Primary Photons Research Articles

An analytic linear accelerator source model for GPU-based Monte Carlo dose calculations

Recently, there has been a lot of research interest in developing fast Monte Carlo (MC) dose calculation methods on graphics processing unit (GPU) platforms. A good linear accelerator (linac) source model is critical for both accuracy and efficiency considerations. In principle, an analytical source model should be more preferred for GPU-based MC dose engines than a phase-space file-based model, in that data loading and CPU-GPU data transfer can be avoided. In this paper, we presented an analytical field-independent source model specifically developed for GPU-based MC dose calculations, associated with a GPU-friendly sampling scheme. A key concept called phase-space-ring (PSR) was proposed. Each PSR contained a group of particles that were of the same type, close in energy and reside in a narrow ring on the phase-space plane located just above the upper jaws. The model parameterized the probability densities of particle location, direction and energy for each primary photon PSR, scattered photon PSR and electron PSR. Models of one 2D Gaussian distribution or multiple Gaussian components were employed to represent the particle direction distributions of these PSRs. A method was developed to analyze a reference phase-space file and derive corresponding model parameters. To efficiently use our model in MC dose calculations on GPU, we proposed a GPU-friendly sampling strategy, which ensured that the particles sampled and transported simultaneously are of the same type and close in energy to alleviate GPU thread divergences. To test the accuracy of our model, dose distributions of a set of open fields in a water phantom were calculated using our source model and compared to those calculated using the reference phase-space files. For the high dose gradient regions, the average distance-to-agreement (DTA) was within 1 mm and the maximum DTA within 2 mm. For relatively low dose gradient regions, the root-mean-square (RMS) dose difference was within 1.1% and the maximum dose difference within 1.7%. The maximum relative difference of output factors was within 0.5%. Over 98.5% passing rate was achieved in 3D gamma-index tests with 2%/2 mm criteria in both an IMRT prostate patient case and a head-and-neck case. These results demonstrated the efficacy of our model in terms of accurately representing a reference phase-space file. We have also tested the efficiency gain of our source model over our previously developed phase-space-let file source model. The overall efficiency of dose calculation was found to be improved by ~1.3–2.2 times in water and patient cases using our analytical model.

Implementation and validation of collapsed cone superposition for radiopharmaceutical dosimetry of photon emitters

Two collapsed cone (CC) superposition algorithms have been implemented for radiopharmaceutical dosimetry of photon emitters. The straight CC (SCC) superposition method uses a water energy deposition kernel (EDKw) for each electron, positron and photon components, while the primary and scatter CC (PSCC) superposition method uses different EDKw for primary and once-scattered photons. PSCC was implemented only for photons originating from the nucleus, precluding its application to positron emitters. EDKw are linearly scaled by radiological distance, taking into account tissue density heterogeneities. The implementation was tested on 100, 300 and 600 keV mono-energetic photons and 18F, 99mTc, 131I and 177Lu. The kernels were generated using the Monte Carlo codes MCNP and EGSnrc. The validation was performed on 6 phantoms representing interfaces between soft-tissues, lung and bone. The figures of merit were γ (3%, 3 mm) and γ (5%, 5 mm) criterions corresponding to the computation comparison on 80 absorbed doses (AD) points per phantom between Monte Carlo simulations and CC algorithms. PSCC gave better results than SCC for the lowest photon energy (100 keV). For the 3 isotopes computed with PSCC, the percentage of AD points satisfying the γ (5%, 5 mm) criterion was always over 99%. A still good but worse result was found with SCC, since at least 97% of AD-values verified the γ (5%, 5 mm) criterion, except a value of 57% for the 99mTc with the lung/bone interface. The CC superposition method for radiopharmaceutical dosimetry is a good alternative to Monte Carlo simulations while reducing computation complexity.

Increasing efficiency of BEAMnrc-simulated Co-60 beams using directional source biasing.

This study describes the implementation of a directional source biasing (DSB) scheme for efficiently simulating Cobalt-60 treatment heads using the BEAMnrc Monte Carlo code. Previous simulation of Co-60 beams with BEAMnrc was impractical because of the time required to track photons not directed into the treatment field and to simulate secondary charged particles. In DSB, efficiency is increased by splitting each photon emitted by the Co-60 source a user-defined number of times. Only those split primary photons directed into a user-defined splitting field (encompassing the treatment field) are sampled, yielding many low-weight photons directed into the field. Efficiency can be further increased by taking advantage of radial symmetry at the top of the treatment head to reduce the number of split primary photons tracked in this portion. There is also an option to generate contaminant electrons in DSB. The DSB scheme in BEAMnrc increases the photon fluence calculation efficiency in a 10 × 10 cm(2) Co-60 beam by a factor of 1800 with a concurrent increase in contaminant electron fluence calculation efficiency by a factor of 1200. Implementation of DSB in beampp, a C++ code for accelerator simulations based on EGSnrc and the C++ class library, egspp, increases photon fluence efficiency by a factor of 2800 and contaminant electron fluence efficiency by a factor of 1600. Optimum splitting numbers are in the range of 20,000-40,000. For dose calculations in a water phantom (0.5 × 0.5 × 0.5 cm(3) voxels) this translates into a factor of ∼400 increase in dose calculation efficiency (all doses > 0.5 × Dmax). An example calculation of the ratio of dose to water to dose to chamber (the basis of the beam quality correction factor) to within 0.2% in a realistic chamber using a full simulation of a Co-60 treatment head as a source indicates the practicality of Co-60 simulations with DSB. The efficiency improvement resulting from DSB makes Monte Carlo commissioning of Co-60 beams and calculation of beam quality correction factors feasible.

Transmission Imaging in Lymphoscintigraphy with a 153Gd Flood Source.

Lymphoscintigraphy uses intradermal or interstitial injections of (99m)Tc-labeled tracers to produce images of focal lymph nodes. Because there is little or no anatomic information in the (99m)Tc images, a (57)Co flood source is sometimes used to provide transmission data along with the emission data. The anatomic shadow from the transmission scan generally improves interpretation and surgical planning. However, the (57)Co transmission photons contribute to background on the (99m)Tc images, reducing contrast and signal-to-noise ratio (SNR). SNR is related to lesion detection, and some lymph nodes that would be detected in an emission-only scan might not be detected if acquired with a (57)Co flood source. An alternative to a (57)Co flood source is a (153)Gd flood source, which has primary photon emissions well below the (99m)Tc emission window, allowing the shadow to be acquired in a separate transmission window. Significantly smaller crosstalk from (153)Gd should improve SNR and therefore would be expected to improve lymph node detection. We hypothesized that the use of a (153)Gd flood source would reduce background and improve SNR for these studies. Phantom studies simulating lymphoscintigraphy were performed to compare performance with a (153)Gd flood source, a (57)Co flood source, and no flood source. SNR in the (99m)Tc emission images was measured using a water phantom to simulate patient body and point sources of various activities to simulate nodes and injection site. The encouraging phantom studies prompted use of the (153)Gd flood source in routine clinical breast lymphoscintigraphy, melanoma lymphoscintigraphy, and lymphedema studies. Because emission and transmission data were acquired in separate energy windows, fused planar images of emission and transmission data were available to the physician. SNR was highest with no flood source and was lowest with the (57)Co flood source by a significant margin. SNR with the (153)Gd flood source was similar to that with no flood source on the anterior (transmission) view. SNR was reduced somewhat in the posterior (nontransmission) view because of attenuation of signal by the flood source itself. Minor crosstalk in the (99m)Tc window was observed with the (153)Gd flood source, attributed to simultaneous detection of x-ray photons and gamma-photons. This crosstalk was reduced by introducing thin metal filters to absorb most x-ray photons, at the expense of more attenuation in the posterior view. Unlike with the (57)Co flood source, a usable posterior view (with anatomic shadow derived from the anterior view) was generated with the (153)Gd flood source. Clinical lymphoscintigraphy images with the (153)Gd flood source were of high quality. Interpretation was aided by the ability to control image mixing and brightness and contrast of separate color scales. By producing fused images with reduced crosstalk and improved image quality, a (153)Gd flood source offers advantages over a conventional (57)Co flood source for anatomic shadowing in lymphoscintigraphy. Lymph nodes in emission images have higher SNR, indicating a likely improvement in clinical lesion detection. Separate emission and transmission images provide additional flexibility in image display during interpretation.

SU‐E‐T‐557: Monte Carlo Modeling of Philips RT‐250 Orthovoltage Unit for Beam Spectrum Modulation

Purpose:To develop and validate a Monte Carlo (MC) model of a Phillips RT‐250 orthovoltage unit to test various beam spectrum modulation strategies for in vitro/vivo studies. A model of this type would enable the production of unconventional beams from a typical orthovoltage unit for novel therapeutic applications such as gold nanoparticle‐aided radiotherapy.Methods:The MCNP5 code system was used to create a MC model of the head of RT‐250 and a 30 × 30 × 30 cm3 water phantom. For the x‐ray machine head, the current model includes the vacuum region, beryllium window, collimators, inherent filters and exterior steel housing. For increased computational efficiency, the primary x‐ray spectrum from the target was calculated from a well‐validated analytical software package. Calculated percentage‐depth‐dose (PDD) values and photon spectra were validated against experimental data from film and Compton‐scatter spectrum measurements.Results:The model was validated for three common settings of the machine namely, 250 kVp (0.25 mm Cu), 125 kVp (2 mm Al), and 75 kVp (2 mm Al). The MC results for the PDD curves were compared with film measurements and showed good agreement for all depths with a maximum difference of 4 % around dmax and under 2.5 % for all other depths. The primary photon spectra were also measured and compared with the MC results showing reasonable agreement between the two, validating the input spectra and the final spectra as predicted by the current MC model.Conclusion:The current MC model accurately predicted the dosimetric and spectral characteristics of each beam from the RT‐250 orthovoltage unit, demonstrating its applicability and reliability for beam spectrum modulation tasks. It accomplished this without the need to model the bremsstrahlung xray production from the target, while significantly improved on computational efficiency by at least two orders of magnitude.Supported by DOD/PCRP grant W81XWH‐12‐1‐0198

TH‐A‐213‐02: IAEA/AAPM Code of Practice for the Dosimetry of Static Small Photon Fields

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation

TH‐EF‐BRD‐02: An Analytic Linear Accelerator Source Model and Automatic Source Commissioning for GPU‐Based Fast Monte Carlo Dose Calculation

Purpose:GPU‐based Monte Carlo (MC) dose calculations have become available. Linac source modeling for it is still a problem to be addressed. The model should be carefully designed for sufficient accuracy, ease of automatic commissioning, and more importantly suitability for parallelization. This abstract presents an analytical source model for GPU‐based MC dose engines with these factors considered.Methods:Analytical source models are preferred for GPU comparing to phase‐space‐file based models due to the avoidance of loading and transferring large data. In our model, we introduced a phase‐space‐ring (PSR) concept. A PSR contains particles of the same type and close in energy and radial distance on a plane above jaws. We parameterized probability densities of particle location, energy, and direction for three sub‐source types: primary photon PSR, scattered photon PSR and electron PSR. Model parameters were derived by analyzing reference phase‐space files. To incorporate this model into our GPU dose engine, we designed an efficient sampling scheme to reduce GPU thread divergence. Automatic commissioning was achieved by solving an optimization problem to adjust PSR weight.Results:Our model built for Varian Truebeam 6MV photon beam was able to represent the reference phase‐space. Comparing open‐field doses in water computed using our model and the phase‐space file, average distance‐to‐agreement(DTA)≤1mm for high‐gradient regions, root‐mean‐square(RMS) difference≤1.1% for low‐gradient regions and output factor difference≤0.5% were observed. Good agreements were seen in IMRT cases. Our model improved efficiency by 1.70–4.41 times compared to phase‐space‐based source model. It took ∼20sec to perform commissioning. Compared to uncommissioned model, DTA was reduced from 0.04–0.28cm to 0.04–0.12cm for depth dose at build‐up region, and RMS from 0.32%–0.67% to 0.21%–0.48%, 0.31%–2.0% to 0.06%–0.78% and 0.20%–1.25% to 0.10%–0.51% for depth dose after build‐up, lateral profiles of inner beam and outer beam, respectively.Conclusion:The analytic source model achieved high accuracy, efficiency, and suitability for GPU‐based dose calculations.

TH‐A‐213‐00: Small Field Dosimetry: Overview of AAPM TG‐155 and the IAEAAAPM Code of Practice

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation

SU‐E‐T‐560: Monte Carlo Simulation of the Neutron Radiation Field Around a Medical 18 MV Linac

Purpose:Today the majority of radiation therapy treatments are performed at medical electron linear accelerators (linacs). The accelerated electrons are used for the generation of bremsstrahlung photons. The use of higher electron respectively photon energies has some advantages over lower energies such as the longer dose build‐up. However photons with energies higher than ∼7 MeV can additionally to the interaction with bound electrons undergo inelastic reactions with nuclei. These photonuclear reactions lead to the emission of fast neutrons which contaminate the primary photon field. The neutrons might penetrate through the collimators and deliver out‐of‐field dose to the patient. Furthermore the materials inside the linac head as well as the air inside the treatment room get activated which might deliver dose to the medical employees even when the linac is not in operation. A detailed knowledge of these effects is essential for adequate radiation protection of the employees and an optimal patient treatment.Methods:It is a common method to study the radiation fields of such linacs by means of Monte Carlo simulations. For the investigation of the effects caused by photonuclear reactions a typical linac in high energy mode (Varian Clinac 18 MV‐X) as well as the surrounding bunker were modelled and simulated using the Monte Carlo code FLUKA which includes extensive nuclear reaction and neutron transport models additional to electron‐photon transport as well as capabilities for a detailed study of effective dose distributions and activation yields.Results:Neutron spectra as well as neutron effective dose distributions within the bunker were obtained, reaching up to some mSv/Gy in the patient's plane. The results are normalized per Gy in the depth dose maximum at 10×10 cm2 field size. Therefore an absolute interpretation is possible.Conclusion:The obtained data gives a better understanding of the photonuclear reaction caused effects.

TH‐A‐213‐01: Small Field Dosimetry: Overview of AAPM TG‐155

Modern radiotherapy techniques such as SRS/SRT, SBRT, IMRT, VMAT as well as specialized machines such as Tomotherapy, CyberKnife and Gamma Knife use small photon fields with at least one dimension <3 cm. Dosimetry in such small fields is challenging because of large detector perturbations due to non‐equilibrium conditions, occlusion of the primary photon source and large dose gradients across the field. Many small‐size dosimeters have been proposed for use in small fields. However, their characteristics especially the volume averaging and fluence perturbations have only recently been adequately understood. An IAEA‐AAPM working group has provided a framework for reference dosimetry in non‐compliant beams and the measurement of field output factors small fields (1). The AAPM TG‐155 (2) has adopted this framework to provide guidelines on relative dosimetry. This course explains the code of practice for absolute dosimetry that is under review and discusses the availability of correction factors to convert detector readings to doses. TG‐155 defines small field conditions, provides recommendations for suitable detectors and recommendations for good working practice for relative dosimetry (PDD, TMR, output factor, etc.) and dose calculations based on the new formulation. It also discusses beam modeling and dose calculations as a critical step in clinical utilization of small field radiotherapy. Alfonso et al, Med Phys 35, 5179–5186 (2008). Das et al, Med Phys (under review, 2014) Learning Objectives: Concepts and recommended procedures in the IAEA‐AAPM code of practice for dosimetry of small fields. Physics of dosimetry in non‐equilibrium conditions and definition of small fields Recommended procedures for relative dosimetry in TG‐1554. Choice of detectors for small field dosimetry, perturbations and corrections for dosimetry Understand the detector properties required for small field measurements Understand the advantages and limitations of commercially available detectors for small field dosimetry Research supported with operating grants from CIHR, NSERC, and BIOWin, a program funded at the Universite Catholique Louvain by the Walloon Government (Belgium). Student stipends supported by Medical Physics Research Training Network funded by the Collaborative Research and Training Experience program of the NSERC.Seuntjens: support by Sun Nuclear Corporation

SU-E-J-69: Iterative Deconvolution of the Initial Photon Fluence for EPID Dosimetry: A Monte Carlo Based Study

Purpose: Developing a fast and accurate calculation model to reconstruct the applied photon fluence from an external photon radiation therapy treatment based on an image recorded by an electronic portal image device (EPID). Methods: To reconstruct the initial photon fluence the 2D EPID image was corrected for scatter from the patient/phantom and EPID to generate the transmitted primary photon fluence. This was done by an iterative deconvolution using precalculated point spread functions (PSF). The transmitted primary photon fluence was then backprojected through the patient/phantom geometry considering linear attenuation to receive the initial photon fluence applied for the treatment.The calculation model was verified using Monte Carlo simulations performed with the EGSnrc code system. EPID images were produced by calculating the dose deposition in the EPID from a 6 MV photon beam irradiating a water phantom with air and bone inhomogeneities and the ICRP anthropomorphic voxel phantom. Results: The initial photon fluence was reconstructed using a single PSF and position dependent PSFs which depend on the radiological thickness of the irradiated object. Appling position dependent point spread functions the mean uncertainty of the reconstructed initial photon fluence could be reduced from 1.13 % to 0.13 %. Conclusion: This study presents a calculationmore » model for fluence reconstruction from EPID images. The{sup Result} show a clear advantage when position dependent PSF are used for the iterative reconstruction. The basic work of a reconstruction method was established and further evaluations must be made in an experimental study.« less

MC‐PEPTITA: A Monte Carlo model for Photon, Electron and Positron Tracking In Terrestrial Atmosphere—Application for a terrestrial gamma ray flash

AbstractTerrestrial gamma ray flashes are natural bursts of X and gamma rays, correlated to thunderstorms, that are likely to be produced at an altitude of about 10 to 20 km. After the emission, the flux of gamma rays is filtered and altered by the atmosphere and a small part of it may be detected by a satellite on low Earth orbit (RHESSI or Fermi, for example). Thus, only a residual part of the initial burst can be measured and most of the flux is made of scattered primary photons and of secondary emitted electrons, positrons, and photons. Trying to get information on the initial flux from the measurement is a very complex inverse problem, which can only be tackled by the use of a numerical model solving the transport of these high‐energy particles. For this purpose, we developed a numerical Monte Carlo model which solves the transport in the atmosphere of both relativistic electrons/positrons and X/gamma rays. It makes it possible to track the photons, electrons, and positrons in the whole Earth environment (considering the atmosphere and the magnetic field) to get information on what affects the transport of the particles from the source region to the altitude of the satellite. We first present the MC‐PEPTITA model, and then we validate it by comparison with a benchmark GEANT4 simulation with similar settings. Then, we show the results of a simulation close to Fermi event number 091214 in order to discuss some important properties of the photons and electrons/positrons that are reaching satellite altitude.

Scatter correction improves concordance in SPECT MPI with a dedicated cardiac SPECT solid-state camera

PurposeCorrection for photon attenuation and scatter improves image quality with conventional NaI-based gamma cameras but evaluation of these corrections for novel solid-state dedicated cardiac cameras is limited. In this study, we assess the accuracy of dual-energy-window (DEW) scatter correction (SC) applied to clinically acquired 99mTc-tetrofosmin myocardial perfusion images obtained on a dedicated multi-pinhole camera with cadmium-zinc-telluride (CZT) detectors (GE Discovery NM530) compared to DEW scatter-corrected images from our conventional SPECT camera (GE Infinia Hawkeye 4; INF). MethodsA modified DEW SC method was formulated to account for the detection of primary photons in the lower energy window (120 keV ± 5%) with CZT detectors, in addition to estimating the scattered photons detected in the photopeak window (140 keV ± 10%). Phantom experiments were used to estimate the DEW correction parameters. Data from 108 patients, acquired using a standard rest/stress Tc-99m-tetrofosmin SPECT/CT protocol on both cameras, were reconstructed with no correction (NC), attenuation correction (AC), and AC with DEW-SC. Images were compared based on the summed stress/rest/difference scores (SSS/SRS/SDS) calculated by clinical software. ResultsThe correlation between SSS/SRS for the two cameras was excellent (r ≥ 0.94). The mean difference between cameras was <0.4 for SSS/SRS/SDS scores. Since datasets did not follow a normal distribution, non-parametric tests were used to show significant differences between datasets. Classification of disease (SSS) was highly correlated, as ranked by the two cameras (kendall’s tau = 0.72, P < .001). AC significantly reduced the mean difference between the two cameras for SSS/SRS compared to NC. AC without SC on the CZT introduced a bias towards higher scores when compared to the INF, which was reduced after applying SC. Although SC increased noise, the scores for the AC/SC images were not significantly different between the two cameras (P > .1). ConclusionsDEW-SC on the CZT camera was feasible and produced images that are not significantly different from those acquired on the INF camera. Although use of SC on CZT images does increase noise, the resultant noise does not introduce bias relative to the INF camera.

An improved measurement of electron-ion recombination in high-pressure xenon gas

We report on results obtained with the NEXT-DEMO prototype of the NEXT-100 high-pressure xenon gas time projection chamber (TPC), filled with pure xenon gas at 10 bar pressure and exposed to an alpha decay calibration source. Compared to our previous measurements with alpha particles, an upgraded detector and improved analysis techniques have been used. We measure event-by-event correlated fluctuations between ionization and scintillation due to electron-ion recombination in the gas, with correlation coefficients between −0.80 and −0.56 depending on the drift field conditions. By combining the two signals, we obtain a 2.8% FWHM energy resolution for 5.49 MeV alpha particles and a measurement of the optical gain of the electroluminescent TPC. The improved energy resolution also allows us to measure the specific activity of the radon in the gas due to natural impurities. Finally, we measure the average ratio of excited to ionized atoms produced in the xenon gas by alpha particles to be 0.561± 0.045, translating into an average energy to produce a primary scintillation photon of Wex=(39.2± 3.2) eV.

Monte Carlo simulations and benchmark measurements on the response of TE(TE) and Mg(Ar) ionization chambers in photon, electron and neutron beams

The paired ionization chambers (ICs) technique is commonly employed to determine neutron and photon doses in radiology or radiotherapy neutron beams, where neutron dose shows very strong dependence on the accuracy of accompanying high energy photon dose. During the dose derivation, it is an important issue to evaluate the photon and electron response functions of two commercially available ionization chambers, denoted as TE(TE) and Mg(Ar), used in our reactor based epithermal neutron beam. Nowadays, most perturbation corrections for accurate dose determination and many treatment planning systems are based on the Monte Carlo technique. We used general purposed Monte Carlo codes, MCNP5, EGSnrc, FLUKA or GEANT4 for benchmark verifications among them and carefully measured values for a precise estimation of chamber current from absorbed dose rate of cavity gas. Also, energy dependent response functions of two chambers were calculated in a parallel beam with mono-energies from 20keV to 20MeV photons and electrons by using the optimal simple spherical and detailed IC models. The measurements were performed in the well-defined (a) four primary M-80, M-100, M120 and M150 X-ray calibration fields, (b) primary 60Co calibration beam, (c) 6MV and 10MV photon, (d) 6MeV and 18MeV electron LINACs in hospital and (e) BNCT clinical trials neutron beam. For the TE(TE) chamber, all codes were almost identical over the whole photon energy range. In the Mg(Ar) chamber, MCNP5 showed lower response than other codes for photon energy region below 0.1MeV and presented similar response above 0.2MeV (agreed within 5% in the simple spherical model). With the increase of electron energy, the response difference between MCNP5 and other codes became larger in both chambers. Compared with the measured currents, MCNP5 had the difference from the measurement data within 5% for the 60Co, 6MV, 10MV, 6MeV and 18MeV LINACs beams. But for the Mg(Ar) chamber, the derivations reached 7.8–16.5% below 120kVp X-ray beams. In this study, we were especially interested in BNCT doses where low energy photon contribution is less to ignore, MCNP model is recognized as the most suitable to simulate wide photon–electron and neutron energy distributed responses of the paired ICs. Also, MCNP provides the best prediction of BNCT source adjustment by the detector’s neutron and photon responses.

Evaluation of Compton attenuation and photoelectric absorption coefficients by convolution of scattering and primary functions and counts ratio on energy spectra.

Purpose:Estimation of Compton attenuation and the photoelectric absorption coefficients were explored at various depths.Methods:A new method was proposed for estimating the depth based on the convolution of two exponential functions, namely convolution of scattering and primary functions (CSPF), which the convolved result will conform to the photopeak region of energy spectrum with the variable energy-window widths (EWWs) and a theory on the scattering cross-section. The triple energy-windows (TEW) and extended triple energy-windows scatter correction (ETEW) methods were used to estimate the scattered and primary photons according to the energy spectra at various depths due to a better performance than the other methods in nuclear medicine. For this purpose, the energy spectra were employed, and a distinct phantom along with a technetium-99 m source was simulated by Monte Carlo method.Results:The simulated results indicate that the EWW, used to calculate the scattered and primary counts in terms of the integral operators on the functions, was proportional to the depth as an exponential function. The depth will be calculated by the combination of either TEW or ETEW and proposed method resulting in the distinct energy-window. The EWWs for primary photons were in good agreement with those of scattered photons at the same as depths. The average errors between these windows for both methods TEW, and ETEW were 7.25% and 6.03% at different depths, respectively. The EWW value for functions of scattered and primary photons was reduced by increasing the depth in the CSPF method.Conclusions:This coefficient may be an index for the scattering cross-section.

Analysis of Scattered Radiation in an Irradiated Body by Means of the Monte Carlo Simulation: Variation of the Subjective Contrast Due to Difference in the Location of Inhomogeneous Region

When an inhomogeneous medium such as bone, whose composition or density are clearly different from that of soft tissue of human body, exist in irradiated body, a subjective contrast of X-ray image changes by the location of these inhomogeneous medium. This cause due to the change of behavior of scattered photons in the body depends on the location of inhomogeneous medium besides due to the influence of a penumbra. But this mechanism is not explained clearly yet. In this paper, it was analyzed by means of the Monte Carlo simulation that what kind of difference occurs to a subjective contrast by the difference in location of inhomogeneous medium in water phantom and that a change in behavior of scattered photons in the phantom influences a subjective contrast by what kind of mechanism. In this case the inhomogeneous medium is bone, whose effective atomic number and density are higher than that of water, the subjective contrast of X-ray image degrades when bone is located near the entrance surface (upper position) than located near the exit surface (lower position). This is caused by the number of scattered photons, originated in primary photons incident upon the zone besides the region from entrance surface to exit surface including inhomogeneous medium and incident on the area of shadow of inhomogeneous medium on the image detector, is greater in case of the upper position than in case of the lower position. In the lower position, many of these scattered photons are interacted in bone located near the exit surface by the photo-electric absorption and only a small amount is incident on the image detector.

A hybrid phase-space and histogram source model for GPU-based Monte Carlo radiotherapy dose calculation33Presented as part of the International Workshop on Monte Carlo Techniques in Medical Physics

In GPU-based Monte Carlo simulations for radiotherapy dose calculation, source modelling from a phase-space source can be an efficiency bottleneck. Previously, this has been addressed using phase-space-let (PSL) sources, which provided significant efficiency enhancement. We propose that additional speed-up can be achieved through the use of a hybrid primary photon point source model combined with a secondary PSL source. A novel phase-space derived and histogram-based implementation of this model has been integrated into gDPM v3.0. Additionally, a simple method for approximately deriving target photon source characteristics from a phase-space that does not contain inheritable particle history variables (LATCH) has been demonstrated to succeed in selecting over 99% of the true target photons with only ~0.3% contamination (for a Varian 21EX 18 MV machine). The hybrid source model was tested using an array of open fields for various Varian 21EX and TrueBeam energies, and all cases achieved greater than 97% chi-test agreement (the mean was 99%) above the 2% isodose with 1% / 1 mm criteria. The root mean square deviations (RMSDs) were less than 1%, with a mean of 0.5%, and the source generation time was 4–5 times faster. A seven-field intensity modulated radiation therapy patient treatment achieved 95% chi-test agreement above the 10% isodose with 1% / 1 mm criteria, 99.8% for 2% / 2 mm, a RMSD of 0.8%, and source generation speed-up factor of 2.5.

SYNCHROTRON AND INVERSE-COMPTON EMISSIONS FROM PAIRS FORMED IN GRB AFTERGLOWS (ANALYTICAL TREATMENT)

We calculate the synchrotron and inverse-Compton emissions from pairs formed in GRB afterglows from high-energy photons (above 100 MeV), assuming a power-law photon spectrum C_nu ~ nu^{-2} and considering only the pairs generated from primary high-energy photons. The essential properties of these pairs (number, minimal energy, cooling energy, distribution with energy) and of their emission (peak flux, spectral breaks, spectral slope) are set by the observables GeV fluence Phi (t) = Ft and spectrum, and by the Lorentz factor Gamma and magnetic field B of the source of high-energy photons, at observer-time t. Optical and X-ray pseudo--light-curves F_nu (Gamma) are calculated for given B; proper synchrotron self-Compton light-curves are calculated by setting the dynamics Gamma(t) of the high-energy photons source to be that of a decelerating, relativistic shock. It is found that the emission from pairs can accommodate the flux and decays of the optical flashes measured during the prompt (GRB) phase and of the faster-decaying X-ray plateaus observed during the delayed (afterglow) phase. The brightest pair optical emission is obtained for 100 < Gamma < 500, and depends mostly on the GeV fluence, being independent of the source redshift. Emission from pairs formed during the GRB phase offers an alternate explanation to reverse-shock optical flashes. These two models may be distinguished based on their corresponding flux decay index--spectral slope relations, different correlations with the LAT fluence, or through modeling of the afterglow multiwavelength data.

First results from HAWC: monitoring the TeV gamma-ray sky

AbstractThe High Altitude Water Cherenkov (HAWC) Observatory is a wide-field gamma-ray detector sensitive to primary energies between 100 GeV and 100 TeV. The array is being built at an altitude of 4100 m a.s.l. on the Sierra Negra volcano near Puebla, Mexico. Data taking has already started while construction continues, with the completion projected for early 2015. The design is optimized to detect extended air showers induced by gamma rays that pass through the array and to reconstruct the directions and energies of the primary photons. With a duty cycle close to 100% and a daily coverage of ~8 sr of the sky, HAWC will perform a survey of TeV emissions from many different sources. The northern active galactic nuclei will be monitored for up to 6 hours each day, providing unprecedented light curve coverage at energies comparable to those of imaging air Cherenkov telescopes. HAWC has been in scientific operation with more than 100 detector modules since August 2013. Here we present a preliminary look at the first results and discuss the efforts to integrate HAWC in multi-wavelength studies of extragalactic jets.