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Comparison of measured and calculated PDD curves. 6MV photon beam and different circular cone sizes. For depiction reasons, PDD curves for cones 5, 10 and 30 mm were scaled with 0.6, 0.8 and 1.2 respectively.
Source publication
Background:
In stereotactic radiosurgery, sharp beam edges have clear advantages to spare normal tissues. In general, the dose gradient is a limiting factor in minimizing dose to nearby critical structures for clinical cases. Therefore the penumbral width should be diminished.
Methods:
A Varian Clinac 2100 linear accelerator equipped with in-hou...
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Stereotactic Radiosurgery (SRS): is a special form of radiation therapy-it`s not a form of open surgery. Unlike open surgery, SRS is outpatient, relatively painless and non-invasive treatment stream, which delivering prescribed high doses of radiation to precisely located volumes of approximately 0.6 to more than 10.0 ml within the brain using high...
Citations
... Increased dose rates allow for faster delivery, reduce the impact of patient and/or target movement, and improve patient comfort. [10]. The advantage of FFF beams has been shown clearly in small fields, especially for stereotactic radiotherapy/radiosurgery (SRT / SRS) treatment, which is suitable for volumemodulated arc radiotherapy (VMAT). ...
Introduction: This work aimed to evaluate the accuracy of using parallel plane against thimble chambers in beam data commissioning of the high dose gradients region for versa HD linear accelerator performing clinical advanced modulated radiation treatment techniques. Material and Methods: All clinical commissioning data were collected from Elekta Versa HD for energies of 6 MV, 10 MV, 6 MV FFF, and 10 MV FFF for different field sizes using thimble ionization chamber CC13, some from the pool of the measured data were rescanned using parallel plate chamber PPC05 and Gafchromic films and compared to those collected using the thimble ionization chamber. Results: The skin doses differences measured by thimble chamber against reference films were (0.8%, 0.5%, 1.2% 4.7%) and for the parallel plane chamber against films were (8.4%, 9.7%, 9%, 12%) for 6 MV, 10 MV, 6 MV FFF, 10 MV FFF, respectively. The parried test-test showed a highly significant difference (p> 0.001) between the two chambers in measurements of penumbra regions taking over all the investigated field sizes and depths in both inline and crossline datasets. The parallel plate showed a wider and broader penumbra than the thimble chamber and films. Conclusion: Robust and consistent scans were obtained for the thimble chamber compared to the parallel plane chamber in the highest dose gradient of buildup and penumbra regions. Using a parallel plane chamber might bring dosimetric clinical uncertainties affecting the modeling of the gradient regions in the treatment planning system.
... Our results reported for the ionization chamber ( Figures 6-9) were in close agreement with the K NR values reported by Wulff [12] and da Rosa [30] calculated by Monte Carlo simulation. However, the increasing response estimated at MV energy ranges in our study is likely due to the relativistic density effect which affects the stopping power of water more than that of air [31,32]. In other experiments in which TLD-100 (LiF:Mg,Ti) chips have been served as the reference detectors to observe the changes of ionization chamber responses, similar findings to ours are reported [28,30]. ...
Background:
Estimating dosimetric parameters for small fields under non-reference conditions leads to significant errors if done based on conventional protocols used for large fields in reference conditions. Hence, further correction factors have been introduced to take into account the influence of spectral quality changes when various detectors are used in non-reference conditions at different depths and field sizes.
Objective:
Determining correction factors (KNR and KNCSF) recommended recently for small field dosimetry formalism by American Association of Physicists in Medicine (AAPM) for different detectors at 6 and 18 MV photon beams.
Methods:
EGSnrc Monte Carlo code was used to calculate the doses measured with different detectors located in a slab phantom and the recommended KNR and KNCSF correction factors for various circular small field sizes ranging from 5-30 mm diameters. KNR and KNCSF correction factors were determined for different active detectors (a pinpoint chamber, EDP-20 and EDP-10 diodes) in a homogeneous phantom irradiated to 6 and 18 MV photon beams of a Varian linac (2100C/D).
Results:
KNR correction factor estimated for the highest small circular field size of 30 mm diameter for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.993, 1.020 and 1.054; and 0.992, 1.054 and 1.005 for the 6 and 18 MV beams, respectively. The KNCSF correction factor estimated for the lowest circular field size of 5 mm for the pinpoint chamber, EDP-20 and EDP-10 diodes were 0.994, 1.023, and 1.040; and 1.000, 1.014, and 1.022 for the 6 and 18 MV photon beams, respectively.
Conclusion:
Comparing the results obtained for the detectors used in this study reveals that the unshielded diodes (EDP-20 and EDP-10) can confidently be recommended for small field dosimetry as their correction factors (KNR and KNCSF) was close to 1.0 for all small field sizes investigated and are mainly independent from the electron beam spot size.
... Yarahmadi et al. [40] evaluated the penumbral width of the VarianClinac2100 linear accelerator using the EGSnrc/ BEAMnrc Monte Carlo code and reported a 0.3 mm /0.2 mm reduction in penumbra for 10×10 cm 2 /20×20 cm 2 fields by removing the flattening filter. Comparing this with our work this reduction was 0.1 mm less than our results for 10×10 cm 2 field size while for the 20×20 cm 2 field size had a similar value. ...
The aim of our work is to provide the up-to-now missing information on the Siemens Artiste FFF 7 MV beam line using a Monte-Carlo model fit to the realistic dosimetric measurements at the linear accelerator in clinical use at our department. The main Siemens Artiste 6MV and FFF 7MV beams were simulated using the Geant4 toolkit. The simulations were compared with the measurements with an ionization chamber in a water phantom to verify the validation of simulation and tuning the primary electron parameters. Hereafter, other parameters such as surface dose, spectrum, electron contamination, symmetry, flatness/unflatness, slope, and characteristic off-axis changes were discussed for both Flat and FFF mode. The mean electron energy for the FFF beam was 8.8 MeV and 7.5 MeV for Flat 6 MV, the spread energy and spot size of the selected Gaussian distribution source were 0.4 MeV and 1mm, respectively. The dose rate of the FFF beam was 2.8 (2.96) times higher than for the flattened beam for a field size of 10×10 (20×20) cm². The electron contamination has significant contribution to the surface dose especially for the flattened beam. The penumbra, surface dose and the mean energy of photons decrease by removing the flattening filter. Finally, the results show that off-axis changes have no strong effect on the mean energy of FFF beams, while this effect was more considerable for the flattened beam.
... Here, r 0 is the PTV radius and the normalization point at the same time. In comparison, the GI for isocentric and isotropic pencil beams or a point source under assumption of no absorption leads to GI ¼ Circular cones dedicated to linac stereotactic radiosurgery (SRS), as found in Ref.[22][23][24] vary in their penumbra properties and moreover were characterized under different conditions. Yarahmadi et al.(Fig. ...
Purpose:
To quantify the contribution of penumbra in the improvement of healthy tissue sparing at reduced source-to-axis distance (SAD) for simple spherical target and different prescription isodoses (PI).
Method:
A TPS-independent method was used to estimate three-dimensional (3D) dose distribution for stereotactic treatment of spherical targets of 0.5 cm radius based on single beam two-dimensional (2D) film dosimetry measurements. 1 cm target constitutes the worst case for the conformation with standard Multi-Leaf Collimator (MLC) with 0.5 cm leaf width. The measured 2D transverse dose cross-sections and the profiles in leaf and jaw directions were used to calculate radial dose distribution from isotropic beam arrangement, for both quadratic and circular beam openings, respectively. The results were compared for standard (100 cm) and reduced SAD 70 and 55 cm for different PI.
Results:
For practical reduction of SAD using quadratic openings, the improvement of healthy tissue sparing (HTS) at distances up to 3 times the PTV radius was at least 6%-12%; gradient indices (GI) were reduced by 3-39% for PI between 40% and 90%. Except for PI of 80% and 90%, quadratic apertures at SAD 70 cm improved the HTS by up to 20% compared to circular openings at 100 cm or were at least equivalent; GI were 3%-33% lower for reduced SAD in the PI range 40%-70%. For PI = 80% and 90% the results depend on the circular collimator model.
Conclusion:
Stereotactic treatments of spherical targets delivered at reduced SAD of 70 or 55 cm using MLC spare healthy tissue around the target at least as good as treatments at SAD 100 cm using circular collimators. The steeper beam penumbra at reduced SAD seems to be as important as perfect target conformity. The authors argue therefore that the beam penumbra width should be addressed in the stereotactic studies.
... This is caused by the fact that imparting a high dose in the SRS method leads to a high dose penumbra region. 6,7 This penumbra results in considerable out-of-field dose in tumour-adjacent normal tissues especially in critical structures and also reduces the dose homogeneity in the radiation filed. 8 Therefore, due to these challenges, penumbra width measurement and considering it, is very important to estimate the dose and possible radiation damage to the organs at risk, which are located in the penumbra region and then achieve the appropriate treatment plan. ...
... Due to the similarity of profiles along both X and Y axes, only X-profiles were considered. 7 The irradiations were performed three times for each collimator size to calculate the measurement uncertainty. ...
Background
One of the stereotactic radiosurgery techniques is Gamma Knife radiosurgery, in which intracranial lesions that are inaccessible or inappropriate for surgery are treated using 201 cobalt-60 sources in one treatment session. In this conformal technique, the penumbra width, which results in out-of-field dose in tumour-adjacent normal tissues should be determined accurately. The aim of this study is to calculate the penumbra widths of single and 201 beams for different collimator sizes of Gamma Knife machine model 4C using EGSnrc/BEAMnrc Monte Carlo simulation code and comparison the results with EBT3 film dosimetry data.
Methods and materials
In this study, simulation of Gamma Knife machine model 4C was performed based on the Monte Carlo codes of EGSnrc/BEAMnrc. To investigate the physical penumbra width (80−20%), the single beam and 201 beams profiles were obtained using EGSnrc/DOSXYZnrc code and EBT3 films located at isocentre point in a spherical Plexiglas head phantom.
Results
Based on the results, the single beam penumbra widths obtained from simulation data for 4, 8, 14 and 18 mm collimator sizes along X axis were 0·75, 0·77, 0·90 and 0·92 mm, respectively. The data for 201 beams obtained from simulation were 2·61, 4·80, 7·92 and 9·81 mm along X axis and 1·31, 1·60, 1·91 and 2·14 mm along Z axis and from film dosimetry were 3·21, 4·90, 8·00 and 10·61 mm along X axis and 1·22, 1·69, 2·01 and 2·25 mm along Z axis, respectively.
Conclusion
The differences between measured and simulated penumbra widths are in an acceptable range. However, for more precise measurement in the penumbra region in which dose gradient is high, Monte Carlo simulation is recommended.
... Certainly, there is no standard dosimeter for small fields because no detector has all the aforementioned properties. Commonly used dosimeters in small fields are ionization chambers, [23,24] films, [25] thermoluminescent dosimeters (TLDs), [26] polymer gels, [27] metal oxide semiconductor field effect transistors (MOSFETs), [28] diamond detectors, [29] silicon diodes, [30] alanine dosimeters, [31] and Monte Carlo (MC) simulations, [32,33] among others. The advantages and disadvantages of these detectors will be discussed next. ...
With the advent of complex and precise radiation therapy techniques, the use of relatively small fields is needed. Using such field sizes can cause uncertainty in dosimetry; therefore, special attention is required both in dose calculations and measurements. There are several challenges in small-field dosimetry such as the steep gradient of the radiation field, volume averaging effect, lack of charged particle equilibrium, partial occlusion of radiation source, beam alignment, and unable to use a reference dosimeter. Due to these challenges, special dosimeters are needed for small-field dosimetry, and this review article discusses this topic.
... Certainly, there is no standard dosimeter for small fields because no detector has all the aforementioned properties. Commonly used dosimeters in small fields are ionization chambers, [23,24] films, [25] thermoluminescent dosimeters (TLDs), [26] polymer gels, [27] metal oxide semiconductor field effect transistors (MOSFETs), [28] diamond detectors, [29] silicon diodes, [30] alanine dosimeters, [31] and Monte Carlo (MC) simulations, [32,33] among others. The advantages and disadvantages of these detectors will be discussed next. ...
With the advent of complex and precise radiation therapy techniques, the use of relatively small fields is needed. Using such field sizes can cause uncertainty in dosimetry; therefore, special attention is required both in dose calculations and measurements. There are several challenges in small-field dosimetry such as the steep gradient of the radiation field, volume averaging effect, lack of charged particle equilibrium, partial occlusion of radiation source, beam alignment, and unable to use a reference dosimeter. Due to these challenges, special dosimeters are needed for small-field dosimetry, and this review article discusses this topic. © 2018 Journal of Medical Signals & Sensors | Published by Wolters Kluwer - Medknow
... For the other vendors, recently Sangeetha et al.[61] reported a relative dose rate of 2.52 (2.60) for the 10×10 cm² (20×20 cm²) field for a Varian 600C/D and Dalaryd et al.[16], observed that the dose rate of the FFF beams increased by a factor of 2.23 for an ElektaPrecise. Notably, the values for our Siemens Artiste were larger than previous ones with dose rate of 2.8 (2.96) for the 10×10 cm² (20×20 cm²) fields.Yarahmadi et al.[78] evaluated the penumbral width of the VarianClinac2100 linear accelerator using the EGSnrc/ BEAMnrc Monte Carlo code and reported a 0.3 mm /0.2 mm reduction in penumbra for 10×10 cm²/20×20 cm² fields by removing the flattening filter.Comparing this with our work this reduction was 0.1 mm less than our results for 10×10 cm² field size while for the 20×20 cm² field size had a similar value.Our obtained relative surface dose of the flat 6 MV beam line tended to decrease by about 21% (22%) for 10×10 cm² (20×20 cm²) field size when the flattening filter was removed from the beam line. Contrarily, Mohammed et al.[46] showed an increase in relative surface dose by about 19 % (12%) for 10×10 cm² ( 20 × 20 cm²) field size by removing the flattening filter from a Varian 2100 linac included to low energy. ...
... A similar result was reported by different investigators. 32,33 Yarahmadi et al. 34 used EGSnrc/BEAMnrc MC code to calculate output factors for in-house SRS cones with different field sizes. However, they have considered SRS cone with hole diameter of 30 mm as a reference field size. ...
The Geant4 toolkit was used to develop a Monte Carlo (MC)-based engine for accurate dose calculations in small radiation field sizes. The Geant4 toolkit (version 10.1.p02) was used to simulate 6-MV photon beam of a Varian2100C linear accelerator that is being used for stereotactic radiosurgery (SRS) treatment with small radiation fields. Geometric models of 3 in-house designed radiosurgical divergent cones, with the diameters of their projections at the isocenter being 10, 20, and 30 mm, were simulated. The accuracy of the MC simulation technique was examined by reproducing several different simulated dosimetric parameters of the primary beams with the experimental data. The dose distributions are first checked for single beams for each cone, then standard multiple field (SMF) techniques are applied. A sample set of DICOM files from computed tomography (CT) scan imaging of a patient's head was converted to the Geant4 geometry format to implement MC-based engine for a clinical test. To validate the accuracy of the MC-based calculations for SMF arrangements, the isodose lines from MC simulation in water phantom were compared with the measured isodose lines using EBT3 Gafchromic film in Solid Water phantoms. Agreements between measured and simulated depth dose values and beam profiles for SRS cones were generally within 2%/2 mm. For output factors, the largest discrepancy was observed for 10 mm SRS cone, which was 1.7%. For SMF techniques, in SRS cones, the MC simulation and EBT3 Gafchromic film dosimetry were in acceptable agreement (5%/5 mm). Excellent agreement between the results of the MC-based and measured dose values for both single and SMF techniques in SRS cones indicates the ability of the Geant4 toolkit to be applied as the platform for treatment planning of advanced radiotherapy techniques.
... [1][2][3] Because of the very small size of the sensitive volume, diodes are increasingly used for small-field dosimetry in modern radiotherapy techniques such as stereotactic radiosurgery (SRS), Cyberknife, Gammaknife, Tomotherapy and intensity-modulated radiotherapy (IMRT). [4][5][6] In general, a diode has a high spatial resolution and high sensitivity, but its response is energy-dependent. 7 Silicon detectors over-respond to photons of low energy because of the increasing cross-section of the photoelectric effect in silicon compared to water. ...
Purpose:
To investigate the energy dependence/spectral sensitivity of silicon diodes designed for small-field dosimetry and obtain response factors (RFs) for arbitrary photon spectra using Monte Carlo (MC) simulations.
Methods:
The EGSnrc user-code DOSRZnrc was used to calculate the dose deposition in water and in the active volume of a stereotactic diode field detector (SFD). Then the RFs of the SFD were calculated for several circular field sizes and energies at 5 cm depth in water. Several low-energy photon spectra (mean energy 55 to 200 keV), as well as Co-60 radiation (mean energy 1.25 MeV) and a 6 MV Elekta Synergy beam (mean energy 2.9 MeV), in 10×10 cm(2) field size were used to validate the MC calculations, using a simple beam model. The RFs of the SFD detector for a 6 MV Elekta Synergy linac photon beam in different field sizes were calculated. These were also measured with an EBT3 Gafchromic film and the SFD detector.
Results:
For the reference field size differences between measured and calculated RFs were less than 5% at mean energies below 1 MeV and less than 1% at energies above 1 MeV. The calculated RFs for a 6 MV Elekta Synergy linac photon beam as a function of different field sizes showed a good agreement between the measurements and previously reported results. This agreement was within 2% for all considered field sizes.
Conclusion:
While at high photon energies the change of response of the SFD is marginal, it is extreme at low energies. Therefore, it is desirable to benchmark response calculations also in the low energy domain. Our results, with a simple beam model and geometry, indicate that a validation of the simulations by experimental results is achievable. The present work provides a comprehensive table that can be used to calculate SFD detector response factors depending on both, field size and photon energy. This article is protected by copyright. All rights reserved.
