Collimator Size Research Articles

A new emittance selection system to maximize beam transmission for low-energy beams in cyclotron-based proton therapy facilities with gantry.

PurposeIn proton therapy, the potential of using high‐dose rates in the cancer treatment is being explored. High‐dose rates could improve efficiency and throughput in standard clinical practice, allow efficient utilization of motion mitigation techniques for moving targets, and potentially enhance normal tissue sparing due to the so‐called FLASH effect. However, high‐dose rates are difficult to reach when lower energy beams are applied in cyclotron‐based proton therapy facilities, because they result in large beam sizes and divergences downstream of the degrader, incurring large losses from the cyclotron to the patient position (isocenter). In current facilities, the emittance after the degrader is reduced using circular collimators; however, this does not provide an optimal matching to the acceptance of the following beamline, causing a low transmission for these energies. We, therefore, propose to use a collimation system, asymmetric in both beam size and divergence, resulting in symmetric emittance in both beam transverse planes as required for a gantry system. This new emittance selection, together with a new optics design for the following beamline and gantry, allows a better matching to the beamline acceptance and an improvement of the transmission.MethodsWe implemented a custom method to design the collimator sizes and shape required to select high emittance, to be transported by the following beamline using new beam optics (designed with TRANSPORT) to maximize acceptance matching. For predicting the transmission in the new configuration (new collimators + optics), we used Monte Carlo simulations implemented in BDSIM, implementing a model of PSI Gantry 2 which we benchmarked against measurements taken in the current clinical scenario (circular collimators + clinical optics).ResultsFrom the BDSIM simulations, we found that the new collimator system and matching beam optics results in an overall transmission from the cyclotron to the isocenter for a 70 MeV beam of 0.72%. This is an improvement of almost a factor of 6 over the current clinical performance (0.13% transmission). The new optics satisfies clinical beam requirements at the isocenter.ConclusionsWe developed a new emittance collimation system for PSI's PROSCAN beamline which, by carefully selecting beam size and divergence asymmetrically, increases the beam transmission for low‐energy beams in current state‐of‐the‐art cyclotron‐based proton therapy gantries. With these improvements, we could predict almost 1% transmission for low‐energy beams at PSI's Gantry 2. Such a system could easily be implemented in facilities interested in increasing dose rates for efficient motion mitigation and FLASH experiments alike.

Acceptance Test of Physical Properties of Perfexion Gamma Knife

Objective The acceptance test of the physical performance for the newly installed PFX Gamma knife was carried out to provide a reference for the quality assurance of the equipment. Method According to the manufacturer's acceptance manual and Chinese health industry standards, the dose rate of the PFX Gamma knife with maximum collimators in the calibration center point, the focal spot dose distribution with all three collimator sizes, the distance between the RFP and the PPS calibration center point, and the multi-target position accuracy were measured by means of the acceptance tools. Results The average absorbed dose rate, as measured in April 29, 2019, was 3.295 Gy/min in the calibration center point by the maximum collimators, which was better than acceptance standard of 2.5 Gy/min. The focal spot dosimetry data measured by the film were basically the same as the TMR-10 data stored in the LGP. The difference in FWHM was 0.18±0.13 mm, and the difference in penumbra was 0.15±0.25 mm, which was normal and as expected. The distance between the RFP and PPS calibration center point in the three-dimensional space was 0.24 mm, which was better than the recommended value of the factory standard and the national standard. The maximum position deviation of multiple targets was 0.16 mm, which was also better than the factory standard of 0.5 mm. Conclusion The various physical performance indicators of the PFX Gamma knife met the acceptance standards of manufacturers and industries.

Commissioning of an ultra-high dose rate pulsed electron beam medical LINAC for FLASH RT preclinical animal experiments and future clinical human protocols.

To present the acceptance and the commissioning, to define the reference dose, and to prepare the reference data for a quality assessment (QA) program of an ultra-high dose rate (UHDR) electron device in order to validate it for preclinical animal FLASH radiotherapy (FLASH RT) experiments and for FLASH RT clinical human protocols. The Mobetron® device was evaluated with electron beams of 9MeV in conventional (CONV) mode and of 6 and 9MeV in UHDR mode (nominal energy). The acceptance was performed according to the acceptance protocol of the company. The commissioning consisted of determining the short- and long-term stability of the device, the measurement of percent depth dose curves (PDDs) and profiles at two different positions (with two different dose per pulse regimen) and for different collimator sizes, and the evaluation of the variability of these parameters when changing the pulse width and pulse repetition frequency. Measurements were performed using a redundant and validated dosimetric strategy with alanine and radiochromic films, as well as Advanced Markus ionization chamber for some measurements. The acceptance tests were all within the tolerances of the company's acceptance protocol. The linearity with pulse width was within 1.5% in all cases. The pulse repetition frequency did not affect the delivered dose more than 2% in all cases but 90Hz, for which the larger difference was 3.8%. The reference dosimetry showed a good agreement within the alanine and films with variations of 2.2% or less. The short-term (resp. long-term) stability was less than 1.0% (resp. 1.8%) and was the same in both CONV and UHDR modes. PDDs, profiles, and reference dosimetry were measured at two positions, providing data for two specific dose rates (about 9Gy/pulse and 3Gy/pulse). Maximal beam size was 4 and 6cm at 90% isodose in the two positions tested. There was no difference between CONV and UHDR mode in the beam characteristics tested. The device is commissioned for FLASH RT preclinical biological experiments as well as FLASH RT clinical human protocols.

An automated optimization strategy to design collimator geometry for small field radiation therapy systems

Purpose. To develop an automated optimization strategy to facilitate collimator design for small-field radiotherapy systems. Methods and Materials. We developed an objective function that links the dose profile characteristics (FWHM, penumbra, and central dose rate) and the treatment head geometric parameters (collimator thickness/radii, source-to-distal-collimator distance (SDC)) for small-field radiotherapy systems. We performed optimization using a downhill simplex algorithm. We applied this optimization strategy to a linac-based radiosurgery system to determine the optimal geometry of four pencil-beam collimators to produce 5, 10, 15, and 20 mm diameter photon beams (from a 6.7 MeV, 2.1 mm FWHM electron beam). Two different optimizations were performed to prioritize minimum penumbra or maximum central dose rate for each beam size. We compared the optimized geometric parameters and dose distributions to an existing clinical system (CyberKnife). Results. When minimum penumbra was prioritized, using the same collimator thickness and SDC (40 cm) as a CyberKnife system, the optimized collimator upstream and downstream radii agreed with the CyberKnife system within 3%–14%, the optimized output factors agreed within 0%–8%, and the optimized transverse and percentage depth dose profiles matched those of the CyberKnife with the penumbras agreeing within 2%. However, when maximum dose rate was prioritized, allowing both the collimator thickness and SDC to change, the central dose rate for larger collimator sizes (10, 15, 20 mm) could be increased by about 1.5–2 times at the cost of 1.5–2 times larger penumbras. No further improvement in central dose rate for the 5 mm beam size could be achieved. Conclusions. We developed an automated optimization strategy to design the collimator geometry for small-field radiation therapy systems. Using this strategy, the penumbra-prioritized dose distribution and geometric parameters agree well with the CyberKnife system as an example, suggesting that this system was designed to prioritize sharp penumbra. This represents proof-of-principle that an automated optimization strategy may apply to more complex collimator designs with multiple optimization parameters.

Determination of neutron image quality of compact collimator facility for non-destructive testing

Neutron imaging, based on a digital-based detector system, was developed for non-destructive internal inspection at the Thai Research Reactor-1/Modification 1 (TRR-1/M1). Neutron activation analysis was employed to determine thermal-neutron flux at the imaging position. The facility provided flux of thermal neutrons at 2.5× 105 cm−2s−1. Several standard samples were used to evaluate the collimation ratio (L/D), resolution, and unsharpness for the current setup of neutron imaging. An analytical method was employed to determine image quality parameters to moderate neutron flux. The L/D ratio depends on the aperture size of the divergent collimator and can be amended by using appropriate sample shape, resulting in more accurate estimate of collimation parameters. Resolution and unsharpness for neutron tomography were 0.5 mm and 1 mm, respectively. Results suggested the possibility of evaluating image quality using samples with non-complex structure.

Experimental Validation of Monte Carlo Simulation for the Leksell Gamma Knife Perfexion Using Gafchromic EBT3 Dosimetry Film and Diamond Detector T60019 PTW

This work presents experimental measurements performed using GAFChromic EBT3 dosimetry film and diamond detector T60019 to validate penEasy Monte Carlo based model developed previously for the Leksell Gamma Knife Perfexion. The relative dose profiles along X and Z coordinate axes, for each collimator size 4, 8, and 16 mm, and the relative output factors have been investigated. The penEasy model has been validated comparing the calculated output factors and dose profiles with the corresponding experimental results. The differences between the calculated output factors and the corresponding data measured with diamond detector were found to be 1.8 and 2.1% for the 8 and 4 mm collimator, respectively. Excellent agreement was found between penEasy and EBT3 results. The differences in the output factors were found to be 0.4% and −0.7% for the 8 and 4 mm collimators, respectively. The relative dose profiles were found to be in good agreement for both detectors and penEasy data. The gamma index test returns values (3%, 1 mm) < 1 for all profiles comparison calculated between penEasy and EBT3 film.

Impact of bowtie filter and detector collimation on multislice CT scatter profiles: A simulation study

To investigate via Monte Carlo simulations, the impact of scan subject size, antiscatter grid (ASG), collimator size, and bowtie filter on the distribution of scatter radiation in a typical realistically modeled third generation 16 slice diagnostic computed tomography (CT) scanner. Full radiation transport was simulated with Geant4 in a realistic CT scanner geometric model, including the imaging phantom, bowtie filter (BTF), collimators and detector assembly, except for the ASGs. An analytical method was employed to quantify the probable transmission through the ASG of each photon intersecting the detector array. Normalized scatter profiles (NSP) and scatter-to-primary-ratio (SPR) profiles were simulated for 90 and 140kVp beams for different size phantoms and slice thicknesses. The impact of CT scatter on the reconstructed attenuation coefficient factor was also studied as were the modulating effects of phantom- and patient-tissue heterogeneities on scatter profiles. A method to characterize the relative spatial frequency content of sinogram signals was developed to assess the latter. For the 21.4-cm diameter phantom, NSP and SPR increase linearly with collimator opening for both tube potentials, with the 90kVp scan exhibiting slightly larger NSP and SPR. The BTF modestly modulates scatter under the phantom center, reducing the prominent off-axis lobes by factors of 1.1-1.3. The ASG reduces scatter on the central axis NSP threefold, and reduces scatter at the detectors outside the phantom shadow by factors of 25 to 500. For the phantoms with diameters of 27 and 32cm, the scatter increases roughly three- and fourfold, respectively, demonstrating that scatter monotonically increases with phantom size, despite deployment of the ASG and BTF. In the absence of a scan subject, the ASG reduces the signal profile arising photons scattered by the BTF. Without ASG, the in-air scatter profile is relatively flat compared to the scatter profile when the ASG is present. For both 90 and 140 kVp photon spectra, the calculated attenuation coefficient decreases linearly with increasing collimation size. For both homogeneous and heterogeneous objects, NSPs are dominated by low spatial frequency content compared to the primary signal. However, the SPR, which quantifies the local magnitude of nonlinear detector response and is dominated by the high frequency content of the primary profile, can contribute strongly to high-spatial frequency streaking artifacts near high-density structures in reconstructed image artifacts. Public-domain Monte Carlo codes, Geant-4 in particular, is a feasible method for characterizing CT detector response to scattered- and off-focal radiation. Our study demonstrates that the ASG substantially reduces the scatter radiation and reshapes scatter-radiation profiles and affects the accuracy with which the detector array can measure narrow-beam attenuation due its inability to distinguish between true uncollided primary and narrow-angle coherently scattered photons. Hence, incorporating the impact of detector array collimation into the forward-projection signal formation models used by iterative reconstruction algorithms is necessary to use CT for accurately characterizing material properties. While tissue heterogeneities exercise a modest influence on local NPS shape and magnitude, they do not add significant high spatial frequency content.

The impact of treatment parameter variation on secondary neutron spectra in high-energy electron beam radiotherapy.

High-energy electron treatment procedures in radiotherapy pose a potential iatrogenic cancer risk as well as a critical health risk to patients with cardiac implantable electronic devices due to the generation of secondary neutrons in the linac head, the treatment room, and the patient. It may be argued that the neutron production from photons is well characterized, but the same is not true for electrons. Therefore, to assess the risk involved in an electron treatment, one must determine the neutron flux spectrum generated by the treatment procedure. The neutron spectrum depends on the treatment parameters used and therefore it is crucial to study its dependence on these parameters. In this work, eight experiments were devised to analyze how eight electron treatment parameters impacted the neutron spectrum. The parameters we considered were the electron beam energy, location of measurement, cutout size, collimator size, applicator size, collimator angle, choice of treatment room, and the presence or absence of a solid water phantom. For each experiment, we used a Nested Neutron Spectrometer™ (NNS) to measure the neutron flux spectra for multiple settings of the treatment parameter of interest. The resulting spectra were plotted and compared. We found that the electron beam energy and the location of measurement had the most impact on the neutron flux spectra, while the other parameters had a smaller or insignificant impact. This report may serve as a reference tool for medical physicists to help estimate the risk associated with a particular high-energy electron treatment procedure.

Efek Perubahan Dosis Radiasi Dan Kualitas Gambar Pada Hasil Radiograph Dengan Luas Kolimasi Berbeda

Background: The use of variations of collimation will provide a low dose to the patient and radiographic results that provide a good quality image. This research is to examine the effects of various collimation sizes on the image quality and radiation doses when using x-ray machine at Radiology laboratory of Al-Islam Polytechnic, Bandung. How much the change in density values that occur and to find out of scattering radiation received by the patient in the Radiology laboratory of Al-Islam Polytechnic, Bandung.Methods: This type of research is an experimental study with participatory observation data collection. at the Radiology Laboratory of the Al-Islam Polytechnic, Bandung The independent variable is variations of collimation, the dependent variables are quality image and doses radiation. Data collection was carried out by exposing a Stepwedge stored on a 35 x 35 cm radiographic cassette with different irradiation areas and pendose as a measure of radiation. Radiograph results were analyzed by a densitometer. Then the results will be analyzed using digraph to know the movement of each different irradiation field area.Results: The results showed that there were influences of changes in the variation of collimation. Density value and scattered radiation of each different irradiation field area produces are different values. The smaller area of the irradiation field. Is, the higher density value and low dose. Thus, to get the best density value and the low dose is required to reduce the width of the collimation field conducted.Conclusions: There was an influence variation of collimation on the quality of the image and dose to the patient.

Verification of dose distribution on the gamma knife perfexion radiosurgery using gafchromic EBT3 film: RANDO phantom study

Leksell gamma knife (LGK) is an advanced modality of radiation therapy sourced Co-60 radioactive for treating patient with intracranial lesion. Small field techniques with highly integrated radiation delivering to patients in single session must be calculated accurately and verified carefully. This study illustrates a procedure to verify the accuracy of dose distribution associated with Leksell Gamma Plan (LGP) using RANDO phantom and gafchromic EBT3 film dosimetry. First, we assessed the profile dose on LGK standard phantom with collimators size 4, 8 and 16 mm and compared the results with the profile dose based on RANDO to obtained Full Width Half Maximum (FWHM) and beam-symmetry. Absorbed-dose distributions on RANDO with various combinations of lesion volume, collimator size, location and number of shots assessed by EBT3 film using LGK Perfexion. Scanned images of the measured films were processed following standard EBT3 film-handling procedures. Dose distribution calculation were performed using ImageJ software and MATLAB in-house software. The study shows different FHWM and beam symmetry of collimator size of 16 mm on standard phantom between LGP and measurement is 1.83 % and 1.58 % respectively whereas the discrepancy in RANDO phantom is 2.15 %, and 1.64 % respectively. Verification of max dose shows the smallest discrepancy on collimator size 16 single shot is 1.9 % on 7.61 cc superior lesion volume, where 12.12 Gy obtained by measurement and 12.36 Gy from LGP. In conclusion, collimator size 16 mm have a highest accuracy of FWHM and beam-symmetry value. However, on smaller lesion volume, collimator 16 mm with single shot give higher deviation dose value.

Dose evaluation of organs at risk during treatment using gamma knife stereotactic radiosurgery (GKSRS): phantom study

Gamma Knife Stereotactic Radiosurgery (GKSRS) treatment requires the high accuracy of delivering single high dose without negative impact on the surrounding tissue. During radiosurgery treatments using GKSRS, the surrounding tissue will receive high scatter radiation because the single dose treatment will employed long irradiation time. Therefore, radiation doses in critical organs of patients during GKSRS treatment is important to be evaluated. In this study, we investigated the radiation dose to the lens, thyroid, breast, uterine fundus, ovaries and testes during treatment using Leksell Gamma Knife Perfexion (LGK; Elekta AB, Stockholm, Sweden). The dose was simulated using the variation of tumour target from 5 to 20 cc. The measurement were performed using variation of collimator size of 4 mm, 8 mm, 16 mm and employed thermoluminescent dosimeters (TLDs) to be placed at the surface of Rando anthropomorphic phantom. TLDs reader Harshaw model 3500 were employed to evaluate the measurement result. The result of this study explains that the dose of organs at risk decrease linearly depend on the distance of the target, target volume and the size of collimator.

Investigating the impact of collimator size variation on the single beam radiation of Gamma Knife PerfexionTM based on Monte Carlo simulation

The Gamma Knife PerfexionTM (Elekta Oncology Systems) is a specialized stereotactic radiotherapy device that uses 192 Cobalt-60 radioactive sources. A dynamic collimator system enables three circular field sizes of 4 mm, 8 mm, and 16 mm diameter to be delivered. This study investigates the dosimetric characteristics of the three different sized collimated beams using Monte Carlo (MC) simulations. The MC simulation made use of previously calculated phase space from a Monte-Carlo model of the encapsulated Cobalt-60 sources. The collimators were modelled using the FLATFILT component module of the BEAMnrc user code. The output of the single beam radiation was stored in a circular scoring plane with a radius of 1.4 cm at source surface distance (SSD) 17.20 cm in the phase space file. The simulation parameters were used 2.1 x 109 particles, electron cut off energy (ECUT) of 0.7 MeV, and photon cut off energy (PCUT) of 0.01 MeV. To investigate the physical full width at half maximum (FWHM) and penumbra width (80-20%), the single beam profiles and mean energy were obtained using BEAMnrc/BEAMDP code. Based on the results that there was an increase of the full width at half maximum (FWHM) with the size of the collimator on the beam profile and the mean energy. The FHWM of the energy fluence profile for the collimator 4, 8, and 16 mm are 2.22, 3.90, and 7.80 mm, respectively. The FHWM of the fluence profile for the collimator sizes 4, 8, and 16 mm are 1.87, 3.87, and 7.80 mm. The FHWM of mean energy as a function of position for the collimator sizes 4, 8, and 16 mm are 2.56, 4.20, and 7.94 mm, respectively. The maximum mean energy for the collimators decreases as much as 0.0185 MeV.

Online dose delivery verification in small animal image-guided radiotherapy.

To accomplish novel radiation treatment techniques in preclinical radiation research, small animal image-guided radiotherapy systems have been increasingly integrated into preclinical radiation research over the last decade. Although such systems have sophisticated tools (such as cone-beam computed tomography-based image guidance, robotic couch, treatment planning system (TPS), and electronic portal imaging devices [EPIDs]). To our knowledge, no established technique can perform independent and online verification of the delivered dose during radiotherapy. In this study, we implement an online EPID dosimetry for each administered SA-IGRT fraction in a rat orthotopic model of prostate cancer. To verify the accuracy of delivered dose to rat, we compared the two-dimensional (2D) calculated dose distribution by TPS as the planned dose, with online dose distribution estimated using an EPID as the delivered dose. Since image acquisition software was not capable of acquiring integrated images over a long period of time, we used the EPID to estimate dose rate rather than dose. The central axis (CAX) dose rate values at the beam's exit surface were compared. In addition, 2D dose distributions were also compared under different gamma criteria. To verify the accuracy of our EPID dosimetry technique, we measured transit and exit doses with film during animal treatment. In this study, 20-mm cone was used to collimate beam. We previously observed that the EPID response was independent of collimator size for collimator size ≥15-mm, we did not apply for additional correction factor. Comparison of exit CAX dose rate values of TPS-calculated and EPID-estimated showed that the average difference was 3.1%, with a maximum of 9.3%. Results of gamma analysis for 2D comparison indicated an average of 90% passing rate with global gamma criterion of 2mm/5%. We observed that TPS could not calculate dose accurately in peripheral regions in which the penumbra effect was dominant. Dose rate values estimated by EPID were within 2.1% agreement with film at both the imager plane and the beam's exit surface for 4 randomly selected animals for which film measurement verification was performed. The small animal radiation research platform (SARRP) system's built-in EPID was utilized to estimate dose delivered to rats at kilovoltage energy x-rays. The results of this study suggest that the EPID is an invaluable tool for verifying delivered dose to small animal to help validate conclusions made from preclinical radiation research.

Эффективный способ моделирования системы Leksell Gamma Knife Perfexion методом поворота в файле фазового пространства

Purpose: To develop an effective method of Monte Carlo simulation of the GammaKnife Perfexion system by rotating particles in the phase space file (PSF). This method does not require simulating of all 192 sources that are distributed in the conical form of the Perfexion collimator. The simulation was performed only for 5 out of 192 sources for each collimator size.&#x0D; Material and methods: Monte Carlo simulation of dose distribution for previous models of GammaKnife system requires phase space file for only one source, since this phase space is identical for all the 201 sources. The Perfexion model is more complex due to the non-coaxial positions of the sources and the complexity of the collimator system itself.&#x0D; In this work, we present an effective method to simulate the Perfexion model using a phase space file. Penelope Monte Carlo code was used to perform this simulation. In this method, the PSF was obtained for one source in each ring, resulting in five files for each collimator size. PSF for other sources were created by azimuthal redistribution of particles, in the obtained PSF, by rotation around the Z-axis. The phase space files of the same ring were then stored together in a single file.&#x0D; Results: The paper presented MC simulation using the azimuthal redistribution of particles in the phase space file by rotation around the Z-axis. The simulation has been validated comparing the dose profiles and output factors with the data of the algorithm TMR10 planning system Leksell Gamma Plan (LGP) in a homogeneous environment. The acceptance criterion between TMR10 and Monte Carlo calculations for the profiles was based on the gamma index (GI). Index values more than one were not detected in all cases, which indicates a good agreement of results. The differences between the output factors obtained in this work and the TMR10 data for collimators 8 mm and 4 mm are 0.74 and 0.73 %, respectively.&#x0D; Conclusion: In this work successfully implemented an effective method of simulating the Leksell Gamma knife Perfexion system. The presented method does not require modeling for all 192 sources distributed in the conical form of the Perfexion collimator. The simulation was performed for only five sources for each collimator and their files PSF were obtained. These files were used to create the PSF files for other sources by azimuthal redistribution of particles, in these files, by rotation around the Z-axis providing correct calculations of dose distributions in a homogeneous medium for 16, 8 and 4 mm collimators.

Dosimetric evaluation of the Leksell GammaPlan™ Convolution dose calculation algorithm

The dosimetric accuracy of the Leksell GammaPlan Convolution calculation algorithm was evaluated through comparison with corresponding Monte Carlo (MC) dosimetric results. MC simulations were based on generated sector phase space files for the 4 mm, 8 mm and 16 mm collimator sizes, using a previous comprehensive Gamma Knife Perfexion™ source model and validated using film dosimetry. Test cases were designed for the evaluation of the Convolution algorithm involving irradiation of homogeneous and inhomogeneous phantom geometries mimicking clinical cases, with radiation fields created using one sector (single sector), all sectors with the same (single shot) or different (composite shot) collimator sizes. Dose calculations using the Convolution algorithm were found to be in excellent agreement (gamma pass rate greater than 98%, applying 1%/1 mm local dose difference and distance agreement criteria), with corresponding MC calculations, indicating the accuracy of the Convolution algorithm in homogeneous and heterogeneous model geometries. While of minor clinical importance, large deviations were observed for the voxels laying inside air media. The calculated beam on times using the Convolution algorithm were found to increase (up to 7%) relative to the TMR 10 algorithm currently used in clinical practice, especially in a test case mimicking a brain metastasis close to the skull, in excellent agreement with corresponding MC calculations.

MR-Linac Collimator Size Limitation and Impact on the Treatment of GYN Patients

MR-Linac Collimator Size Limitation and Impact on the Treatment of GYN Patients

MCNP optimization of fast neutron beam thermalization device based on D-T neutron generator

The 14 MeV neutrons generated by Deuterium-Tritium (D-T) fusion reaction usually need to be moderated for Prompt Gamma Neutron Activation Analysis (PGNAA) applications. A fast neutron beam thermalization device was developed in Northeast Normal University based on a D-T neutron generator (model NG-9). A simple experimental system was set up firstly to validate the feasibility of simulation. The materials and dimensions of moderator and the size of multiplier, reflector and collimator were then optimized through MCNP5 to improve the efficiency of the thermalization device. The thermal, epithermal, fast and total neutron flux across the output surface in final system were increased for a factor of 9.65, 8.70, 5.84 and 6.94 times compared to the initial experimental system, respectively. And the optimized thermalization efficiency is 13.28 times higher than that of the original model of experimental system.

A new Monte Carlo model of a Cyberknife® system for the precise determination of out-of-field doses

In a previous work, a PENELOPE Monte Carlo model of a Cyberknife system equipped with fixed collimator was developed and validated for in-field dose evaluation. The aim of this work is to extend it to evaluate peripheral doses and to determine the precision of the treatment planning system (TPS) Multiplan in evaluating the off-axis doses.The Cyberknife® head model was completed with surrounding components based on manufacturer drawings. The contribution of the different head parts on the out-of-field dose was studied. To model the attenuation and the modification of particle energy caused by components not modelled, the photon transport was modified in one of the added components. The model was iteratively adjusted to fit dose profiles measured with EBT3 films and an ionization chamber for several collimator sizes. Finally, dose profiles were calculated using the two Multiplan TPS algorithms and were compared to our simulations.The contributions to out-of-field dose were identified as scattered radiation from the phantom and head leakage and scatter originating at the secondary collimator level. Particle transport in the additional pieces was modified to model this radiation. The maximum differences between simulated and measured doses are of 20.4%. Regarding the detector responses away from axis, EBT3 films and the Farmer chamber give similar response (less than 20% difference). The TPS Monte Carlo algorithm underestimates the doses away from axis more importantly for the smaller field sizes (up to 98%). Besides, RayTracing simplifies peripheral dose to a constant value with no inclusion of particle transport.A Monte Carlo model of a Cyberknife system for the determination of out-of-field doses up to 14 cm off-axis was successfully developed and validated for different depths and field sizes in comparison with measurements. This study also confirms that TPS algorithms do not model peripheral dose properly.

Locating events in a segment of a segmented HPGe detector at KRISS

Abstract A specific segment of the 24 fold segmented HPGe detector was studied for improving the interaction position resolution from several cm to mm. The segmented HPGe detector was modeled by CAD program with their electrodes which was isolated by the Boolean function. The Shockley-Ramo theorem changed the Gauss law to Laplace equation problem for calculating the induced charge on each electrode. Finite Element Method solved the Poisson and Laplace equation with their boundary conditions for Electric field and potential. The drift velocities of charge carriers were compared between both along the 100 > and one between 100 > and 110 > Ge crystal axis at a specific segment and used to simulate the pulse shapes with 1 mm position resolution. Coincidence system between a specific segment and 3 × 3 HPGe detector verified the simulated pulse shapes by comparing these with the experimental ones which are generated at the specific target region with a fixed scattering angle. The experimental and simulated ones were χ 2 fitted to find the least χ 2 value and locate the interaction positions. The interaction positions of experimental pulse shapes were analyzed statistically after the rise time correction along 100 > and 110 > Ge crystal axis. The position resolution as FWHM on x, y, and z axis is the 3.12, 4.38 and 8.69 mm with the collimator size 3, 6, and 6 mm, respectively.

Angular distribution of multiple Compton scattered gamma rays of 662 keV in tin

In multiple Compton scattering, the incident gamma photon is scattered more than once due to finite dimensions of the sample. It serves as a noise for the exact evaluation of electron momentum distribution in Compton profile, Compton cross section and non-destructive testing of in-assessable large samples. To overcome these difficulties, there is a need to characterise the contribution of multiple scattering as a function of atomic number, scattering angle and incident gamma photon energy. In the present work, measurements are carried out to investigate the multiple scattering of 662 keV photons with tin sample of different thickness in the forward and backward hemispheres. The scattered photon flux is recorded by a NaI(Tl) scintillation detector and with an HPGe solid-state detector to check for the trend of saturation thickness as a function of resolution of the detector. The number of multiply scattered photons is found to be increasing with the sample thickness up to a particular thickness value called saturation thickness beyond which it remains constant with increase in target thickness. The effect of detector collimator size on multiply scattered photons is also studied. Both the detectors show the same trend of saturation thickness as a function of thickness. Angular distribution of multiply scattered photons shows symmetry around 90° scattering angle. The low energy side of multiple scattered photon spectrums is also characterised as a function of sample thickness and order of scattering. Monte Carlo calculations support the present experimental results.