The Experts below are selected from a list of 23403 Experts worldwide ranked by ideXlab platform

Oleg N Vassiliev - One of the best experts on this subject based on the ideXlab platform.

  • validation of a new grid based boltzmann equation solver for dose calculation in radiotherapy with Photon Beams
    Physics in Medicine and Biology, 2010
    Co-Authors: Oleg N Vassiliev, Mohammad Salehpour, Todd A. Wareing, Gregory Failla, John M. Mcghee, Firas Mourtada
    Abstract:

    A new grid-based Boltzmann equation solver, Acuros™, was developed specifically for performing accurate and rapid radiotherapy dose calculations. In this study we benchmarked its performance against Monte Carlo for 6 and 18 MV Photon Beams in heterogeneous media. Acuros solves the coupled Boltzmann transport equations for neutral and charged particles on a locally adaptive Cartesian grid. The Acuros solver is an optimized rewrite of the general purpose Attila© software, and for comparable accuracy levels, it is roughly an order of magnitude faster than Attila. Comparisons were made between Monte Carlo (EGSnrc) and Acuros for 6 and 18 MV Photon Beams impinging on a slab phantom comprising tissue, bone and lung materials. To provide an accurate reference solution, Monte Carlo simulations were run to a tight statistical uncertainty (σ ≈ 0.1%) and fine resolution (1–2 mm). Acuros results were output on a 2 mm cubic voxel grid encompassing the entire phantom. Comparisons were also made for a breast treatment plan on an anthropomorphic phantom. For the slab phantom in regions where the dose exceeded 10% of the maximum dose, agreement between Acuros and Monte Carlo was within 2% of the local dose or 1 mm distance to agreement. For the breast case, agreement was within 2% of local dose or 2 mm distance to agreement in 99.9% of voxels where the dose exceeded 10% of the prescription dose. Elsewhere, in low dose regions, agreement for all cases was within 1% of the maximum dose. Since all Acuros calculations required less than 5 min on a dual-core two-processor workstation, it is efficient enough for routine clinical use. Additionally, since Acuros calculation times are only weakly dependent on the number of Beams, Acuros may ideally be suited to arc therapies, where current clinical algorithms may incur long calculation times.

  • validation of a new grid based boltzmann equation solver for dose calculation in radiotherapy with Photon Beams
    Physics in Medicine and Biology, 2010
    Co-Authors: Oleg N Vassiliev, Mohammad Salehpour, Todd A. Wareing, Gregory Failla, John M. Mcghee, Firas Mourtada
    Abstract:

    A new grid-based Boltzmann equation solver, Acuros, was developed specifically for performing accurate and rapid radiotherapy dose calculations. In this study we benchmarked its performance against Monte Carlo for 6 and 18 MV Photon Beams in heterogeneous media. Acuros solves the coupled Boltzmann transport equations for neutral and charged particles on a locally adaptive Cartesian grid. The Acuros solver is an optimized rewrite of the general purpose Attila software, and for comparable accuracy levels, it is roughly an order of magnitude faster than Attila. Comparisons were made between Monte Carlo (EGSnrc) and Acuros for 6 and 18 MV Photon Beams impinging on a slab phantom comprising tissue, bone and lung materials. To provide an accurate reference solution, Monte Carlo simulations were run to a tight statistical uncertainty (sigma approximately 0.1%) and fine resolution (1-2 mm). Acuros results were output on a 2 mm cubic voxel grid encompassing the entire phantom. Comparisons were also made for a breast treatment plan on an anthropomorphic phantom. For the slab phantom in regions where the dose exceeded 10% of the maximum dose, agreement between Acuros and Monte Carlo was within 2% of the local dose or 1 mm distance to agreement. For the breast case, agreement was within 2% of local dose or 2 mm distance to agreement in 99.9% of voxels where the dose exceeded 10% of the prescription dose. Elsewhere, in low dose regions, agreement for all cases was within 1% of the maximum dose. Since all Acuros calculations required less than 5 min on a dual-core two-processor workstation, it is efficient enough for routine clinical use. Additionally, since Acuros calculation times are only weakly dependent on the number of Beams, Acuros may ideally be suited to arc therapies, where current clinical algorithms may incur long calculation times.

  • su ff t 324 modifications of megavoltage Photon Beams for gold nanoparticle aided radiation therapy gnrt a monte carlo study
    Medical Physics, 2006
    Co-Authors: Sang Hyun Cho, Oleg N Vassiliev, S Jang, Sunil Krishnan
    Abstract:

    Purpose: To produce megavoltage Photon Beams capable of achieving clinically significant (> 10%) macroscopic tumordose enhancement during goldnanoparticle‐aidedradiation therapy (GNRT). Method and Materials: GNRT is an emerging treatment modality currently under development, based on the following observations: a) high tumor specificity of goldnanoparticles due to passive extravasation; b) significant tumordose enhancement during x‐ray irradiation as a result of increased photoelectric absorption due to high atomic number (Z) of gold. A previous Monte Carlo study found that no meaningful tumordose enhancement would occur during GNRT with typical megavoltage Photon Beams, even after the removal of the flattening filter from linear accelerators. Therefore, the current Monte Carlo study investigated a number of ways to further increase the amount of low energy Photons in the beam and consequently to achieve clinically significant tumordose enhancement with Photon Beams in megavoltage range. Specifically, the macroscopic tumordose enhancement under the identical geometry was calculated using the BEAMnrc/DOSXYZnrc code as the following conditions changed: the energy of electron pencil beam incident on the target, the target thickness, and the target material.Results: The current results showed that the macroscopic dose enhancement up to 40 and 18% across the tumor volume could be achievable with unflattened 2 and 4 MV Photon Beams, respectively, at a reasonable gold concentration of 3% within the tumor, after the proposed changes in target thickness and material. These Beams were found capable of producing clinically acceptable treatment plans for GNRT, in spite of their softer Photon energy spectra and larger buildup doses, compared to conventional megavoltage Beams at the same nominal Photon energies. Conclusion: Clinically significant tumordose enhancement could be achievable during GNRT with megavoltage Photon Beams, provided that the proposed modifications to linear accelerators are made.

  • dosimetric properties of Photon Beams from a flattening filter free clinical accelerator
    Physics in Medicine and Biology, 2006
    Co-Authors: Oleg N Vassiliev, Uwe Titt, Falk Ponisch, Stephen F Kry, Radhe Mohan, Michael Gillin
    Abstract:

    Basic dosimetric properties of 6 MV and 18 MV Photon Beams from a Varian Clinac 21EX accelerator operating without the flattening filter have been measured. These include dose rate data, depth dose dependencies and lateral profiles in a water phantom, total scatter factors and transmission factors of a multileaf collimator. The data are reviewed and compared with measurements for the flattened Beams. The unflattened Beams have the following: a higher dose rate by factors of 2.3 (6 MV) and 5.5 (18 MV) on the central axis; lower out-of-field dose due to reduced head scatter and softer spectra; less variation of the total scatter factor with field size; and less variation of the shape of lateral dose profiles with depth. The findings suggest that with a flattening filter free accelerator better radiation treatments can be developed, with shorter delivery times and lower doses to normal tissues and organs.

Dietmar Georg - One of the best experts on this subject based on the ideXlab platform.

  • radiation therapy with unflattened Photon Beams dosimetric accuracy of advanced dose calculation algorithms
    Radiotherapy and Oncology, 2011
    Co-Authors: G Kragl, David Albrich, Dietmar Georg
    Abstract:

    Abstract Purpose To compare the dosimetric accuracy of advanced dose calculation algorithms for flattened (FF) and unflattened (FFF) Photon Beams. Material and methods We compared the enhanced collapsed cone (eCC) algorithm implemented in OncentraMasterplan and the XVMC (MC) code in Monaco. Test plans were created for 10MV FF and FFF Beams. Single beam tests were delivered to radiochromic films positioned within a solid water phantom and evaluated with 1D γ-index analysis. Conformal plans were verified with ion chambers in an anthropomorphic thorax phantom. IMRT plans were applied to the Delta4 system and evaluated with γ-criteria of 3% and 3mm. Results 1D γ-index evaluation revealed significantly lower ( p γ mean -values of 0.46±0.22 for MC calculated FFF profiles compared to average values of 0.53±0.27 detected for FF Beams. Respective values for eCC were 0.42±0.27/0.38±0.26 (FF/FFF). When considering off-axis profiles separately, we found significantly reduced average γ mean -values for FFF and both algorithms (MC: 0.55±24 vs. 0.45±0.21, eCC: 0.41±0.24 vs. 0.35±0.22). No significant differences were detected on-axis. Absolute dosimetry in the anthropomorphic phantom revealed superior results for MC based dose calculation, with mean deviations of 0.8±0.8/0.0±1.0% compared to −0.1±1.7/−0.5±0.1.7% (FF/FFF) for the eCC algorithm. IMRT plans showed similar results for both linac modes. Conclusions The dose calculation accuracy for unflattened Beams was found to be at least as high as for flattened Beams. The slightly improved dose calculation accuracy observed for off-axis profiles for single FFF Beams did not directly translate into better verification results for composite IMRT plans.

  • dosimetric characteristics of 6 and 10 mv unflattened Photon Beams
    Radiotherapy and Oncology, 2009
    Co-Authors: G Kragl, Tommy Knoos, Sacha Af Wetterstedt, B Knausl, Marten Lind, Patrick Mccavana, Brendan Mcclean, Dietmar Georg
    Abstract:

    Purpose: To determine dosimetric properties of unflattened megavoltage Photon Beams. Materials and methods: Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it. Results: Depths of dose maxima for flattened and unflattened Beams did not deviate by more than 2 mm and penumbral widths agreed within 1 mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened Beams showed on average 40% and 56% less variation for 6 and 10 MV Beams than conventional Beams. Phantom scatter factors for FFF Beams differed up to 4% from the published reference data. For field sizes smaller than 15 cm, surface doses relative to the dose at d(max) increased for unflattened Beams with maximum differences of 7% at 6 MV and 25% at 10 MV for a 5 x 5 cm(2) field. For a 30 x 30 cm(2) field, relative surface dose decreased by about 10% for FFF Beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10 MV Beams, respectively. Leakage radiation was reduced by 52% for 6 MV and by 65% for 10 MV unflattened Beams. Conclusions: The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter. (C) 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 93 (2009) 141-146 (Less)

  • dosimetric characteristics of 6 and 10 mv unflattened Photon Beams
    Radiotherapy and Oncology, 2009
    Co-Authors: G Kragl, Tommy Knoos, Sacha Af Wetterstedt, B Knausl, Marten Lind, Patrick Mccavana, Brendan Mcclean, Dietmar Georg
    Abstract:

    Purpose: To determine dosimetric properties of unflattened megavoltage Photon Beams. Materials and methods: Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it. Results: Depths of dose maxima for flattened and unflattened Beams did not deviate by more than 2 mm and penumbral widths agreed within 1 mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened Beams showed on average 40% and 56% less variation for 6 and 10 MV Beams than conventional Beams. Phantom scatter factors for FFF Beams differed up to 4% from the published reference data. For field sizes smaller than 15 cm, surface doses relative to the dose at d(max) increased for unflattened Beams with maximum differences of 7% at 6 MV and 25% at 10 MV for a 5 x 5 cm(2) field. For a 30 x 30 cm(2) field, relative surface dose decreased by about 10% for FFF Beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10 MV Beams, respectively. Leakage radiation was reduced by 52% for 6 MV and by 65% for 10 MV unflattened Beams. Conclusions: The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter. (C) 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 93 (2009) 141-146 (Less)

  • a methodology for tld postal dosimetry audit of high energy radiotherapy Photon Beams in non reference conditions
    Radiotherapy and Oncology, 2007
    Co-Authors: J Izewska, Dietmar Georg, Pranabes Bera, D I Thwaites, Mehenna Arib, Margarita Saravi, Katia Sergieva, Kaibao Li, Ashok Kumar Mahant, Wojciech Bulski
    Abstract:

    Abstract Background and purpose A strategy for national TLD audit programmes has been developed by the International Atomic Energy Agency (IAEA). It involves progression through three sequential dosimetry audit steps. The first step audits are for the beam output in reference conditions for high-energy Photon Beams. The second step audits are for the dose in reference and non-reference conditions on the beam axis for Photon and electron Beams. The third step audits involve measurements of the dose in reference, and non-reference conditions off-axis for open and wedged symmetric and asymmetric fields for Photon Beams. Through a co-ordinated research project the IAEA developed the methodology to extend the scope of national TLD auditing activities to more complex audit measurements for regular fields. Materials and methods Based on the IAEA standard TLD holder for high-energy Photon Beams, a TLD holder was developed with horizontal arm to enable measurements 5 cm off the central axis. Basic correction factors were determined for the holder in the energy range between Co-60 and 25 MV Photon Beams. Results New procedures were developed for the TLD irradiation in hospitals. The off-axis measurement methodology for Photon Beams was tested in a multi-national pilot study. The statistical distribution of dosimetric parameters (off-axis ratios for open and wedge beam profiles, output factors, wedge transmission factors) checked in 146 measurements was 0.999 ± 0.012. Conclusions The methodology of TLD audits in non-reference conditions with a modified IAEA TLD holder has been shown to be feasible.

Firas Mourtada - One of the best experts on this subject based on the ideXlab platform.

  • validation of a new grid based boltzmann equation solver for dose calculation in radiotherapy with Photon Beams
    Physics in Medicine and Biology, 2010
    Co-Authors: Oleg N Vassiliev, Mohammad Salehpour, Todd A. Wareing, Gregory Failla, John M. Mcghee, Firas Mourtada
    Abstract:

    A new grid-based Boltzmann equation solver, Acuros™, was developed specifically for performing accurate and rapid radiotherapy dose calculations. In this study we benchmarked its performance against Monte Carlo for 6 and 18 MV Photon Beams in heterogeneous media. Acuros solves the coupled Boltzmann transport equations for neutral and charged particles on a locally adaptive Cartesian grid. The Acuros solver is an optimized rewrite of the general purpose Attila© software, and for comparable accuracy levels, it is roughly an order of magnitude faster than Attila. Comparisons were made between Monte Carlo (EGSnrc) and Acuros for 6 and 18 MV Photon Beams impinging on a slab phantom comprising tissue, bone and lung materials. To provide an accurate reference solution, Monte Carlo simulations were run to a tight statistical uncertainty (σ ≈ 0.1%) and fine resolution (1–2 mm). Acuros results were output on a 2 mm cubic voxel grid encompassing the entire phantom. Comparisons were also made for a breast treatment plan on an anthropomorphic phantom. For the slab phantom in regions where the dose exceeded 10% of the maximum dose, agreement between Acuros and Monte Carlo was within 2% of the local dose or 1 mm distance to agreement. For the breast case, agreement was within 2% of local dose or 2 mm distance to agreement in 99.9% of voxels where the dose exceeded 10% of the prescription dose. Elsewhere, in low dose regions, agreement for all cases was within 1% of the maximum dose. Since all Acuros calculations required less than 5 min on a dual-core two-processor workstation, it is efficient enough for routine clinical use. Additionally, since Acuros calculation times are only weakly dependent on the number of Beams, Acuros may ideally be suited to arc therapies, where current clinical algorithms may incur long calculation times.

  • validation of a new grid based boltzmann equation solver for dose calculation in radiotherapy with Photon Beams
    Physics in Medicine and Biology, 2010
    Co-Authors: Oleg N Vassiliev, Mohammad Salehpour, Todd A. Wareing, Gregory Failla, John M. Mcghee, Firas Mourtada
    Abstract:

    A new grid-based Boltzmann equation solver, Acuros, was developed specifically for performing accurate and rapid radiotherapy dose calculations. In this study we benchmarked its performance against Monte Carlo for 6 and 18 MV Photon Beams in heterogeneous media. Acuros solves the coupled Boltzmann transport equations for neutral and charged particles on a locally adaptive Cartesian grid. The Acuros solver is an optimized rewrite of the general purpose Attila software, and for comparable accuracy levels, it is roughly an order of magnitude faster than Attila. Comparisons were made between Monte Carlo (EGSnrc) and Acuros for 6 and 18 MV Photon Beams impinging on a slab phantom comprising tissue, bone and lung materials. To provide an accurate reference solution, Monte Carlo simulations were run to a tight statistical uncertainty (sigma approximately 0.1%) and fine resolution (1-2 mm). Acuros results were output on a 2 mm cubic voxel grid encompassing the entire phantom. Comparisons were also made for a breast treatment plan on an anthropomorphic phantom. For the slab phantom in regions where the dose exceeded 10% of the maximum dose, agreement between Acuros and Monte Carlo was within 2% of the local dose or 1 mm distance to agreement. For the breast case, agreement was within 2% of local dose or 2 mm distance to agreement in 99.9% of voxels where the dose exceeded 10% of the prescription dose. Elsewhere, in low dose regions, agreement for all cases was within 1% of the maximum dose. Since all Acuros calculations required less than 5 min on a dual-core two-processor workstation, it is efficient enough for routine clinical use. Additionally, since Acuros calculation times are only weakly dependent on the number of Beams, Acuros may ideally be suited to arc therapies, where current clinical algorithms may incur long calculation times.

G Kragl - One of the best experts on this subject based on the ideXlab platform.

  • radiation therapy with unflattened Photon Beams dosimetric accuracy of advanced dose calculation algorithms
    Radiotherapy and Oncology, 2011
    Co-Authors: G Kragl, David Albrich, Dietmar Georg
    Abstract:

    Abstract Purpose To compare the dosimetric accuracy of advanced dose calculation algorithms for flattened (FF) and unflattened (FFF) Photon Beams. Material and methods We compared the enhanced collapsed cone (eCC) algorithm implemented in OncentraMasterplan and the XVMC (MC) code in Monaco. Test plans were created for 10MV FF and FFF Beams. Single beam tests were delivered to radiochromic films positioned within a solid water phantom and evaluated with 1D γ-index analysis. Conformal plans were verified with ion chambers in an anthropomorphic thorax phantom. IMRT plans were applied to the Delta4 system and evaluated with γ-criteria of 3% and 3mm. Results 1D γ-index evaluation revealed significantly lower ( p γ mean -values of 0.46±0.22 for MC calculated FFF profiles compared to average values of 0.53±0.27 detected for FF Beams. Respective values for eCC were 0.42±0.27/0.38±0.26 (FF/FFF). When considering off-axis profiles separately, we found significantly reduced average γ mean -values for FFF and both algorithms (MC: 0.55±24 vs. 0.45±0.21, eCC: 0.41±0.24 vs. 0.35±0.22). No significant differences were detected on-axis. Absolute dosimetry in the anthropomorphic phantom revealed superior results for MC based dose calculation, with mean deviations of 0.8±0.8/0.0±1.0% compared to −0.1±1.7/−0.5±0.1.7% (FF/FFF) for the eCC algorithm. IMRT plans showed similar results for both linac modes. Conclusions The dose calculation accuracy for unflattened Beams was found to be at least as high as for flattened Beams. The slightly improved dose calculation accuracy observed for off-axis profiles for single FFF Beams did not directly translate into better verification results for composite IMRT plans.

  • dosimetric characteristics of 6 and 10 mv unflattened Photon Beams
    Radiotherapy and Oncology, 2009
    Co-Authors: G Kragl, Tommy Knoos, Sacha Af Wetterstedt, B Knausl, Marten Lind, Patrick Mccavana, Brendan Mcclean, Dietmar Georg
    Abstract:

    Purpose: To determine dosimetric properties of unflattened megavoltage Photon Beams. Materials and methods: Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it. Results: Depths of dose maxima for flattened and unflattened Beams did not deviate by more than 2 mm and penumbral widths agreed within 1 mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened Beams showed on average 40% and 56% less variation for 6 and 10 MV Beams than conventional Beams. Phantom scatter factors for FFF Beams differed up to 4% from the published reference data. For field sizes smaller than 15 cm, surface doses relative to the dose at d(max) increased for unflattened Beams with maximum differences of 7% at 6 MV and 25% at 10 MV for a 5 x 5 cm(2) field. For a 30 x 30 cm(2) field, relative surface dose decreased by about 10% for FFF Beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10 MV Beams, respectively. Leakage radiation was reduced by 52% for 6 MV and by 65% for 10 MV unflattened Beams. Conclusions: The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter. (C) 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 93 (2009) 141-146 (Less)

  • dosimetric characteristics of 6 and 10 mv unflattened Photon Beams
    Radiotherapy and Oncology, 2009
    Co-Authors: G Kragl, Tommy Knoos, Sacha Af Wetterstedt, B Knausl, Marten Lind, Patrick Mccavana, Brendan Mcclean, Dietmar Georg
    Abstract:

    Purpose: To determine dosimetric properties of unflattened megavoltage Photon Beams. Materials and methods: Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it. Results: Depths of dose maxima for flattened and unflattened Beams did not deviate by more than 2 mm and penumbral widths agreed within 1 mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened Beams showed on average 40% and 56% less variation for 6 and 10 MV Beams than conventional Beams. Phantom scatter factors for FFF Beams differed up to 4% from the published reference data. For field sizes smaller than 15 cm, surface doses relative to the dose at d(max) increased for unflattened Beams with maximum differences of 7% at 6 MV and 25% at 10 MV for a 5 x 5 cm(2) field. For a 30 x 30 cm(2) field, relative surface dose decreased by about 10% for FFF Beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10 MV Beams, respectively. Leakage radiation was reduced by 52% for 6 MV and by 65% for 10 MV unflattened Beams. Conclusions: The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter. (C) 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 93 (2009) 141-146 (Less)

D W O Rogers - One of the best experts on this subject based on the ideXlab platform.

  • addendum to the aapm s tg 51 protocol for clinical reference dosimetry of high energy Photon Beams
    Medical Physics, 2014
    Co-Authors: Malcolm Mcewen, D W O Rogers, Larry A Dewerd, G Ibbott, David S Followill, Stephen M Seltzer, J Seuntjens
    Abstract:

    An addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water in megavoltage Photon Beams is presented. This addendum continues the procedure laid out in TG-51 but new k Q data for Photon Beams, based on Monte Carlo simulations, are presented and recommendations are given to improve the accuracy and consistency of the protocol's implementation. The components of the uncertainty budget in determining absorbed dose to water at the reference point are introduced and the magnitude of each component discussed. Finally, the consistency of experimental determination of N D,w coefficients is discussed. It is expected that the implementation of this addendum will be straightforward, assuming that the user is already familiar with TG-51. The changes introduced by this report are generally minor, although new recommendations could result in procedural changes for individual users. It is expected that the effort on the medical physicist's part to implement this addendum will not be significant and could be done as part of the annual linac calibration.

  • beam quality conversion factors for parallel plate ionization chambers in mv Photon Beams
    Medical Physics, 2012
    Co-Authors: B R Muir, Malcolm Mcewen, D W O Rogers
    Abstract:

    Purpose: To investigate the behavior of plane-parallel ion chambers in high-energy Photon Beams through measurements and Monte Carlo simulations. Methods: Ten plane-parallel ion chamber types were obtained from the major ion chamber manufacturers. Absorbed dose-to-water calibration coefficients are measured for these chambers and k Q factors are determined. In the process, the behaviors of the chambers are characterized through measurements of leakage currents, chamber settling in cobalt-60, polarity and ion recombination behavior, and long-term stability. Monte Carlo calculations of the absorbed dose to the air in the ion chamber and absorbed dose to water are obtained to calculate k Q factors. Systematic uncertainties in Monte Carlo calculated k Q factors are investigated by varying material properties and chamber dimensions. Results: Chamber behavior was variable in MV Photon Beams, especially with regard to chamber leakage and ion recombination. The plane-parallel chambers did not perform as well as cylindrical chambers. Significant differences up to 1.5% were observed in calibration coefficients after a period of eight months although k Q factors were consistent on average within 0.17%. Chamber-to-chamber variations in k Q factors for chambers of the same type were at the 0.2% level. Systematic uncertainties in Monte Carlo calculated k Q factors ranged between 0.34% and 0.50% depending on the chamber type. Average percent differences between measured and calculated k Q factors were − 0.02%, 0.18%, and − 0.16% for 6, 10, and 25 MV Beams, respectively. Conclusions: Excellent agreement is observed on average at the 0.2% level between measured and Monte Carlo calculated k Q factors. Measurements indicate that the behavior of these chambers is not adequate for their use for reference dosimetry of high-energy Photon Beams without a more extensive QA program than currently used for cylindrical reference-class ion chambers.

  • the replacement correction factors for cylindrical chambers in high energy Photon Beams
    Physics in Medicine and Biology, 2009
    Co-Authors: L L W Wang, D W O Rogers
    Abstract:

    The values of the replacement correction factors (Prepl, or in the IAEA's notation pdispcav) used for cylindrical chambers in high-energy Photon Beams represent one of the most significant differences between the AAPM and the IAEA dosimetry protocols. In a previous study (Wang L L W and Rogers D W O 2008 Med. Phys. 35 1747–55), we found that the AAPM protocol adopted incorrect values of Prepl for cylindrical chambers in Photon Beams. For a 60Co beam, the calculated Prepl value is 0.5% higher than the AAPM value and about 1% higher than the IAEA value. It was still not clear why the IAEA values, which are based on measurements by Johansson et al, are incorrect. In this study, EGSnrc Monte Carlo simulation codes are used to simulate Johansson et al's experimental procedures for determining Prepl values. The simulation results agree well with the measurements if the chamber responses versus depth are normalized at dmax as was apparently done in the experiments as it was believed that the chambers of different radii gave the same maximum reading at the respective dmax. However, if the chamber responses are not normalized, then the simulated experimental results lead to a result which agrees well with the Prepl values calculated by the standard Monte Carlo methods. This demonstrates that the normalization procedure used in the experiments is incorrect as is based on an incorrect assumption, and thus the interpretation of Johansson et al's experimental values as Prepl (pdis) in the IAEA TRS-398 Code of Practice is wrong. The values of Prepl for cylindrical chambers of different radii in various high-energy Photon Beams are calculated and an empirical formula is given.

  • absorbed dose beam quality conversion factors for cylindrical chambers in high energy Photon Beams
    Medical Physics, 2000
    Co-Authors: J Seuntjens, C K Ross, K R Shortt, D W O Rogers
    Abstract:

    Recent working groups of the AAPM [Almond et al., Med. Phys. 26, 1847 (1999)] and the IAEA (Andreo et al., Draft V.7 of “An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water,” IAEA, 2000) have described guidelines to base reference dosimetry of high energy Photon Beams on absorbed dose to water standards. In these protocols use is made of the absorbed-dose beam quality conversion factor, k Q which scales an absorbed-dose calibration factor at the reference quality 60 Co to a quality Q, and which is calculated based on state-of-the-art ion chamber theory and data. In this paper we present the measurement and analysis of beam quality conversion factors k Q for cylindrical chambers in high-energy Photon Beams. At least three chambers of six different types were calibrated against the Canadian primary standard for absorbed dose based on a sealed water calorimeter at 60 Co [ TPR 10 20 =0.572, % dd (10) x =58.4], 10 MV [ TPR 10 20 =0.682, % dd (10) x =69.6), 20 MV ( TPR 10 20 =0.758, % dd (10) x =80.5] and 30 MV [ TPR 10 20 =0.794, % dd (10) x =88.4]. The uncertainty on the calorimetric determination of k Q for a single chamber is typically 0.36% and the overall 1σ uncertainty on a set of chambers of the same type is typically 0.45%. The maximum deviation between a measured k Q and the TG-51 protocol value is 0.8%. The overall rms deviation between measurement and the TG-51 values, based on 20 chambers at the three energies, is 0.41%. When the effect of a 1 mm PMMA waterproofing sleeve is taken into account in the calculations, the maximum deviation is 1.1% and the overall rms deviation between measurement and calculation 0.48%. When the beam is specified using TPR 10 20 , and measurements are compared with k Q values calculated using the version of TG-21 with corrected formalism and data, differences are up to 1.6% when no sleeve corrections are taken into account. For the NE2571 and the NE2611A chamber types, for which the most literature data are available, using % dd (10) x , all published data show a spread of 0.4% and 0.6%, respectively, over the entire measurement range, compared to spreads of up to 1.1% for both chambers when the k Q values are expressed as a function of TPR 10 20 . For the PR06-C chamber no clear preference of beam quality specifier could be identified. When comparing the differences of our k Q measurements and calculations with an analysis in terms of air-kerma protocols with the same underlying calculations but expressed in terms of a compound conversion factor C Q , we observe that a system making use of absorbed-dose calibrations and calculated k Q values, is more accurate than a system based on air-kerma calibrations in combination with calculated C Q (rms deviation of 0.48% versus 0.67%, respectively).