Primary Beam Parameters Research Articles

Application of Conditional Generative Adversarial Networks to Efficiently Generate Photon Phase Space in Medical Linear Accelerators of Different Primary Beam Parameters

Successful application of external photon beam therapy in oncology requires that the dose delivered by a medical linear accelerator and distributed within the patient’s body is accurately calculated. Monte Carlo simulation is currently the most accurate method for this purpose but is computationally too extensive for routine clinical application. A very elaborate and time-consuming part of such Monte Carlo simulation is generation of the full set (phase space) of ionizing radiation components (mainly photons) to be subsequently used in simulating dose delivery to the patient. We propose a method of generating phase spaces in medical linear accelerators through learning, by artificial intelligence models, the joint multidimensional probability density distribution of the photon properties (their location in space, energy, and momentum). The models are conditioned with respect to the parameters of the primary electron beam (unique to each medical accelerator), which, through Bremsstrahlung, generates the therapeutical beam of ionizing radiation. Two variants of conditional generative adversarial networks are chosen, trained, and compared. We also present the second-best type of deep learning architecture that we studied: a variational autoencoder. Differences between dose distributions obtained in a water phantom, in a test phantom, and in real patients using generative-adversarial-network-based and Monte-Carlo-based phase spaces are very close to each other, as indicated by the values of standard quality assurance tools of radiotherapy. Particle generation with generative adversarial networks is three orders of magnitude faster than with Monte Carlo. The proposed GAN model, together with our earlier machine-learning-based method of tuning the primary electron beam of an MC simulator, delivers a complete solution to the problem of tuning a Monte Carlo simulator against a physical medical accelerator.

Monte Carlo modelling and validation of the elekta synergy medical linear accelerator equipped with radiosurgical cones

Monte Carlo simulations of medical linear accelerator heads help in visualizing the energy spectrum and angular spread of photons and electrons, energy deposition, and scattering from each of the head components. Hence, the purpose of this study was to validate the Monte Carlo model of the Elekta synergy medical linear accelerator equipped with stereotactic radio surgical connical collimators. For this, the Elekta synergy medical linear accelerator was modelled using the EGSnrc Monte Carlo code. The model results were validated using the measured data. The primary electron beam parameters, beam size, and energy were tuned to match the measured data; a dose profile with a field size of 40 × 40 cm2 and percentage depth dose with a field size of 10 × 10 cm2 were matched during tuning. The validation of the modelled data with the measurement results was performed using gamma analysis, point dose, and field size comparisons. For small radiation fields, relative output factors were also compared. The gamma analysis revealed good agreement between the Monte Carlo modeling results and the measured data. A gamma pass rate of more than 95% was obtained for field sizes of 40 × 40 cm2 to 2 × 2 cm2 with gamma criteria of 1% and 1 mm for the dose difference (DD) and distance to agreement (DTA), respectively; this gamma pass rate was more than 98% for the corresponding values of 2% and 2 mm for the DD and DTA, respectively. A gamma pass rate of more than 99% was obtained for a percentage depth dose with 1 mm and 1% criteria. The field size was also in good agreement with the measurement results, and the maximum deviation observed was 1.1%. The stereotactic cone field also passed this analysis with a gamma pass rate of more than 98% for dose profiles and 99% for the percentage depth dose. The small field output factor exhibited a deviation of 4.3%, 3.4%, and 1.9% for field sizes of 5 mm, 7.5 mm, and 10 mm, respectively. Thus, the Monte Carlo model of the Elekta Linear accelerator was successfully validated. The validation of radio surgical cones passed the analysis in terms of the dose profiles and percentage depth dose. The small field relative output factors exhibited deviations of up to 4.3%, and to resolve this, detector-specific and field-specific correction factors must be derived.

Critical-dimension scanning electron microscope characterization of smoothly varying wave structures with a Monte Carlo simulation

Scanning electron microscopy (SEM) characterization of a smoothly varying nanograting structure (a Pt-coated Cr grid on a Si substrate) with a sinusoidal waveform has been carried out by a Monte Carlo (MC) simulation technique. Previous studies with critical-dimension (CD) SEM (CD-SEM) have mostly concerned line structures with sharp edges so that there is an obvious edge bloom in the linescan profile of secondary electrons. In contrast, the present grating structures prepared by a laser-focused atomic deposition technique have a smoothly varying waveform in the cross-sectional profile, which pose greater difficulty for quantitative structural characterization by SEM. The grating structure, with a fixed pitch of λ/2 as the period of the standing-wave laser light field, where λ is the wavelength of the corresponding laser light, can be used as an ideal nanoscale metrological length tool; therefore, it is important to characterize its structural features over a large deposited area by SEM imaging for quality control, with the aim of mass reproduction. The present work extends the CD characterization of MC simulation methods to more complex structures. Taking into account different experimental factors, i.e. primary electron beam parameters, geometrical parameters and material properties, the unknown geometrical parameters (i.e. base height, peak height, linewidth shrinkage and peak tilt angle) of the grating lines have been successfully extracted from the experimental linescan profiles of SEM images.

Multiscale simulation of the focused electron beam induced deposition process

Focused electron beam induced deposition (FEBID) is a powerful technique for 3D-printing of complex nanodevices. However, for resolutions below 10 nm, it struggles to control size, morphology and composition of the structures, due to a lack of molecular-level understanding of the underlying irradiation-driven chemistry (IDC). Computational modeling is a tool to comprehend and further optimize FEBID-related technologies. Here we utilize a novel multiscale methodology which couples Monte Carlo simulations for radiation transport with irradiation-driven molecular dynamics for simulating IDC with atomistic resolution. Through an in depth analysis of hbox {W(CO)}_6 deposition on hbox {SiO}_2 and its subsequent irradiation with electrons, we provide a comprehensive description of the FEBID process and its intrinsic operation. Our analysis reveals that simulations deliver unprecedented results in modeling the FEBID process, demonstrating an excellent agreement with available experimental data of the simulated nanomaterial composition, microstructure and growth rate as a function of the primary beam parameters. The generality of the methodology provides a powerful tool to study versatile problems where IDC and multiscale phenomena play an essential role.

3D Depth Profile Reconstruction of Segregated Impurities using Secondary Ion Mass Spectrometry.

The presented protocol combines excellent detection limits (1 ppm to 1 ppb) using secondary ion mass spectrometry (SIMS) with reasonable spatial resolution (~1 µm). Furthermore, it describes how to obtain realistic three-dimensional (3D) distributions of segregated impurities/dopants in solid state materials. Direct 3D depth profile reconstruction is often difficult to achieve due to SIMS-related measurement artifacts. Presented here is a method to identify and solve this challenge. Three major issues are discussed, including the i) nonuniformity of the detector being compensated by flat-field correction; ii) vacuum background contribution (parasitic oxygen counts from residual gases present in the analysis chamber) being estimated and subtracted; and iii)performance of all steps within a stable timespan of the primary ion source. Wet chemical etching is used to reveal the position and types of dislocation in a material, then the SIMS result is superimposed on images obtained via scanning electron microscopy (SEM). Thus, the position of agglomerated impurities can be related to the position of certain defects. The method is fast and does not require sophisticated sample preparation stage; however, it requires a high-quality, stable ion source, and the entire measurement must be performed quickly to avoid deterioration of the primary beam parameters.

Local Diagnostic Reference Levels in Interventional Radiology

ObjetLes procédures de cardiologie d'intervention (CI), pendant lesquelles des images en temps réel sont acquises, supposent une exposition aux rayons X. L'utilisation de la fluoroscopie peut exposer le patient et les opérateurs à des doses de radiation élevées en raison de la durée prolongée de l’émission des rayons X. Pour cette raison, une attention particulière et une vigilance élevée présentent des défis pour les commissions et les groupes d'experts du domaine. Le but de cette étude est d’établir des niveaux de référence de diagnostic (NRD) locaux pour ces procédures, dans le but d'améliorer la pratique radiologique et d'optimiser les doses de radiation. MéthodologieCes travaux ont été réalisés dans deux hôpitaux universitaires et deux cliniques privées de Rabat, la capitale du Maroc, en 2017–2018. L’étude a porté sur 657 procédures de cardiologie d'intervention (457 angiographies coronariennes (AC) et 200 angioplasties coronariennes transluminales percutanées (ATP)), effectuées par 11 cardiologues sur différentes installations dans cinq salles de cathétérisme. Les données recueillies pour chacune des procédures comprennent: l’âge, la taille et le poids du patient, les indicateurs de dosimètre comme le produit dose-surface (PKA), le Kerma total dans l'air au point de référence (Kar), le temps de fluoroscopie (FT) et le nombre de cadres (NF), ainsi que les paramètres de base du faisceau (kV et mAs total. Les NRD proposés ont été fixés au 75e centile du PKA et du FT. RésultatsLe PKA moyen pour les procédures d’AC et d’ATP était respectivement de 29,2 Gy∗cm2 et 70,4 Gy∗cm2, la moyenne des temps de fluoroscopie était respectivement de 4,0 min et 12,17 min pour 334 et 685 cadres. Les résultats pour les NDT locaux étaient de 37,3 et 87,1 Gy cm2 pour le PKA et de 4,48 et 16,15 min pour le FT, correspondant aux procédures AC et ATP. ConclusionCe travail mettait l'accent sur la proposition de NRD locaux au Maroc pour les procédures d’AC et d’ATP. Les résultats montrent que les valeurs constatées sont conformes à celles des études internationales.

Monitoring mixed neutron-proton field near the primary proton and deuteron beams in spallation targets

At the Joint Institute for Nuclear Research (JINR) we are involved in the Accelerator-Driven-System (ADS) research. We perform experiments with assemblies composed of a spallation target and a subcritical blanket irradiated with high-energy proton or deuteron beams that generate high-energy neutron fields by spallation and fission reactions. In this paper, three uranium assemblies are presented: Energy plus Transmutation (E+T), QUINTA and BURAN. We discuss the results of the E+T and QUINTA irradiations by 1.6 GeV deuterons and 660 MeV protons, respectively. We have focused on the regions close to the primary beam passage through the targets. The field has been measured using activation detectors of 209Bi, 59Co, and natPb. Monte Carlo simulations using MCNPX 2.7.0 have been performed and compared to the experimental results. We discovered that the field intensity near the primary beam is very dependent on the precision of the accelerator beam settings. Therefore, a Monte Carlo-based study of the influence of the uncertainty of primary proton beam parameters on experimental result accuracy of the QUINTA assembly has been carried out. The usage of MCNPX 2.7.0 in the future BURAN irradiations has been assessed.

Development and validation of a 1.5T MR-Linac full accelerator head and cryostat model for Monte Carlo dose simulations.

To develop, implement, and validate a full 1.5T/7MV magnetic resonance (MR)-Linac accelerator head and cryostat model in EGSnrc for high precision dose calculations accounting for magnetic field effects that are independent from the vendor treatment planning system. Primary electron beam parameters for the implemented model were adapted to be in accordance with measured dose profiles of the Elekta Unity (Elekta AB, Stockholm, Sweden). Parameters to be investigated were the mean electron energy as well as the Gaussian radial intensity and energy distributions. Energy tuning was done comparing depth dose profiles simulated with monoenergetic beams of varying energies to measurements. The optimum radial intensity distribution was found by varying the radial full width at half maximum (FWHM) and comparing simulated and measured lateral profiles. The influence of the energy distribution was investigated by comparing simulated lateral and depth dose profiles with varying energy spreads to measured data. Comparison of simulations and measurements was performed by calculating average and maximum local dose deviations. The model was validated recalculating a clinical intensity-modulated radiation therapy plan for the MR-Linac and comparing the resulting dose distribution with simulations from the commercial treatment planning system Monaco using the gamma criterion. Comparison of simulated and measured data showed that the optimum initial electron beam for MR-Linac simulations was monoenergetic with an electron energy of (7.4±0.2)MeV. The optimum Gaussian radial intensity distribution has a FWHM of (2.2±0.3)mm. The average relative deviations were smaller than 1% for all simulated profiles with optimum electron parameters, whereas the largest maximum deviation of 2.07% was found for the 22×22cm cross-plane profile. Profiles were insensitive to energy spread variations. The IMRT plan recalculated with the final MR-Linac model with optimized initial electron beam parameters showed a gamma pass rate of 99.83 % using a gamma criterion of 3%/3mm. The EGSnrc MR-Linac model developed in this study showed good accordance with measurements and was successfully used to recalculate a first full clinical IMRT treatment plan. Thus, it shows the general possibility for future secondary dose calculations of full IMRT plans with EGSnrc, which needs further detailed investigations before clinical use.

Laser wakefield acceleration in Kr–He plasmas and its application to positron beam generation

We present the generation of quasi-monoenergetic electron beams via ionization injection in a laser wakefield acceleration by using a krypton–helium plasma. We obtained higher-energy electron beams of low divergence as compared with beams from the nitrogen–helium plasma at low plasma densities. This is attributed to an immediate ionization, trapping and acceleration of many electrons from several inner shells of the krypton atoms. On the other hand, there are very high ionization potentials of the K-shell nitrogen electrons which require the pulse (in our laser-plasma parameters) to experience a self-focusing (after 1.6 mm of propagation) at first before any ionization injection and acceleration of those two electrons per atom can occur. Based on those high-quality electron beams from the Kr–He plasmas, we generated ultra-relativistic positron beams with energies up to 100 MeV by placing a lead slab converter with a thickness of the order of its radiation length in the electron beam path after the gas jet. Experimental and Mont-Carlo simulated energy spectra of the positron beams are compared for the energy range of 30 MeV ≤ Ee+ ≤ 100 MeV along with the positron yields. In addition, various parameters of the positrons are investigated against the primary electron beam parameters. Such electron–positron cascading experiment could be helpful in future development for the composition of an all-optical electron–positron collider.

Primo software as a tool for Monte Carlo simulations of intensity modulated radiotherapy: a feasibility study

BackgroundIMRT provides higher dose conformation to the target and dose sparing to surrounding tissues than 3DCRT. Monte Carlo method in Medical Physics is not a novelty to approach dosimetric problems. A new PENELOPE based code named PRIMO recently was published. The most intriguing features of PRIMO are the user-friendly approach, the stand-alone property and the built-in definition of different linear accelerators models. Nevertheless, IMRT simulations are not yet implemented.MethodsA Varian Trilogy with a Millennium120 MLC and a Varian Novalis with 120HD MLC were studied. A RW3 multi-slab phantom was irradiated with Gafchromic films inserted between slabs. An Expression 10000XL scanner (Seiko Epson Corp., Nagano, Japan) was used to digitalize the films. PTW-Verisoft software using the global Gamma Function (2%, 2 mm) was used to compare simulated and experimental results.The primary beam parameters were adjusted to best match reference data previously obtained in a water phantom. Static MLC simulations were performed to validate the MLC models in use. Two Dynamic IMRT preliminary tests were performed with leaves moving with constant and variable speed. A further test of an in phantom delivery of a real IMRT field allowed simulating a clinical-like MLC modulation.ResultsSimulated PDD, X- and Y-profiles in reference conditions showed respectively 100.0%, 100.0% and 99.4% of Gamma points < 1 (2%, 2 mm). Static MLC simulations showed 100.0% of Gamma points < 1 with the 120HD MLC and 99.1% with the Millennium compared with the scanned images.The fixed speed test showed 99.5 and 98.9% of Gamma points < 1 respectively with two different MLC configuration-sampling algorithms when the 120HD MLC was used. The higher modulation MLC motion simulation showed 99.1% of Gamma points < 1 with respect to the experimental. This result depends on the number of the fields to reproduce the MLC motion, as well as calculation time. The clinical-like simulation showed 96.2% of Gamma points < 1 using the same analysis conditions.ConclusionsThe numerical model of the Varian Trilogy and Novalis in the PRIMO software was validated. The algorithms to simulate MLC motion were considered reliable. A clinical-like procedure was successfully simulated.

Abstract ID: 185 Monte Carlo simulation of “Out-of-Field” dose distributions in a pediatric phantom in external radiotherapy – Comparison with Gafchromic film measurements

Introduction Patients undergoing radiotherapy are exposed to out-of-field scattered and leakage radiation, which may induce secondary cancer in long-term survivors. The induction of secondary cancer in pediatric patients is an increased concern, because they have increasingly higher life expectancy, and tend to receive higher secondary organ doses than adults due to geometrical factors. Radiation-induced thyroid and breast cancers have been observed in pediatrics after receiving doses as low as 100 mGy [1] . While the dosimetric accuracy of treatment planning systems (TPS) is known to decrease in the out-of-field region, Monte Carlo (MC) is nowadays the most reliable method in dose calculation. A PENELOPE based MC code named PRIMO was tested for out-of-field dose estimation in pediatric irradiation. Material and methods a 5-year pediatric anthropomorphic phantom was irradiated by Varian Triology linac, using three 6MV static fields beams for a brain tumor. The treatment plan was calculated in Varian Eclipse TPS, using Analytic Anisotropic Algorithm (AAA). The 2D dose distributions at thyroid and lung levels were measured using Gafcromic EBT3 films. The plan was simulated in PRIMO after adjusting the primary beam parameters based on reference data in water phantom. PRIMO output data was reshaped, manipulated and re-written as DICOM files for comparisons with both measured and TPS calculated dose using 2D Gamma Function (3%, 3 mm) on DoseLab Pro software. Results The maximum measured doses per 1 Gy at isocenter (0 cm) were 1.5 cGy and 8 mGy at thyroid (10.25 cm) and lung levels (15.25 cm) respectively. Comparing simulated and measured dose distribution one obtained 82.6% and 88.8% of gamma points 1 at thyroid and lung levels respectively. The difference between measured and TPS calculated dose distributions could easily be accessed visually. Conclusion For secondary induced cancer risk estimation, the present TPS, even using a reliable algorithm as AAA, is not generally satisfactory for out-of-field dosimetry. Alternative tools, like MC simulations are necessary if correct dose estimation must be obtained for risk models application.

Monte Carlo simulations of ³He ion physical characteristics in a water phantom and evaluation of radiobiological effectiveness.

(3)He ions may hold great potential for clinical therapy because of both their physical and biological properties. In this study, the authors investigated the physical properties, i.e., the depth-dose curves from primary and secondary particles, and the energy distributions of helium ((3)He) ions. A relative biological effectiveness (RBE) model was applied to assess the biological effectiveness on survival of multiple cell lines. In light of the lack of experimental measurements and cross sections, the authors used Monte Carlo methods to study the energy deposition of (3)He ions. The transport of (3)He ions in water was simulated by using three Monte Carlo codes-FLUKA, GEANT4, and MCNPX-for incident beams with Gaussian energy distributions with average energies of 527 and 699 MeV and a full width at half maximum of 3.3 MeV in both cases. The RBE of each was evaluated by using the repair-misrepair-fixation model. In all of the simulations with each of the three Monte Carlo codes, the same geometry and primary beam parameters were used. Energy deposition as a function of depth and energy spectra with high resolution was calculated on the central axis of the beam. Secondary proton dose from the primary (3)He beams was predicted quite differently by the three Monte Carlo systems. The predictions differed by as much as a factor of 2. Microdosimetric parameters such as dose mean lineal energy (y(D)), frequency mean lineal energy (y(F)), and frequency mean specific energy (z(F)) were used to characterize the radiation beam quality at four depths of the Bragg curve. Calculated RBE values were close to 1 at the entrance, reached on average 1.8 and 1.6 for prostate and head and neck cancer cell lines at the Bragg peak for both energies, but showed some variations between the different Monte Carlo codes. Although the Monte Carlo codes provided different results in energy deposition and especially in secondary particle production (most of the differences between the three codes were observed close to the Bragg peak, where the energy spectrum broadens), the results in terms of RBE were generally similar.

SU‐E‐T‐138: Automated Chart Review Module Including Cross‐Vendor Data Transfer Verification Developed for IHE‐RO Initiative

Purpose:To follow the Integrating Healthcare Enterprise ‐ Radiation Oncology (IHE‐RO) initiative of proper cross‐vendor technology integration, an automated chart checker (ACC) was developed. ACC compares extracted data from an approved patient plan in the Eclipse treatment planning system (TPS) against data existing in the Mosaiq treatment management system (TMS). ACC automatically analyzes these parameters using built‐in quality checklists to provide further aid in chart review.Methods:Eclipse TPS data are obtained using Eclipse scripting API (ESAPI) while Mosaiq TMS data are obtained from a radiotherapy‐treatment‐planning (RTP) file. Using this information, ACC identifies TPS‐TMS discrepancies in 18 primary beam parameters including MU, energy, jaw positions, gantry angle, table angle, accessories, and bolus for up to 31 beams. Next, approximately 40 items from traditional quality checklists are evaluated such as prescription consistency, DRR graticule placement, plan approval status, global max dose, and dose tracking coefficients. Parameters were artificially modified to determine if ACC would detect an error in data transfer and to test each component of quality checklists.Results:Using ESAPI scripting and RTP file‐processing, ACC was able to properly aggregate data from TPS and TMS for up to 31 beams. Errors were artificially introduced into each plan parameter, and ACC was able to successfully detect all of them within seconds. Next, ACC was able to successfully detect mistakes in the chart by identifying deviations with its quality checklists, within seconds.Conclusion:ACC effectively addresses the potential issue of faulty cross‐vendor data transfer, as described by IHE‐RO. In addition, ACC was also able to detect deviations from its built‐in quality checklists. ACC is already an invaluable tool for efficient and standardized chart review and will continue to improve as its incorporated checklists become more comprehensive.

A Dosimetry Methodology for Two-Photon Absorption Induced Single-Event Effects Measurements

A pulsed-laser dosimetry approach for two-photon absorption (TPA) single-event effects (SEE) measurements is presented. Development and implementation of three online beam monitors is described. The beam monitors permit characterization of the primary laser beam parameters of interest: the pulse energy delivered to the device under test, the pulse duration, and the focused laser spot size. A direct consequence of this methodology is the ability to monitor continuously the operating point of the TPA SEE pulsed-laser beamline and to make the necessary adjustments when parameters drift, either during an experiment or between experiments.

Ion beam sputtering of Ag – Angular and energetic distributions of sputtered and scattered particles

Ion beam sputter deposition (IBD) provides intrinsic features which influence the properties of the growing film, because ion properties and geometrical process conditions generate different energy and spatial distribution of the sputtered and scattered particles. A vacuum deposition chamber is set up to measure the energy and spatial distribution of secondary particles produced by ion beam sputtering of different target materials under variation of geometrical parameters (incidence angle of primary ions and emission angle of secondary particles) and of primary ion beam parameters (ion species and energies).A representative set of Ag thin films is deposited arranged on a substrate holder equatorial to the Ag target in steps of 10° and characterized concerning their film thickness by profilometry to determine the angular distribution of the sputtered particles. The film thickness distributions show a tilted, cosine-like shape and a shifting of the maximum position depending on the primary particle energy and incidence angle of the primary ions.The energy distributions of sputtered and scattered ions and of sputtered neutrals are measured with an energy-selective mass spectrometer. The average energy of the sputtered ions increases with increasing emission angle and also increases with increasing incidence angle of the primary ions. In contrast, the average energy of the sputtered ions is nearly unaffected by the primary particle energy and particle species. The energy distribution of the scattered Ar ions reveals high energetic maxima which shift with increasing emission angle to higher energies. These maxima are not observed for Xe bombardment. The total energies of sputtered and scattered ions show significant differences between the two bombarding species. The maximum of the energy distribution of sputtered Ag neutrals is used to conclude on the surface binding energy of Ag (2.72eV).All experimental data are compared with Monte Carlo simulations done with the well-known TRIM.SP.

An investigation into the use of MMCTP to tune accelerator source parameters and testing its clinical application

This paper presents an alternative method to tune Monte Carlo electron beam parameters to match measured data using a minimal set of variables in order to reduce the model setup time prior to clinical implementation of the model. Monte Carlo calculations provide the possibility of a powerful treatment planning verification technique. The nonstandardized and nonautomated process of tuning the required accelerator model is one of the reasons for delays in the clinical implementation of Monte Carlo techniques. This work aims to establish and verify an alternative tuning method that can be carried out in a minimal amount of time, allowing it to be easily implemented in a clinical setting by personnel with minimal experience with Monte Carlo methods. This tuned model can then be incorporated into the MMCTP system to allow the system to be used as a second dose calculation check for IMRT plans. The technique proposed was used to establish the primary electron beam parameters for accelerator models for the Varian Clinac 2100 6 MV photon beam using the BEAMnrc Monte Carlo system. The method is intended to provide a clear, direct, and efficient process for tuning an accelerator model using readily available clinical quality assurance data. The tuning provides a refined model, which agrees with measured dose profile curves within 1.5% outside the penumbra or 3 mm in the penumbra, for square fields with sides of 3 cm up to 30 cm. These models can then be employed as the basis for Monte Carlo recalculations of dose distributions, using the MMCTP system, for clinical treatment plans, providing an invaluable assessment tool. This was tested on six IMRT plans and compared to the measurements performed for the pretreatment QA process. These Monte Carlo values for the average dose to the chamber volume agreed with measurements to within 0.6%.PACS number: 87.55.km

Investigations of secondary ion yield‐enhancing methods in combination

AbstractToday, several methods are used for secondary ion yield enhancement in time‐of‐flight SIMS (ToF‐SIMS). Some deal with the modification of sample properties, e.g. with monolayer preparation of analytes on metal substrates instead of thick layers, or the metalization of thick‐layer samples with noble metals like gold or silver (MetA‐SIMS). Other methods deal with primary ion beam parameters, e.g. the use of heavier ions or the use of polyatomic instead of monoatomic primary ions.To investigate the yield‐enhancing effects of the combination of these techniques, (sub‐) monolayers of polyethylene imine, both pristine and metalized with different amounts of gold, were prepared on substrate materials with different stopping powers. For primary ion (PI) bombardment, monoatomic (Ne+, Ar+, Kr+, Xe+, Bi+) and polyatomic (Bin+, Bin++) primary ions were used.It was found that all techniques used in this contribution increase the secondary ion yield mainly by increasing the stopping power, thereby increasing the amount of PI energy that is deposited near the sample surface (upper 5 nm). Only a small additional yield enhancement was observed when to a yield‐enhancing technique such as polyatomic PI bombardment, which deposits nearly all the PI energy near the surface, another technique was added that is based on the same mechanism for yield enhancement. In other words, the yield enhancement achieved by two enhancement techniques used in combination was much smaller than the product of the yield enhancements obtainable with each technique taken by itself. Copyright © 2010 John Wiley &amp; Sons, Ltd.

On Monte Carlo modeling of megavoltage photon beams: A revisited study on the sensitivity of beam parameters

To commission Monte Carlo beam models for five Varian megavoltage photon beams (4, 6, 10, 15, and 18 MV). The goal is to closely match measured dose distributions in water for a wide range of field sizes (from 2 x 2 to 35 x 35 cm2). The second objective is to reinvestigate the sensitivity of the calculated dose distributions to variations in the primary electron beam parameters. The GEPTS Monte Carlo code is used for photon beam simulations and dose calculations. The linear accelerator geometric models are based on (i) manufacturer specifications, (ii) corrections made by Chibani and Ma ["On the discrepancies between Monte Carlo dose calculations and measurements for the 18 MV Varian photon beam," Med. Phys. 34, 1206-1216 (2007)], and (iii) more recent drawings. Measurements were performed using pinpoint and Farmer ionization chambers, depending on the field size. Phase space calculations for small fields were performed with and without angle-based photon splitting. In addition to the three commonly used primary electron beam parameters (E(AV) is the mean energy, FWHM is the energy spectrum broadening, and R is the beam radius), the angular divergence (theta) of primary electrons is also considered. The calculated and measured dose distributions agreed to within 1% local difference at any depth beyond 1 cm for different energies and for field sizes varying from 2 x 2 to 35 x 35 cm2. In the penumbra regions, the distance to agreement is better than 0.5 mm, except for 15 MV (0.4-1 mm). The measured and calculated output factors agreed to within 1.2%. The 6, 10, and 18 MV beam models use theta = 0 degrees, while the 4 and 15 MV beam models require theta = 0.5 degrees and 0.6 degrees, respectively. The parameter sensitivity study shows that varying the beam parameters around the solution can lead to 5% differences with measurements for small (e.g., 2 x 2 cm2) and large (e.g., 35 x 35 cm2) fields, while a perfect agreement is maintained for the 10 x 10 cm2 field. The influence of R on the central-axis depth dose and the strong influence of theta on the lateral dose profiles are demonstrated. Dose distributions for very small and very large fields were proved to be more sensitive to variations in E(AV), R, and theta in comparison with the 10 x 10 cm2 field. Monte Carlo beam models need to be validated for a wide range of field sizes including small field sizes (e.g., 2 x 2 cm2).

SU‐GG‐T‐402: On Monte Carlo Modeling of Megavoltage Photon Beams: A Revisited Study on Beam Parameters Sensitivity

Purpose: To commission Monte Carlo beam models for 5 Varian megavoltage photon beams (4, 6, 10, 15 and 18 MV). The goal is to match measured dose distributions in a water phantom within 1% local difference for a wide range of field sizes. Another objective is to re‐investigate the sensitivity of dose distributions to the primary electron beam parameters. Material and Methods: The GEPTS Monte Carlo code is used for photon beam simulation and dose calculations. Linac geometry models are based on (i) manufacturer specifications, (ii) corrections made by Chibani and Ma (Med. Phys. Vol. 34, 2007), and (iii) recent drawings of the W/Pb shields. Measurements were performed using pinpoint and Farmer ionization chambers depending on the field size. In addition to the three commonly used primary electron beam parameters (EAV: average energy, FWHM: energy spectrum broadening, and R: beam radius), we also included the primary electron angular divergence (q). Results: We were able to match within 1% local difference measured depth dose distributions and lateral dose profiles (within 1%/1mm in penumbra), at any depth beyond 1 cm. Varying primary beam parameters away from the solution can lead to 5% differences with measurements for small (e.g. 2×2 cm2) and large (e.g. 35×35 cm2) field size WHILE a perfect agreement still stands for 10×10. Very small and very large field sizes prove to be more sensitive to FWHM, R and q variations than the 10×10 field size. Conclusion: GEPTS beam models reproduce measured dose distributions within 1% local difference and simultaneously for field sizes varying from 2×2 to 35×35. Achieving 1% agreement between measured and calculated dose distributions for the 10×10 field size is relatively easy. However, this does not guaranty a similar agreement for other field sizes. Therefore, Monte Carlo beam models need to be commissioned for multiple field sizes..

Ion-optical layout of the focusing system in front of the Super-FRS target

Two ion-optical layouts of the focusing system in front of the Super-FRS production target are described. The first option uses dedicated single magnet units adapted for the large range of primary beam parameters. The second one employs magnets designed for other components of FAIR (Super-FRS, SIS100). The latter option is favored for the construction phase.