Prompt Gamma Measurements Research Articles

Measuring prompt gamma-ray emissions from elements found in tissue during passive-beam proton therapy

Prompt gamma detection during proton radiotherapy for range verification purposes will need to operate in both active and passive treatment beam environments. This paper describes prompt gamma measurements using a high resolution 2″ × 2″ LaBr3 detector for a 200 MeV clinical passive-scatter proton beam. These measurements examine the most likely discrete prompt gamma rays emitted from tissue by detecting gammas produced in water, Perspex, carbon and liquid-nitrogen targets. Measurements were carried out at several positions around the depth corresponding to the location of the Bragg peak for water and Perspex targets in order to investigate prompt gamma emission as a function of depth along the beam path. This work also focused on validating the Geant4 Monte Carlo model of the passive-scatter proton beam line and LaBr3 detector by making a direct comparison between the simulated and experimental results. The initial prompt gamma measurements were overwhelmed by the high amount of scattered radiation when measuring at isocenter, shifting the target further downstream from the final collimator significantly reduced the background radiation. Prompt gamma peaks were then clearly identified for the water, Perspex and graphite targets. The developed Geant4 Monte Carlo model was able to replicate the measured prompt gamma ray energy spectra, including production for important photopeaks to within 10%, except for the 4.44 MeV peak from the water target, which had more than a 50% overestimation of the number of produced prompt gamma rays. The prompt gamma measurements at various depths correlated well with the proton dose deposition; the 4.44 and 6.13 MeV photopeak profiles peaked within 1 cm of the Bragg peak and the R50% value for the 3–7 MeV energy range predicted the proton range within 8 mm.

Photo-peak area ratios for estimation of elemental concentration in aqueous solutions using prompt gamma measurements

The use of photo-peak area ratios are investigated for quantitative estimation of elements, particularly chlorine, in aqueous solutions using neutron induced prompt gamma measurements. A ratio of prompt gamma intensities avoids the need for estimating the incident total neutron fluence and is demonstrated for chlorine concentration estimation in NaCl solutions. Monte Carlo simulation results validated with experimental measurements support the present analysis. Use of several prompt gamma intensities instead of a single one improves the accuracy of the estimated results.

Moisture effect in prompt gamma measurements from soil samples

The variation in intensity of 1.78MeV silicon, 6.13MeV oxygen, and 2.22MeV hydrogen prompt gamma rays from soil samples due to the addition of 5.1, 7.4, 9.7, 11.9 and 14.0wt% water was studied for 14MeV incident neutron beams utilizing a LaBr3:Ce gamma ray detector. The intensities of 1.78MeV and 6.13MeV gamma rays from silicon and oxygen, respectively, decreased with increasing sample moisture. The intensity of 2.22MeV hydrogen gamma rays increases with moisture. The decrease in intensity of silicon and oxygen gamma rays with moisture concentration indicates a loss of 14MeV neutron flux, while the increase in intensity of 2.22MeV gamma rays with moisture indicates an increase in thermal neutron flux due to increasing concentration of moisture. The experimental intensities of silicon, oxygen and hydrogen prompt gamma rays, measured as a function of moisture concentration in the soil samples, are in good agreement with the theoretical results obtained through Monte Carlo calculations.

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.

SU‐F‐J‐65: Prediction of Patient Setup Errors and Errors in the Calibration Curve from Prompt Gamma Proton Range Measurements

Purpose:To understand the extent to which the prompt gamma camera measurements can be used to predict the residual proton range due to setup errors and errors in the calibration curve.Methods:We generated ten variations on a default calibration curve (CC) and ten corresponding range maps (RM). Starting with the default RM, we chose a square array of N beamlets, which were then rotated by a random angle θ and shifted by a random vector s. We added a 5% distal Gaussian noise to each beamlet in order to introduce discrepancies that exist between the ranges predicted from the prompt gamma measurements and those simulated with Monte Carlo algorithms. For each RM, s, θ, along with an offset u in the CC, were optimized using a simple Euclidian distance between the default ranges and the ranges produced by the given RM.Results:The application of our method lead to the maximal overrange of 2.0mm and underrange of 0.6mm on average. Compared to the situations where s, θ, and u were ignored, these values were larger: 2.1mm and 4.3mm. In order to quantify the need for setup error corrections, we also performed computations in which u was corrected for, but s and θ were not. This yielded: 3.2mm and 3.2mm. The average computation time for 170 beamlets was 65 seconds.Conclusion:These results emphasize the necessity to correct for setup errors and the errors in the calibration curve. The simplicity and speed of our method makes it a good candidate for being implemented as a tool for in‐room adaptive therapy. This work also demonstrates that the Prompt gamma range measurements can indeed be useful in the effort to reduce range errors. Given these results, and barring further refinements, this approach is a promising step towards an adaptive proton radiotherapy.

SU‐E‐J‐142: Prompt Gamma Emission Measurements From a Passively Scattered Proton Beam On Targets Containing 16O, 12C and 14N

Purpose:To measure the prompt gamma emission from the important elements found in tissue (1 6O,1 2C and1 4N) in a clinical passive‐scatter treatment environment.Methods:The targets (composed of water, Perspex, graphite and liquid nitrogen) were irradiated with a 200 MeV passive‐scatter proton beam and the discrete prompt gamma energy spectra was detected by a high resolution 2′ × 2′ LaBr. detector. In order to reduce the high level of radiation produced by the beam line elements, the detector was surrounded by 10 cm of lead to attenuate the scattered gamma‐rays entering the detector with an extra 5 cm thick layer of lead added along the beam direction. A 10 cm thick collimator with a 5 cm × 10 cm rectangular opening was also used.Results:The prompt gamma peaks at 6.13 MeV and 4.44 MeV were clearly identified as a Result of the inelastic nuclear reaction between the protons and the 16O atoms found in the water target. The 6.13 MeV peak was 5% higher than the peak at 4.44 MeV for the water target. The 4.44 MeV peak was the only identified emission in the prompt gamma energy spectra from the graphite target (1 2C). The expected 2.313 MeV peak form the1 4N (liquid nitrogen target) was identified, but the other expected1 4N peaks could not be resolved.Conclusion:Prompt gamma measurements with a passive‐scatter proton beam are possible, but the presence of a high amount of background radiation from the patient final collimator presents a challenge at the treatment isocenter. The prominent prompt gamma peaks at 6.13 MeV and 4.44 MeV were identified from the water, Perspex and graphite targets. The prompt gammas from the liquid nitrogen target were difficult to see, but may not be significant in the in‐vivo verification process.

Neutron Based Imaging and Element-mapping at the Budapest Neutron Centre

The Budapest Neutron Centre (BNC) is a consortium of institutes to co-ordinate research activities carried out at the Budapest Research Reactor. It hosts two neutron imaging facilities (RAD and NORMA) operated by the Centre for Energy Research, Hungarian Academy of Sciences and offers access to this scientific infrastructure for the domestic and international users.The radiography station (RAD) at the thermal neutron beamline of the reactor gives a possibility to study relatively large objects by thermal neutron-, gamma- and X-ray radiography, and to benefit from the complementary features of the different radiations. RAD is being extended in 2014 with digital imaging and tomographic capabilities. The image detection is based on suitable converter screens. The static radiography and tomography images are acquired by a new, large area sCMOS camera, whereas the dynamic radiography is accomplished by a low-light-level TV camera and a frame grabber card.The NORMA facility is designed to perform neutron radiography and tomography on small samples using guided cold neutrons. Here two non-destructive techniques are coupled to determine the chemical composition and to visualize the internal structure of heterogeneous objects. The position-sensitive element analysis with prompt-gamma activation analysis (PGAA) and the imaging with neutron radiography/tomography (NR/NT) are integrated into a unique facility called NIPS-NORMA. The goal of such a combination of these methods is to save substantial beam time in the so-called NR/NT-driven PGAI (Prompt Gamma Activation Imaging) mode, in which the interesting regions are first visualized and located, and subsequently the time-consuming prompt-gamma measurements are made only where it is really needed.The paper will give an overview about the technical details of the facilities, and the latest results of selected applications from the fields of archaeometry, engineering and material science.

64: DNA damage, protein expression and migration of melanoma cells irradiated with proton beam

64: DNA damage, protein expression and migration of melanoma cells irradiated with proton beam

Two-dimensional measurement of the prompt-gamma distribution for proton dose distribution monitoring

In proton therapy, accurate monitoring of the in-vivo proton dose distribution is essential in order to deliver the planned dose to the tumor volume within a minimal safety margin. Recently, a strong correlation between the distributions of the proton dose and the prompt gammas was found, and various prompt-gamma distribution-measurement systems, including collimation-based systems, Compton cameras, knife-edge imaging systems, and ion vertex imaging systems, have been proposed. In the present study, the feasibility of proton dose distribution monitoring was tested using a two-dimensional measurement system for prompt gammas. The measurement system, developed in the present study, incorporates a vertically-aligned one-dimensional array of gamma sensors, a parallel multi-hole collimator, a precision movement system, and a digitizer- and LabVIEW-based automatic data acquisition system. A 45-MeV proton beam of 0.5 nA was delivered to a polymethyl methacrylate (PMMA) phantom, and the two-dimensional prompt-gamma distribution was measured using the developed system. The proton beam range could be quantitatively determined to within a 1.6-mm error by sigmoidal curve-fitting with the Boltzmann equation. A comparison of the prompt-gamma distribution as measured by our detection system with the proton dose distribution as measured independently by using Gafchromic EBT films positioned inside the PMMA phantom showed good agreement. Both results imply that it is, indeed, possible to confirm the patient’s proton dose distribution by using two-dimensional prompt-gamma measurements.

MO‐D‐BRC‐04: Prompt Gamma Measurements for the Verification of Dose Deposition in Proton Therapy

The proton beam radiotherapy has a merit in that the dose deposit falls off at the end of the beam range. However, mismatch of the falloff location with the prescribed dose boundary can cause adverse effects. Measurement of gamma‐rays emitted by nuclear reactions of the incident proton beam with target materials is the only method to verify the dose distribution in situ. We have investigated two different methods of measuring the prompt gamma distribution: 1) multi‐layer collimation method, which measures the 1D distribution of gamma production at the target, to assure the endpoint of the dose deposition, 2) electron‐tracking Compton camera, which can reconstruct gamma trajectories, to verify the dose distribution. The collimation method can be effectively used when the background neutron flux is low. Hence, it can be more readily applicable to the scanning method in terms of the therapy beam delivery compared to the scattering method. Secondly, the Compton camera is a powerful tool to image the prompt gamma distribution. A preliminary experiment has been carried out with a water phantom at the beam energy of 150 MeV, and images of prompt gamma generation are attained. The clinical application could be considered with further improvement of detection efficiency.This lecture will review the uses of multi‐layer shielding and the Compton camera in the measurement of prompt gamma‐rays emitted from the target to verify the dose deposition during proton therapy.Learning Objectives:1. Understand the physics of dose deposition by the proton therapy2. Understand multi‐layer shielding techniques to measure the prompt gamma distribution for the verification of the proton dose.3. Understand the use of an electron‐tracking Compton camera for imaging the prompt gamma generation by a proton beam.

Dose profile monitoring with carbon ions by means of prompt-gamma measurements

A key point in the quality control of ion therapy is real-time monitoring and imaging of the dose delivered to the patient. Among the possible signals that can be used to make such a monitoring, prompt gamma-rays issued from nuclear fragmentation are possible candidates, provided the correlation between the emission profile and the primary beam range can be established. By means of simultaneous energy and time-of-flight discrimination, we could measure the longitudinal profile of the prompt gamma-rays emitted by 73 MeV/u carbon ions stopping inside a PMMA target. This technique allowed us to minimize the shielding against neutrons and scattered gamma rays, and to find a good correlation between the prompt-gamma profile and the ion range. This profile was studied as a function of the observation angle. By extrapolating our results to higher energies and realistic detection efficiencies, we showed that prompt gamma-ray measurements make it feasible to control in real time the longitudinal dose during ion therapy treatments.

Prompt gamma measurements for locating the dose falloff region in the proton therapy

The location of the distal falloff in the proton therapy is an important but often uncertain parameter as different tissue elements are traversed by the beam. A multilayered collimator system has been constructed as a practical means to locate the dose ends by measuring prompt gammas. The collimator is designed to moderate and capture fast neutrons and to prevent unwanted gammas from reaching the scintillation detector. The system has been studied using Monte Carlo technique and has been tested in the beam energy range of 100–200MeV. Measurements clearly indicated correlations between the gamma distributions and the distal falloff regions.

The fixed point formulation for large sample PGNAA—Part 2: experimental demonstration

We present composition estimation results using fixed point iteration compared to the true composition of sample for prompt gamma measurements. The analysis showed that the fixed point iteration algorithm converges very rapidly to true composition of the sample independent of the initial guess when there is no significant background contribution. Even in the presence of significant background contribution, the true composition of the sample was estimated with high precision but with slower convergence. Although approximate data for neutron source spectrum and relative efficiency of the detector was used for analysis, the results showed that the method is not very sensitive to the details of the model since it is based on ratios. Despite the approximate data used for computations, the composition estimation results are in excellent agreement with chemical analysis.

The fixed point formulation for large sample PGNAA—Part 1: theory

The determination of large sample composition via prompt gamma measurements is examined as a non-linear inverse problem. We show that this non-linear problem can be formulated as a fixed point problem that always has a physically meaningful solution, even in the presence of significant contributions to photopeak area from gammas emitted by the surroundings. The formulation involves only ratios of measured photopeak areas, and, separately, ratios of modeled photopeak areas. It therefore does not require the absolute comparison of measured or modeled quantities. The proof of the existence of meaningful solutions relies on very simple and natural hypotheses of positivity and continuity. The natural fixed point iteration is examined, and certain physical limits where its global convergence can be guaranteed are examined. Several computational examples are presented.

Cold neutron microprobe for materials analysis using tapered capillary optics

A prototype monolithic capillary lens for focusing neutrons produced by thermally drawing straight multicapillary bundles has been characterized with cold neutrons, and gives an intensity gain of a factor of 25 at a focal distance of 8 mm, over the focal spot area of width 87 μm. This is over an order of magnitude smaller in area than for the multifiber capillary lens. The spatial resolution available with the lens has been tested with prompt gamma measurements on slivers of dysprosium. Background problems that can affect the spatial resolution of measurements taken at the focal position of the lens are addressed. The boron glass of the tapered monolithic lens provides good shielding from unfocused neutrons in the vicinity of the lens focus.

A polycapillary bending and focusing lens for neutrons

A glass polycapillary lens that both bends and focuses a cold neutron beam has been designed and constructed. The bender focuser guides part of the incident beam away from its line of sight and focuses it to a spot of width 0.65 mm at a distance 95 mm from the lens exit and 20 mm below the bottom edge of the beam path, with a gain of 20 in neutron current density. The neutron transmission characteristics of the lens have been determined with two types of position-sensitive detectors, a charge injection device, and an imaging plate. The lens has been tested with prompt gamma measurements on a gadolinium shard and titanium foil.