used to monitor particle therapy treatments and potentially act in real-time if the irradiation does not comply to treatment plan.

Prompt gamma imaging for the identification of regional proton range deviations due to anatomic change in a heterogeneous region.

Prompt gamma (PG) imaging has previously been demonstrated for use in proton range verification of a brain treatment with a homogeneous target region. In this study, the feasibility of PG imaging to detect anatomic change within a heterogeneous region is presented. A prompt gamma camera recorded several fractions of a patient treatment to the base of skull. An evaluation CT revealed a decrease in sinus cavity filling during the treatment course. Comparison of PG profiles between measurement and simulation was performed to investigate range variations between planned and measured pencil beam spot positions. For one field, an average over range of 3 mm due to the anatomic change could be detected for a subset of spots traversing the sinus cavity region. The two other fields appeared less impacted by the change but predicted range variations could not be detected. These results were partially consistent with the simulations of the evaluation CT. We report the first clinical application of PG imaging that detected some of the expected small regional proton range deviations due to anatomic change in a heterogeneous region. However, several limitations exist with the technology that may limit its sensitivity to detect range deviations in heterogeneous regions. We report on the first detection of range variations due to anatomic change in a heterogeneous region using PGI. The results confirm the feasibility of using PG-based range verification in highly heterogeneous target regions to identify deviations from the treatment plan.

Estimation of respiratory phases during proton radiotherapy from a 4D-CT and Prompt gamma detection profiles

Proton radiotherapy has a potential to provide an effective cancer treatment while sparing greater volume of healthy tissue than the conventional X-ray based radiotherapy. However, in lungs this potential is hindered by motion due to breathing. An important quantity in treatment verification is the correlation between the respiratory phases (RP) and the timing of pencil beam scanning (PBS). In this note, we demonstrate how the RP can be estimated using Prompt gamma (PG) detection profiles collected during a treatment. We utilized a 4D-CT of a patient with lung cancer, a treatment plan and a PG simulator. The treatment plan consisted of ten layers corresponding to ten proton energies. The RPs of the 4D-CT were interpolated using a deformable registration algorithm, so as to have fifty RPs in total. Deviations from regular breathing were introduced via time dependent frequency modulation. Fifty unique breathing patterns were generated, for which PG profiles were simulated for each pencil beam. Poisson noise was added to each PG profile to account for photon statistics. The RPs were estimated by comparing the PG profiles with and without Poisson noise via three different methods: the RP associated with each layer was estimated 1) independently of the other layers, 2) using a linear correlation between the layers, and 3) using a quadratic correlation between the layers. The best model, the quadratic model, yielded an average error in RP estimation relative to the breathing period of 5% of the breathing period or less with a 90% confidence interval.

Correlations between the shifts in prompt gamma emission profiles and the changes in daily target coverage during simulated pencil beam scanning proton therapy

The aim of this study was to investigate the feasibility of using prompt gamma (PG) ray emission profiles to monitor changes in dose to the planning target volume (PTV) during pencil beam scanning (PBS) proton therapy as a result of day-to-day variation in patient anatomy.For 11 prostate patients, we simulated treatment plan delivery using the patients’ daily anatomy as observed in the planning CT and 7–9 control CT scans, including the detected PG profiles resulting from the 5%, 10%, and 20% most intense proton pencil beams. For each patient, we determined the changes in dosimetric parameters for the high- and low-dose PTVs between the simulations performed using the planning CT scan and the different control CT scans and correlated these to changes in the PG emission profiles.Changes in coverage of the high- and low-dose PTV correlated most strongly with the median and mean absolute PG emission profile shifts of the 5% most intense pencil beams, respectively. With a mean Pearson correlation coefficient of −0.76 (SD: 0.17) for the high-dose PTV and of −0.60 (SD: 0.51) for the low-dose PTV.We showed, as a proof of principle, that PG emission profiles obtained during PBS proton therapy could be used to detect changes in PTV coverage due to day-to-day anatomical variation.

Correction of Geometrical Effects of a Knife-Edge Slit Camera for Prompt Gamma-Based Range Verification in Proton Therapy

Prompt gamma (PG) based range verification can potentially reduce the safety margins in proton therapy. A knife-edge slit camera has been developed in this context using analytical PG simulations as reference for absolute range verification during patient treatment. Geometrical deviations between measurement and simulation could be observed and have to be corrected for in order to improve the range retrieval of the system. A geometrical correction model is derived from Monte Carlo simulations in water. The influence of different parameters is tested and the model is validated in a dedicated benchmark experiment. We found that the geometrical correction improves the agreement between measured and simulated PG profiles resulting in an improved range retrieval and higher accuracy for absolute range verification. An intrinsic offset of 1.4 mm between measurement and simulation is observed in the experimental data and corrected in the PG simulation. In summary, the absolute range verification capabilities of a PG camera have been improved by applying a geometrical correction model.

Toward a new treatment planning approach accounting for in vivo proton range verification

Protons with modern pencil-beam scanning delivery are widely used in state-of-the-art radiotherapy. To reduce the unwanted effect of proton range uncertainties, prompt gamma (PG) monitoring is investigated and considered one of the most promising methods for real-time, in vivo range verification. Despite good correlation between the penetration depth of the PG signal and proton range in most cases, mismatch can occur especially because of tissue heterogeneities. Moreover, detectability and reproducibility of the prompt gamma signal critically depends on counting statistics. Nowadays, conventional treatment planning systems do not account for the degree of correlation between dose and PG signal nor the expected PG signal counting statistics, which considerably influences the possibility of a reliable verification of the intended beam range. Hence, in this project, we investigate a new treatment planning approach, in which the spot-by-spot conformities between PG and dose profiles (PG-dose correlation) as well as PG signal detectability and precision are taken into account based on a TPS optimizer. To investigate the feasibility of this idea, a research computational platform, combining Monte Carlo (MC, Geant4) pre-calculated pencil beams with the analytical Matlab-based TPS engine CERR, is used for treatment planning. Geant4 is employed for realistic simulation of the dose delivery and PG generation of all spots in the heterogeneous patient anatomy given by CT images. First of all, a treatment plan is created using a charged particle extension of CERR. Secondly, the PG fall-off positions of all individual pencil beams are evaluated and compared to the 80% distal dose fall-off positions. Thirdly, the PG-dose correlations of all spots are quantified. A new plan, in which a few spots with the best PG-dose correlation are boosted to ensure PG detectability with good precision, is then made. Finally, the optimized plan is fully recalculated on the same patient CT using Geant4, and the result is evaluated considering both plan quality and beam range monitorability. The evaluation shows that the re-optimized treatment plan is comparable to the initial plan in terms of dose distribution, dose averaged LET distribution and robustness, while fulfilling the set statistical conditions for reliable PG monitoring of the few automatically or manually selected spots. The method could thus complement, and for the selected pencil beams even overcome limitations of, alternative suggested approach such as pencil beam aggregation to provide sufficient counting statistics for precise PG range retrieval with good correlation to the treatment dose.

Neural Network-Based Range Verification for Proton Therapy Based on Prompt Gamma Emissions

Online dose monitoring in proton therapy is currently being investigated with prompt-gamma detection, which is correlated with proton range and dose deposition. We propose a novel approach based on neural network. Neural networks are very good at pattern recognition and it is able to classify any data with arbitrary accuracy given enough neurons. In our study, the features are selected to be the differences of prompt gamma profiles for different proton energies and Bragg peaks. Simulations were carried out with a clinical spot-scanning proton therapy treatment using the Geant4 V9.4p01 toolkit. The proton beams tested were mono-energetic beams at 130 MeV, 125 MeV, 120 MeV and 115 MeV. A water phantom had a dimension of 10x10x30 cm3 and a voxels size of 1x1x1 mm3. The method implemented a neural network classification model comprising 2 layers and 10 neurons. The classifier’s performances were evaluated using confusion matrix and receiver operating characteristic curves (ROCs). In total, 50 data sets were generated for each proton energy (70% training, 30% testing). Two important parameters were evaluated: kernel size (to model the response of each sensing point to emissions at different depths) and detection efficiency (to model the counting statistics and sensors). Without any kernel and detection model applied, the classifier is able to classify three proton energies/ranges with 100% accuracy indicating their strong correlations with the prompt gamma profiles. No significant difference is found among different kernel sizes ranging between 1 mm and 16 mm, which relives our burden in optimizing the location of sensors as well as collimators in-between sensors in order to maintain spatial information of dose profiles. In addition, the accuracy of the classifier is found to be dependent on the detection efficiency. For a kernel size of 8 mm, the accuracy is found to be ∼78% for a detection efficiency of ∼0.1%. We believe that such efficiency is achievable for a clinical proton therapy system, relying on a number of physical factors such as beam intensity, irradiation time, the design of radiation detectors and collimators. The feasibility of proton range verification using neural network was proved. The proposed method will allow us to develop a cost-effective online range/dose verification system for proton therapy with a limited number of sensors located at different depths. If successful, this novel system may also have potential to help improve the accuracy of patient positioning and beam condition, and ultimately allows for dose-volume adapted proton therapy.

SU‐F‐J‐208: Prompt Gamma Imaging‐Based Prediction of Bragg Peak Position for Realistic Treatment Error Scenarios

Purpose:To quantify the accuracy in predicting the Bragg peak position using simulated in‐room measurements of prompt gamma (PG) emissions for realistic treatment error scenarios that combine several sources of errors.Methods:Prompt gamma measurements by a knife‐edge slit camera were simulated using an experimentally validated analytical simulation tool. Simulations were performed, for 143 treatment error scenarios, on an anthropomorphic phantom and a pencil beam scanning plan for nasal cavity. Three types of errors were considered: translation along each axis, rotation around each axis, and CT‐calibration errors with magnitude ranging respectively, between −3 and 3 mm, −5 and 5 degrees, and between −5 and +5%. We investigated the correlation between the Bragg peak (BP) shift and the horizontal shift of PG profiles. The shifts were calculated between the planned (reference) position and the position by the error scenario. The prediction error for one spot was calculated as the absolute difference between the PG profile shift and the BP shift.Results:The PG shift was significantly and strongly correlated with the BP shift for 92% of the cases (p<0.0001, Pearson correlation coefficient R>0.8). Moderate but significant correlations were obtained for all cases that considered only CT‐calibration errors and for 1 case that combined translation and CT‐errors (p<0.0001, R ranged between 0.61 and 0.8). The average prediction errors for the simulated scenarios ranged between 0.08±0.07 and 1.67±1.3 mm (grand mean 0.66±0.76 mm). The prediction error was moderately correlated with the value of the BP shift (p=0, R=0.64). For the simulated scenarios the average BP shift ranged between −8±6.5 mm and 3±1.1 mm. Scenarios that considered combinations of the largest treatment errors were associated with large BP shifts.Conclusion:Simulations of in‐room measurements demonstrate that prompt gamma profiles provide reliable estimation of the Bragg peak position for complex error scenarios.Yafei Xing and Luiza Bondar are funded by BEWARE grants from the Walloon Region.The work presents simulations results for a prompt gamma camera prototype developed by IBA.

A comparison of two prompt gamma imaging techniques with collimator-based cameras for range verification in proton therapy

In vivo range verification plays an important role in proton therapy to fully utilize the benefits of the Bragg peak (BP) for delivering high radiation dose to tumor, while sparing the normal tissue. For accurately locating the position of BP, camera equipped with collimators (multi-slit and knife-edge collimator) to image prompt gamma (PG) emitted along the proton tracks in the patient have been proposed for range verification. The aim of the work is to compare the performance of multi-slit collimator and knife-edge collimator for non-invasive proton beam range verification. PG imaging was simulated by a validated GATE/GEANT4 Monte Carlo code to model the spot-scanning proton therapy and cylindrical PMMA phantom in detail. For each spot, 108 protons were simulated. To investigate the correlation between the acquired PG profile and the proton range, the falloff regions of PG profiles were fitted with a 3-line-segment curve function as the range estimate. Factors including the energy window setting, proton energy, phantom size, and phantom shift that may influence the accuracy of detecting range were studied. Results indicated that both collimator systems achieve reasonable accuracy and good response to the phantom shift. The accuracy of range predicted by multi-slit collimator system is less affected by the proton energy, while knife-edge collimator system can achieve higher detection efficiency that lead to a smaller deviation in predicting range. We conclude that both collimator systems have potentials for accurately range monitoring in proton therapy. It is noted that neutron contamination has a marked impact on range prediction of the two systems, especially in multi-slit system. Therefore, a neutron reduction technique for improving the accuracy of range verification of proton therapy is needed.

Assessment of Geant4 Prompt-Gamma Emission Yields in the Context of Proton Therapy Monitoring.

Monte Carlo tools have been long used to assist the research and development of solutions for proton therapy monitoring. The present work focuses on the prompt-gamma emission yields by comparing experimental data with the outcomes of the current version of Geant4 using all applicable proton inelastic models. For the case in study and using the binary cascade model, it was found that Geant4 overestimates the prompt-gamma emission yields by 40.2 ± 0.3%, even though it predicts the prompt-gamma profile length of the experimental profile accurately. In addition, the default implementations of all proton inelastic models show an overestimation in the number of prompt gammas emitted. Finally, a set of built-in options and physically sound Geant4 source code changes have been tested in order to try to improve the discrepancy observed. A satisfactory agreement was found when using the QMD model with a wave packet width equal to 1.3 fm2.

First clinical application of a prompt gamma based in vivo proton range verification system

Background and purposeTo improve precision of particle therapy, in vivo range verification is highly desirable. Methods based on prompt gamma rays emitted during treatment seem promising but have not yet been applied clinically. Here we report on the worldwide first clinical application of prompt gamma imaging (PGI) based range verification. Material and methodsA prototype of a knife-edge shaped slit camera was used to measure the prompt gamma ray depth distribution during a proton treatment of a head and neck tumor for seven consecutive fractions. Inter-fractional variations of the prompt gamma profile were evaluated. For three fractions, in-room control CTs were acquired and evaluated for dose relevant changes. ResultsThe measurement of PGI profiles during proton treatment was successful. Based on the PGI information, inter-fractional global range variations were in the range of ±2mm for all evaluated fractions. This is in agreement with the control CT evaluation showing negligible range variations of about 1.5mm. ConclusionsFor the first time, range verification based on prompt gamma imaging was applied for a clinical proton treatment. With the translation from basic physics experiments into clinical operation, the potential to improve the precision of particle therapy with this technique has increased considerably.

Detection of mixed-range proton pencil beams with a prompt gamma slit camera

With increasing availability of proton and particle therapy centers for tumor treatment, the need for in vivo range verification methods comes more into the focus. Imaging of prompt gamma rays emitted during the treatment is one of the possibilities currently under investigation. A knife-edge shaped slit camera was recently proposed for this task and measurements proved the feasibility of range deviation detection in homogeneous and inhomogeneous targets. In the present paper, we concentrate on laterally inhomogeneous materials, which lead to range mixing situations when crossed by one pencil beam: different sections of the beam have different ranges. We chose exemplative cases from clinical irradiation and assembled idealized tissue equivalent targets. One-dimensional emission profiles were obtained by measuring the prompt gamma emission with the slit camera. It could be shown that the resulting range deviations can be detected by evaluation of the measured data with a previously developed range deviation detection algorithm. The retrieved value, however, strongly depends on the target composition, and is not necessarily in direct relation to the ranges of both parts of the beam. By combining the range deviation detection with an analysis of the slope of the distal edge of the measured prompt gamma profile, the origin of the detected range deviation, i.e. the mixed range of the beam, is also identified. It could be demonstrated that range mixed prompt gamma profiles exhibit less steep distal slopes than profiles from beams traversing laterally homogeneous material. For future application of the slit camera to patient irradiation with double scattered proton beams, situations similar to the range mixing cases are present and results could possibly apply.

Monte Carlo study on the sensitivity of prompt gamma imaging to proton range variations due to interfractional changes in prostate cancer patients

Proton range verification based on prompt gamma imaging is increasingly considered in proton therapy. Tissue heterogeneity normal to the beam direction or near the end of range may considerably degrade the ability of prompt gamma imaging to detect proton range shifts. The goal of this study was to systematically investigate the accuracy and precision of range detection from prompt gamma emission profiles for various fractions for intensity modulated proton therapy of prostate cancer, using a comprehensive clinical dataset of 15 different CT scans for 5 patients.Monte Carlo simulations using Geant4 were performed to generate spot-by-spot dose distributions and prompt gamma emission profiles for prostate treatment plans. The prompt gammas were scored at their point of emission. Three CT scans of the same patient were used to evaluate the impact of inter-fractional changes on proton range. The range shifts deduced from the comparison of prompt gamma emission profiles in the planning CT and subsequent CTs were then correlated to the corresponding range shifts deduced from the dose distributions for individual pencil beams. The distributions of range shift differences between prompt gamma and dose were evaluated in terms of precision (defined as half the 95% inter-percentile range IPR) and accuracy (median). In total about 1700 individual proton pencil beams were investigated.The IPR of the relative range shift differences between the dose profiles and the prompt gamma profiles varied between ±1.4 mm and ±2.9 mm when using the more robust profile shifting analysis. The median was found smaller than 1 mm. Methods to identify and reject unreliable spots for range verification due to range mixing were derived and resulted in an average 10% spot rejection, clearly improving the prompt gamma–dose correlation.This work supports that prompt gamma imaging can offer a reliable indicator of range changes due to anatomical variations and tissue heterogeneity in scanning proton treatment of prostate cancer patients when considering prompt gamma emission profiles.

Analytical computation of prompt gamma ray emission and detection for proton range verification

A prompt gamma (PG) slit camera prototype recently demonstrated that Bragg Peak position in a clinical proton scanned beam could be measured with 1–2 mm accuracy by comparing an expected PG detection profile to a measured one. The computation of the expected PG detection profile in the context of a clinical framework is challenging but must be solved before clinical implementation. Obviously, Monte Carlo methods (MC) can simulate the expected PG profile but at prohibitively long calculation times. We implemented a much faster method that is based on analytical processing of precomputed MC data that would allow practical evaluation of this range monitoring approach in clinical conditions.Reference PG emission profiles were generated with MC simulations (PENH) in targets consisting of either 12C, 14N, 16O, 31P or 40Ca, with 10% of 1H. In a given geometry, the local PG emission can then be derived by adding the contribution of each element, according to the local energy of the proton obtained by continuous slowing down approximation and the local composition. The actual incident spot size is taken into account using an optical model fitted to measurements and by super sampling the spot with several rays (up to 113). PG transport in the patient/camera geometries and the detector response are modelled by convolving the PG production profile with a transfer function. The latter is interpolated from a database of transfer functions fitted to MC data (PENELOPE) generated for a photon source in a cylindrical phantom with various radiuses and a camera placed at various positions.As a benchmark, the analytical model was compared to MC and experiments in homogeneous and heterogeneous phantoms. Comparisons with MC were also performed in a thoracic CT. For all cases, the analytical model reproduced the prediction of the position of the Bragg peak computed with MC within 1 mm for the camera in nominal configuration. When compared to measurements, the shape of the profiles was well reproduced and agreement for the estimation of the position of the Bragg peak was within 2.7 mm on average (1.4 mm standard deviation). On a non-optimized MATLAB code, computation time with the analytical model is between 0.3 to 10 s depending on the number of rays simulated per spot.The analytical model can be further used to determine which spots are the best candidates to evaluate the range in clinical conditions and eventually correct for over- and under-shoots depending on the acquired PG profiles.

Measurement of prompt gamma profiles in inhomogeneous targets with a knife-edge slit camera during proton irradiation

Proton and ion beam therapies become increasingly relevant in radiation therapy. To fully exploit the potential of this irradiation technique and to achieve maximum target volume conformality, the verification of particle ranges is highly desirable. Many research activities focus on the measurement of the spatial distributions of prompt gamma rays emitted during irradiation. A passively collimating knife-edge slit camera is a promising option to perform such measurements. In former publications, the feasibility of accurate detection of proton range shifts in homogeneous targets could be shown with such a camera. We present slit camera measurements of prompt gamma depth profiles in inhomogeneous targets. From real treatment plans and their underlying CTs, representative beam paths are selected and assembled as one-dimensional inhomogeneous targets built from tissue equivalent materials. These phantoms have been irradiated with monoenergetic proton pencil beams. The accuracy of range deviation estimation as well as the detectability of range shifts is investigated in different scenarios. In most cases, range deviations can be detected within less than 2 mm. In close vicinity to low-density regions, range detection is challenging. In particular, a minimum beam penetration depth of 7 mm beyond a cavity is required for reliable detection of a cavity filling with the present setup. Dedicated data post-processing methods may be capable of overcoming this limitation.

Technical Note: Experimental carbon ion range verification in inhomogeneous phantoms using prompt gammas.

The purpose of this study was to experimentally assess the possibility to monitor carbon ion range variations--due to tumor shift and/or elongation or shrinking--using prompt-gamma (PG) emission with inhomogeneous phantoms. Such a study is related to the development of PG monitoring techniques to be used in a carbon ion therapy context. A 95 MeV/u carbon ion beam was used to irradiate phantoms with a variable density along the ion path to mimic the presence of bone and lung in homogeneous humanlike tissue. PG profiles were obtained after a longitudinal scan of the phantoms. A setup comprising a narrow single-slit collimator and two detectors placed at 90° with respect to the beam axis was used. The time of flight technique was applied to allow the selection between PG and background events. Using the positions at 50% entrance and 50% falloff of the PG profiles, a quantity called prompt-gamma profile length (PGPL) is defined. It is possible to observe shifts in the PGPL when there are absolute ion range shifts as small as 1-2 mm. Quantitatively, for an ion range shift of -1.33 ± 0.46 mm (insertion of a Teflon slab), a PGPL difference of -1.93 ± 0.58 mm and -1.84 ± 1.27 mm is obtained using a BaF2 and a NaI(Tl) detector, respectively. In turn, when an ion range shift of 4.59 ± 0.42 mm (insertion of a lung-equivalent material slab) is considered, the difference is of 4.10 ± 0.54 and 4.39 ± 0.80 mm for the same detectors. Herein, experimental evidence of the usefulness of employing PG to monitor carbon ion range using inhomogeneous phantoms is presented. Considering the homogeneous phantom as reference, the results show that the information provided by the PG emission allows for detecting ion range shifts as small as 1-2 mm. When considering the expected PG emission from an energy slice in a carbon ion therapy scenario, the experimental setup would allow to retrieve the same PGPL as the high statistics of the full experimental dataset in 58% of the times. However, this success rate increases to 93% when using a better optimized setup by means of Monte Carlo simulations.

Prompt gamma imaging of proton pencil beams at clinical dose rate

In this work, we present experimental results of a prompt gamma camera for real-time proton beam range verification. The detection system features a pixelated Cerium doped lutetium based scintillation crystal, coupled to Silicon PhotoMultiplier arrays, read out by dedicated electronics. The prompt gamma camera uses a knife-edge slit collimator to produce a 1D projection of the beam path in the target on the scintillation detector. We designed the detector to provide high counting statistics and high photo-detection efficiency for prompt gamma rays of several MeV. The slit design favours the counting statistics and could be advantageous in terms of simplicity, reduced cost and limited footprint. We present the description of the realized gamma camera, as well as the results of the characterization of the camera itself in terms of imaging performance. We also present the results of experiments in which a polymethyl methacrylate phantom was irradiated with proton pencil beams in a proton therapy center. A tungsten slit collimator was used and prompt gamma rays were acquired in the 3–6 MeV energy range. The acquisitions were performed with the beam operated at 100 MeV, 160 MeV and 230 MeV, with beam currents at the nozzle exit of several nA. Measured prompt gamma profiles are consistent with the simulations and we reached a precision (2σ) in shift retrieval of 4 mm with 0.5 × 108, 1.4 × 108 and 3.4 × 108 protons at 100, 160 and 230 MeV, respectively. We conclude that the acquisition of prompt gamma profiles for in vivo range verification of proton beam with the developed gamma camera and a slit collimator is feasible in clinical conditions. The compact design of the camera allows its integration in a proton therapy treatment room and further studies will be undertaken to validate the use of this detection system during treatment of real patients.