اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدMonte Carlo investigation into feasibility and dosimetry of flat Flattening Filter Free beams
91
0
0.0
(
0
)
تأليف
Sergei Zavgorodni
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Flattening filter free (FFF) beams due to their non-uniformity, are sub-optimal for larger field sizes. The purpose of this study was to investigate the incident electron beam distributions that would produce flat FFF beams without the use of flattening filter. Monte Carlo (MC) simulations with BEAMnrc and DOSXYZnrc codes have been performed to evaluate the feasibility of this approach. The dose distributions in water for open 6MV beams were simulated using Varian 21EX linac head model, which will be called flattening filter (FF) model. Flattening filter has then been removed from FF model, and MC simulations were performed using (1) 6 MeV electrons incident on the target, (2) 6 MeV electron beam with electron angular distributions optimized to provide as flat dose profiles as possible. Configuration (1) represents FFF beam while configuration (2) allowed producing a flat FFF (F4) beam. Optimizations have also been performed to produce flattest profiles for a set of dose rates (DRs) in the range from 1.25 to 2.4 of the DR of FF beam. Profiles and percentage depth doses PDDs from 6MV F4 beams have been calculated and compared to those from FF beam. Calculated profiles demonstrated improved flatness of the FFF beams. In fact, up to field sizes within the circle of 35 cm diameter the flatness of F4 beam at dmax was better or comparable to that of FF beam. At 20 cm off-axis the dose increased from 52% for FFF to 92% for F4 beam. Also, profiles of F4 beams did not change considerably with depth, and for large fields out-of-field dose was reduced by about a factor of two compared to FF beam. PDDs from F4 beams were similar to those of FFF beam. The DR for the largest modeled (44 cm diameter) F4 beam was higher than the DR from FF beam by a factor of 1.25. It was shown that the DR can be increased while maintaining beam flatness, but at the cost of reduced field size.
قيم البحث
اقرأ أيضاً
Suppressing the effects of scattered radiation in flat panel detector, FPD, based CBCT still remains to be a challenge. To address the scatter problem, we have been investigating the feasibility of a two dimensional antiscatter grid (2D ASG) concept
for FPDs. Although a 2D ASG can potentially provide high scatter rejection capability, primary transmission characteristics of a 2D ASG and its implications in image quality plays a more critical role in implementation of the 2D ASG concept. Thus, in this work, a computational model was developed to investigate the primary transmission properties of the 2D ASG for various grid and FPD pixel geometries, and the improvement in signal to noise ratio,SNR, was calculated analytically to demonstrate the impact of 2D ASGs transmission characteristics on image quality. Computational model showed that average primary transmission fraction (Tp) strongly depends on the septal thickness of 2D ASG, and 2D ASG can provide higher Tp than existing radiographic ASGs at a septal thickness of 0.1 mm. Due to the higher Tp, 2D ASG was also predicted to provide SNR improvements in projections in low to moderate scatter environments typically observed in CBCT imaging. On the other hand, the model also indicated that the shadow or footprint of the 2D ASG leads to spatially nonuniform variations in primary signal in FPD pixels. Reduction of septal thickness and optimization of 2D ASGs pitch may play an essential role in reducing such variations in primary image signal, and avoiding potential image artifacts associated with 2D ASGs footprint.
Purpose: To investigate the validity of two Monte Carlo simulation absolute dosimetry approaches in the case of a small field dedicated `D-shaped collimator used for the retinoblastoma treatment with external photon beam radiotherapy. Methods: The
Monte Carlo code {sc penelope} is used to simulate the linac, the dedicated collimator and a water phantom. The absolute doses (in Gy per monitor unit) for the field sizes considered are obtained within the approach of Popescu {it et al.} in which the tallied backscattered dose in the monitor chamber is accounted for. The results are compared to experimental data, to those found with a simpler Monte Carlo approximation for the calculation of absolute doses and to those provided by the analytical anisotropic algorithm. Our analysis allows for the study of the simulation tracking parameters. Two sets of parameters have been considered for the simulation of the particle transport in the linac target. Results: The change in the tracking parameters produced non-negligible differences, of about 10% or larger, in the doses estimated in reference conditions. The Monte Carlo results for the absolute doses differ from the experimental ones by 2.6% and 1.7% for the two parameter sets for the collimator geometries analyzed. For the studied fields, the simpler approach produces absolute doses that are statistically compatible with those obtained with the approach of Popescu {it et al.} The analytical anisotropic algorithm underestimates the experimental absolute doses with discrepancies larger than those found for Monte Carlo results. Conclusions: The approach studied can be considered for absolute dosimetry in the case of small, `D-shaped and off-axis radiation fields. However, a detailed description of the radiation transport in the linac target is mandatory for an accurate absolute dosimetry.
The diagrammatic Monte Carlo (Diag-MC) method is a numerical technique which samples the entire diagrammatic series of the Greens function in quantum many-body systems. In this work, we incorporate the flat histogram principle in the diagrammatic Mon
te method and we term the improved version Flat Histogram Diagrammatic Monte Carlo method. We demonstrate the superiority of the method over the standard Diag-MC in extracting the long-imaginary-time behavior of the Greens function, without incorporating any a priori knowledge about this function, by applying the technique to the polaron problem
Results of a Monte Carlo code intercomparison exercise for simulations of the dose enhancement from a gold nanoparticle (GNP) irradiated by X-rays have been recently reported. To highlight potential differences between codes, the dose enhancement rat
ios (DERs) were shown for the narrow-beam geometry used in the simulations, which leads to values significantly higher than unity over distances in the order of several tens of micrometers from the GNP surface. As it has come to our attention that the figures in our paper have given rise to misinterpretation as showing the DERs of GNPs under diagnostic X-ray irradiation, this article presents estimates of the DERs that would have been obtained with realistic radiation field extensions and presence of secondary particle equilibrium (SPE). These DER values are much smaller than those for a narrow-beam irradiation shown in our paper, and significant dose enhancement is only found within a few hundred nanometers around the GNP. The approach used to obtain these estimates required the development of a methodology to identify and, where possible, correct results from simulations whose implementation deviated from the initial exercise definition. Based on this methodology, literature on Monte Carlo simulated DERs has been critically assessed.
We study the propagation of nucleons and nuclei in tissue-like media within a Monte Carlo Model for Heavy-ion Therapy (MCHIT) based on the GEANT4 toolkit (version 8.2). The model takes into account fragmentation of projectile nuclei and secondary int
eractions of produced nuclear fragments. Model predictions are validated with available experimental data obtained for water and PMMA phantoms irradiated by monoenergetic carbon-ion beams. The MCHIT model describes well (1) the depth-dose distributions in water and PMMA, (2) the doses measured for fragments of certain charge, (3) the distributions of positron emitting nuclear fragments produced by carbon-ion beams, and (4) the energy spectra of secondary neutrons measured at different angles to the beam direction. Radial dose profiles for primary nuclei and for different projectile fragments are calculated and discussed as possible input for evaluation of biological dose distributions. It is shown that at the periphery of the transverse dose profile close to the Bragg peak the dose from secondary nuclear fragments is comparable to the dose from primary nuclei.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
