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Jermain PR, Muir B, McEwen M, Niu Y, Pang D. Accurate machine-specific reference and small-field dosimetry for a self-shielded neuro-radiosurgical system. Med Phys 2024; 51:4423-4433. [PMID: 38695760 DOI: 10.1002/mp.17111] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2024] [Revised: 03/14/2024] [Accepted: 04/18/2024] [Indexed: 06/05/2024] Open
Abstract
BACKGROUND The newly available ZAP-X stereotactic radiosurgical system is designed for the treatment of intracranial lesions, with several unique features that include a self-shielding, gyroscopic gantry, wheel collimation, non-orthogonal kV imaging, short source-axis distance, and low-energy megavoltage beam. Systematic characterization of its radiation as well as other properties is imperative to ensure its safe and effective clinical application. PURPOSE To accurately determine the radiation output of the ZAP-X with a special focus on the smaller diameter cones and an aim to provide useful recommendations on quantification of small field dosimetry. METHODS Six different types of detectors were used to measure relative output factors at field sizes ranging from 4 to 25 mm, including the PTW microSilicon and microdiamond diodes, Exradin W2 plastic scintillator, Exradin A16 and A1SL ionization chambers, and the alanine dosimeter. The 25 mm cone served as the reference field size. Absolute dose was determined with both TG-51-based dosimetry using a calibrated PTW Semiflex ion chamber and measurements using alanine dosimeters. RESULTS The average radiation output factors (maximum deviation from the average) measured with the microDiamond, microSilicon, and W2 detectors were: for the 4 mm cone, 0.741 (1.0%); for the 5 mm cone: 0.817 (1.0%); for the 7.5 mm cone: 0.908 (1.0%); for the 10 mm cone: 0.946 (0.4%); for the 12.5 mm cone: 0.964 (0.2%); for the 15 mm cone: 0.976 (0.1%); for the 20 mm cone: 0.990 (0.1%). For field sizes larger than 10 mm, the A1SL and A16 micro-chambers also yielded consistent output factors within 1.5% of those obtained using the microSilicon, microdiamond, and W2 detectors. The absolute dose measurement obtained with alanine was within 1.2%, consistent with combined uncertainties, compared to the PTW Semiflex chamber for the 25 mm reference cone. CONCLUSION For field sizes less than 10 mm, the microSilicon diode, microDiamond detector, and W2 scintillator are suitable devices for accurate small field dosimetry of the ZAP-X system. For larger fields, the A1SL and A16 micro-chambers can also be used. Furthermore, alanine dosimetry can be an accurate verification of reference and absolute dose typically measured with ion chambers. Use of multiple suitable detectors and uncertainty analyses were recommended for reliable determination of small field radiation outputs.
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Affiliation(s)
- Peter R Jermain
- Department of Radiation Medicine, Medstar Georgetown University Hospital, Washington, District of Columbia, USA
| | - Bryan Muir
- Metrology Research Centre, National Research Council, Ottawa, Ontario, Canada
| | - Malcolm McEwen
- Metrology Research Centre, National Research Council, Ottawa, Ontario, Canada
| | - Ying Niu
- Department of Radiation Medicine, Medstar Georgetown University Hospital, Washington, District of Columbia, USA
| | - Dalong Pang
- Department of Radiation Medicine, Medstar Georgetown University Hospital, Washington, District of Columbia, USA
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Khaledi N, Hayes C, Belshaw L, Grattan M, Khan R, Gräfe JL. Treatment planning with a 2.5 MV photon beam for radiation therapy. J Appl Clin Med Phys 2022; 23:e13811. [PMID: 36300870 PMCID: PMC9797178 DOI: 10.1002/acm2.13811] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2022] [Accepted: 09/23/2022] [Indexed: 01/01/2023] Open
Abstract
PURPOSE The shallow depth of maximum dose and higher dose fall-off gradient of a 2.5 MV beam along the central axis that is available for imaging on linear accelerators is investigated for treatment of shallow tumors and sparing the organs at risk (OARs) beyond it. In addition, the 2.5 MV beam has an energy bridging the gap between kilo-voltage (kV) and mega-voltage (MV) beams for applications of dose enhancement with high atomic number (Z) nanoparticles. METHODS We have commissioned and utilized a MATLAB-based, open-source treatment planning software (TPS), matRad, for intensity-modulated radiation therapy (IMRT) dose calculations. Treatment plans for prostate, liver, and head and neck (H&N), nasal cavity, two orbit cases, and glioblastoma multiforme (GBM) were performed and compared to a conventional 6 MV beam. Additional Monte Carlo calculations were also used for benchmarking the central axis dose. RESULTS Both beams had similar planning target volume (PTV) dose coverage for all cases. However, the 2.5 MV beam deposited 6%-19% less integral doses to the nasal cavity, orbit, and GBM cases than 6 MV photons. The mean dose to the heart in the liver plan was 10.5% lower for 2.5 MV beam. The difference between the doses to OARs of H&N for two beams was under 3%. Brain mean dose, brainstem, and optic chiasm max doses were, respectively, 7.5%-14.9%, 2.2%-8.1%, and 2.5%-19.0% lower for the 2.5 MV beam in the nasal cavity, orbit, and GBM plans. CONCLUSIONS This study demonstrates that the 2.5 MV beam can produce clinically relevant treatment plans, motivating future efforts for design of single-energy LINACs. Such a machine will be capable of producing beams at this energy beneficial for low- and middle-income countries, and investigations on dose enhancement from high-Z nanoparticles.
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Affiliation(s)
- Navid Khaledi
- Department of PhysicsFaculty of ScienceToronto Metropolitan UniversityTorontoOntarioCanada
| | - Chris Hayes
- Radiotherapy PhysicsNorthern Ireland Cancer CentreBelfast Health and Social Care TrustBelfastUK
| | - Louise Belshaw
- Radiotherapy PhysicsNorthern Ireland Cancer CentreBelfast Health and Social Care TrustBelfastUK
| | - Mark Grattan
- Radiotherapy PhysicsNorthern Ireland Cancer CentreBelfast Health and Social Care TrustBelfastUK
| | - Rao Khan
- Department of PhysicsFaculty of ScienceToronto Metropolitan UniversityTorontoOntarioCanada,Department of Physics and Astronomy and Department of Radiation OncologyHoward UniversityWashingtonDistrict of ColumbiaUSA
| | - James L. Gräfe
- Department of PhysicsFaculty of ScienceToronto Metropolitan UniversityTorontoOntarioCanada
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Ding GX. Stopping-power ratios for electron beams used in total skin electron therapy. Med Phys 2021; 48:5472-5478. [PMID: 34287969 DOI: 10.1002/mp.15121] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2021] [Revised: 06/28/2021] [Accepted: 07/12/2021] [Indexed: 11/10/2022] Open
Abstract
PURPOSE The electron beams for total skin electron therapy (TSET) are often degraded by a scatter plate in addition to extended distances. For electron dosimetry, both the AAPM TG-51 and IAEA TRS-398 recommend the use of two formulas developed by Burns et al [Med. Phys. 23, 489-501 (1996)] to estimate the water-to-air stopping-power ratios (SPRs). Both formulas are based on a fit to SPRs calculated for standard electron beams. This study aims to find: (1) if the formulas are applicable to beams used in TSET and (2) the impact of the ICRU report 90 recommendations on the SPRs for these beams. METHODS The EGSnrc Monte Carlo code system is used to generate 6 MeV high dose rate total skin electron (HDTSe) beams used in TSET. The simulated beams are used to calculate dose distributions and SPRs as a function of depth in a water phantom. The fitted SPRs using the empirical formulas are compared with MC-calculated SPRs. RESULTS The electron beam quality specifier, the depth in water at which the absorbed dose falls to 50% of its maximum value, R50 , decreases approximately 1 mm for each additional 100-cm extended distance ranging from 2.24 cm at SSD = 100 to 1.72 cm at SSD = 700 cm. For beams passing through a scatter plate, R50 is 1.76 cm (1.14) at SSD = 300 and 1.48 cm (0.85 cm) at SSD = 600 cm with an Acrylic plate thickness of 3 mm (9 mm), respectively. The discrepancy between fitted and MC-calculated SPRs at dref as a function of R50 is <0.8%, and in many cases <0.4%. The difference between fitted and MC-calculated SPRs as a function of depth and R50 is within 1% at depths <0.8R50 for beams with R50 ≥ 1.14 cm. The ICRU-90 recommendations decrease SPRs by 0.3%-0.4% compared to the use of data recommended in ICRU-37. CONCLUSION The formulas used by the major protocols are accurate enough for clinical beams used in TSET and the error caused using the formulas is <1% to estimate SPRs as a function of depth and R50 for depths <0.8R50 for beams used in TSET with R50 ≥ 1.14 cm. The impact of the ICRU-90 recommendations shows a decrease of SPRs by a fraction of a percent for beams used in TSET.
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Affiliation(s)
- George X Ding
- Department of Radiation Oncology, Vanderbilt University School of Medicine, Nashville, Tennessee, USA
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Detailed Monte Carlo analysis of the secondary photons coming out of the therapeutic X-ray beam of linear accelerator. POLISH JOURNAL OF MEDICAL PHYSICS AND ENGINEERING 2021. [DOI: 10.2478/pjmpe-2021-0018] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Abstract
External photon beam radiotherapy is often used in tumor treatment. The photons are generated from the target which had stricken by the primary electron beam (incident particles). The photon beam contains the primary photons coming directly from the target and secondary photons coming from the photon interactions with head component materials (scattered photons). Altogether is thereafter used in radiotherapy treatment. This Monte Carlo study aims to investigate and evaluate the secondary radiations (photons) in terms of fluence, energy fluence, spectral distribution, mean energy and angular spread distribution.
The secondary photons, which contributed in radiotherapy treatment, are examined and evaluated in number (fluence) and energy. At the phantom surface, the secondary photons originated in the whole linac head are mainly coming from the primary collimator. In 0.45% of secondary photons coming from the whole linac head, the primary collimator contributes by 86% and they are more energetic. However, the flattening filter and the secondary collimator contribute together by less than 14% and their photons are less energetic and then can deteriorate the beam dosimetry quality. To improve the radiotherapy treatment quality, the number of photons of low energy should be as low as possible in the clinical beam. Our work can be a basic investigation to use in the improvement of linac head configuration and specially the beam modifiers.
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Baumann KS, Kaupa S, Bach C, Engenhart-Cabillic R, Zink K. Monte Carlo calculation of perturbation correction factors for air-filled ionization chambers in clinical proton beams using TOPAS/GEANT. Z Med Phys 2021; 31:175-191. [PMID: 33775521 DOI: 10.1016/j.zemedi.2020.08.004] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/29/2020] [Revised: 08/20/2020] [Accepted: 08/31/2020] [Indexed: 10/21/2022]
Abstract
INTRODUCTION Current dosimetry protocols for clinical protons using air-filled ionization chambers assume that the perturbation correction factor is equal to unity for all ionization chambers and proton energies. Since previous Monte Carlo based studies suggest that perturbation correction factors might be significantly different from unity this study aims to determine perturbation correction factors for six plane-parallel and four cylindrical ionization chambers in proton beams at clinical energies. MATERIALS AND METHODS The dose deposited in the air cavity of the ionization chambers was calculated with the help of the Monte Carlo code TOPAS/Geant4 while specific constructive details of the chambers were removed step by step. By comparing these dose values the individual perturbation correction factors pcel, pstem, psleeve, pwall, pcav⋅pdis as well as the total perturbation correction factor pQ were derived for typical clinical proton energies between 80 and 250MeV. RESULTS The total perturbation correction factor pQ was smaller than unity for almost every ionization chamber and proton energy and in some cases significantly different from unity (deviation larger than 1%). The maximum deviation from unity was 2.0% for cylindrical and 1.5% for plane-parallel ionization chambers. Especially the factor pwall was found to differ significantly from unity. It was shown that this is due to the fact that secondary particles, especially alpha particles and fragments, are scattered from the chamber wall into the air cavity resulting in an overresponse of the chamber. CONCLUSION Perturbation correction factors for ionization chambers in proton beams were calculated using Monte Carlo simulations. In contrast to the assumption of current dosimetry protocols the total perturbation correction factor pQ can be significantly different from unity. Hence, beam quality correction factors [Formula: see text] that are calculated with the help of perturbation correction factors that are assumed to be unity come with a corresponding additional uncertainty.
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Affiliation(s)
- Kilian-Simon Baumann
- University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany; University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany; Marburg Ion-Beam Therapy Center (MIT), Marburg, Germany.
| | - Sina Kaupa
- University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany
| | - Constantin Bach
- University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany
| | - Rita Engenhart-Cabillic
- University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany; Marburg Ion-Beam Therapy Center (MIT), Marburg, Germany
| | - Klemens Zink
- University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany; University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany; Marburg Ion-Beam Therapy Center (MIT), Marburg, Germany; Frankfurt Institute of Advanced Studies - FIAS, Frankfurt, Germany
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Becker SJ, Culberson WS, Poirier Y, Mutaf Y, Niu Y, Nichols EM, Yi B. Dosimetry evaluation of the GammaPod stereotactic radiosurgery device based on established AAPM and IAEA protocols. Med Phys 2020; 47:3614-3620. [DOI: 10.1002/mp.14197] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2019] [Revised: 03/23/2020] [Accepted: 04/08/2020] [Indexed: 11/06/2022] Open
Affiliation(s)
- Stewart J. Becker
- Department of Radiation Oncology University of Maryland School of Medicine Baltimore MD 21201 USA
| | - Wesley S. Culberson
- Department of Medical Physics University of Wisconsin–Madison Madison Wisconsin 53705 USA
| | - Yannick Poirier
- Department of Radiation Oncology University of Maryland School of Medicine Baltimore MD 21201 USA
| | - Yildirim Mutaf
- Department of Radiation Oncology University of Maryland School of Medicine Baltimore MD 21201 USA
| | - Ying Niu
- MedStar Georgetown University Hospital Washington DC 20007 USA
| | - Elizabeth M. Nichols
- Department of Radiation Oncology University of Maryland School of Medicine Baltimore MD 21201 USA
| | - Byongyong Yi
- Department of Radiation Oncology University of Maryland School of Medicine Baltimore MD 21201 USA
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Andreo P, Burns DT, Kapsch RP, McEwen M, Vatnitsky S, Andersen CE, Ballester F, Borbinha J, Delaunay F, Francescon P, Hanlon MD, Mirzakhanian L, Muir B, Ojala J, Oliver CP, Pimpinella M, Pinto M, de Prez LA, Seuntjens J, Sommier L, Teles P, Tikkanen J, Vijande J, Zink K. Determination of consensus k Q values for megavoltage photon beams for the update of IAEA TRS-398. ACTA ACUST UNITED AC 2020; 65:095011. [DOI: 10.1088/1361-6560/ab807b] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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Renaud J, Palmans H, Sarfehnia A, Seuntjens J. Absorbed dose calorimetry. ACTA ACUST UNITED AC 2020; 65:05TR02. [DOI: 10.1088/1361-6560/ab4f29] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
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Martin-Martin G, Aguilar PB, Barbés B, Guibelalde E. Assessment of ion recombination correction and polarity effects for specific ionization chambers in flattening-filter-free photon beams. Phys Med 2019; 67:176-184. [PMID: 31734555 DOI: 10.1016/j.ejmp.2019.07.018] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/11/2019] [Revised: 07/23/2019] [Accepted: 07/24/2019] [Indexed: 11/17/2022] Open
Abstract
PURPOSE To investigate ion recombination correction and polarity effects in four ion chamber models in flattening-filter-free (FFF) beams to (1) evaluate their suitability for reference dosimetry; (2) assess the accuracy of the two-voltage technique (TVA) against the Bruggmoser formalism; and (3) examine the influence of the accelerator type on the recombination correction. METHODS Jaffé plots were created for a variety of microchambers, small-volume and Farmer-type chambers to obtain kS, the recombination correction factor, using two different types of accelerators. These values were plotted against dose-per-pulse and Jaffé plots for opposite polarities were created to determine which chambers meet the AAPM TG-51 addendum recombination and polarity specifications. RESULTS Nearly all small-volume chambers exhibited reference-class behavior with respect to ion recombination and polarity effects. The microchambers exhibited anomalous recombination and polarity effects, precluding their use for reference dosimetry in FFF beams. For the reference-class chambers, agreement between TVA-determined kS values and Jaffé and Bruggmoser formalisms-determined kS values was within 0.1%. No significant differences were found between the kS values obtained with the two different accelerators used in this work. CONCLUSIONS This study stresses the need to characterize ion recombination correction and polarity effects for small-volume chambers and microchambers on an individual chamber basis and with the more rigorous criteria of the AAPM TG-51 addendum. Furthermore, the study demonstrated the suitability of the TVA method for chambers that exhibit reference-class behavior in FFF beams. Finally, this work has shown that the recombination correction does not depend on the type of accelerator but on its dose-per-pulse.
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Affiliation(s)
- Guadalupe Martin-Martin
- Medical Physics and Radiation Protection Service, Hospital Universitario de Fuenlabrada, C/ Camino del Molino 2, 28492 Fuenlabrada, Madrid, Spain.
| | - Pedro Borja Aguilar
- Clínica Universidad de Navarra, Department of Radiation Physics, Avenida Pío XII, 31080 Pamplona, Navarra, Spain
| | - Benigno Barbés
- Clínica Universidad de Navarra, Department of Radiation Physics, Avenida Pío XII, 31080 Pamplona, Navarra, Spain
| | - Eduardo Guibelalde
- Medical Physics Group, Department of Radiology, University Complutense of Madrid, 28040 Madrid, Spain
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Baumann K, Horst F, Zink K, Gomà C. Comparison of penh, fluka, and Geant4/topas for absorbed dose calculations in air cavities representing ionization chambers in high-energy photon and proton beams. Med Phys 2019; 46:4639-4653. [PMID: 31350915 PMCID: PMC6851981 DOI: 10.1002/mp.13737] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2018] [Revised: 07/01/2019] [Accepted: 07/16/2019] [Indexed: 12/16/2022] Open
Abstract
PURPOSE The purpose of this work is to analyze whether the Monte Carlo codes penh, fluka, and geant4/topas are suitable to calculate absorbed doses andf Q / f Q 0 ratios in therapeutic high-energy photon and proton beams. METHODS We used penh, fluka, geant4/topas, and egsnrc to calculate the absorbed dose to water in a reference water cavity and the absorbed dose to air in two air cavities representative of a plane-parallel and a cylindrical ionization chamber in a 1.25 MeV photon beam and a 150 MeV proton beam - egsnrc was only used for the photon beam calculations. The physics and transport settings in each code were adjusted to simulate the particle transport as detailed as reasonably possible. From these absorbed doses, f Q 0 factors, f Q factors, andf Q / f Q 0 ratios (which are the basis of Monte Carlo calculated beam quality correction factors k Q , Q 0 ) were calculated and compared between the codes. Additionally, we calculated the spectra of primary particles and secondary electrons in the reference water cavity, as well as the integrated depth-dose curve of 150 MeV protons in water. RESULTS The absorbed doses agreed within 1.4% or better between the individual codes for both the photon and proton simulations. The f Q 0 and f Q factors agreed within 0.5% or better for the individual codes for both beam qualities. The resultingf Q / f Q 0 ratios for 150 MeV protons agreed within 0.7% or better. For the 1.25 MeV photon beam, the spectra of photons and secondary electrons agreed almost perfectly. For the 150 MeV proton simulation, we observed differences in the spectra of secondary protons whereas the spectra of primary protons and low-energy delta electrons also agreed almost perfectly. The first 2 mm of the entrance channel of the 150 MeV proton Bragg curve agreed almost perfectly while for greater depths, the differences in the integrated dose were up to 1.5%. CONCLUSION penh, fluka, and geant4/topas are capable of calculating beam quality correction factors in proton beams. The differences in the f Q 0 and f Q factors between the codes are 0.5% at maximum. The differences in thef Q / f Q 0 ratios are 0.7% at maximum.
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Affiliation(s)
- Kilian‐Simon Baumann
- Department of Radiotherapy and RadiooncologyUniversity Medical Center Giessen‐MarburgMarburgGermany
- Institute of Medical Physics and Radiation ProtectionUniversity of Applied SciencesGiessenGermany
| | - Felix Horst
- Institute of Medical Physics and Radiation ProtectionUniversity of Applied SciencesGiessenGermany
- GSI Helmholtzzentrum für SchwerionenforschungDarmstadtGermany
| | - Klemens Zink
- Department of Radiotherapy and RadiooncologyUniversity Medical Center Giessen‐MarburgMarburgGermany
- Institute of Medical Physics and Radiation ProtectionUniversity of Applied SciencesGiessenGermany
- Frankfurt Institute for Advanced Studies (FIAS)FrankfurtGermany
| | - Carles Gomà
- Department of Oncology, Laboratory of Experimental RadiotherapyKU LeuvenLeuvenBelgium
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de Prez L, de Pooter J, Jansen B, Perik T, Wittkämper F. Comparison of k Q factors measured with a water calorimeter in flattening filter free (FFF) and conventional flattening filter (cFF) photon beams. Phys Med Biol 2018; 63:045023. [PMID: 29461974 DOI: 10.1088/1361-6560/aaaa93] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Recently flattening filter free (FFF) beams became available for application in modern radiotherapy. There are several advantages of FFF beams over conventional flattening filtered (cFF) beams, however differences in beam spectra at the point of interest in a phantom potentially affect the ion chamber response. Beams are also non-uniform over the length of a typical reference ion chamber and recombination is usually larger. Despite several studies describing FFF beam characteristics, only a limited number of studies investigated their effect on k Q factors. Some of those studies predicted significant discrepancies in k Q factors (0.4% up to 1.0%) if TPR20,10 based codes of practice (CoPs) were to be used. This study addresses the question to which extent k Q factors, based on a TPR20,10 CoP, can be applied in clinical reference dosimetry. It is the first study that compares k Q factors measured directly with an absorbed dose to water primary standard in FFF-cFF pairs of clinical photon beams. This was done with a transportable water calorimeter described elsewhere. The measurements corrected for recombination and beam radial non-uniformity were performed in FFF-cFF beam pairs at 6 MV and 10 MV of an Elekta Versa HD for a selection of three different Farmer-type ion chambers (eight serial numbers). The ratio of measured k Q factors of the FFF-cFF beam pairs were compared with the TPR20,10 CoPs of the NCS and IAEA and the %dd(10) x CoP of the AAPM. For the TPR20,10 based CoPs differences less than 0.23% were found in k Q factors between the corresponding FFF-cFF beams with standard uncertainties smaller than 0.35%, while for the %dd(10) x these differences were smaller than 0.46% and within the expanded uncertainty of the measurements. Based on the measurements made with the equipment described in this study the authors conclude that the k Q factors provided by the NCS-18 and IAEA TRS-398 codes of practice can be applied for flattening filter free beams without additional correction. However, existing codes of practice cannot be applied ignoring the significant volume averaging effect of the FFF beams over the ion chamber cavity. For this a corresponding volume averaging correction must be applied.
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Affiliation(s)
- Leon de Prez
- VSL-Dutch Metrology Institute, Delft, Netherlands
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Renaud J, Rossomme S, Sarfehnia A, Vynckier S, Palmans H, Kacperek A, Seuntjens J. Development and application of a water calorimeter for the absolute dosimetry of short-range particle beams. Phys Med Biol 2016; 61:6602-6619. [DOI: 10.1088/0031-9155/61/18/6602] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Renaud J, Sarfehnia A, Marchant K, McEwen M, Ross C, Seuntjens J. Direct measurement of electron beam quality conversion factors using water calorimetry. Med Phys 2015; 42:6357-68. [DOI: 10.1118/1.4931970] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Affiliation(s)
- James Renaud
- Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada
| | - Arman Sarfehnia
- Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada and Department of Radiation Oncology, University of Toronto, Toronto, Ontario M5S 3E2, Canada
| | - Kristin Marchant
- Allan Blair Cancer Centre, Saskatchewan Cancer Agency, Regina, Saskatchewan S4T 7T1, Canada and Department of Oncology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A1, Canada
| | - Malcolm McEwen
- Ionizing Radiation Standards, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada
| | - Carl Ross
- Ionizing Radiation Standards, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada
| | - Jan Seuntjens
- Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada
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Wright T, Lye JE, Ramanathan G, Harty PD, Oliver C, Webb DV, Butler DJ. Direct calibration in megavoltage photon beams using Monte Carlo conversion factor: validation and clinical implications. Phys Med Biol 2015; 60:883-904. [PMID: 25565406 DOI: 10.1088/0031-9155/60/2/883] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Kinoshita N, Kita A, Takemura A, Nishimoto Y, Adachi T. [Uncertainty of cross calibration-applied beam quality conversion factor for the Japan Society of Medical Physics 12]. Nihon Hoshasen Gijutsu Gakkai Zasshi 2014; 70:877-882. [PMID: 25242596 DOI: 10.6009/jjrt.2014_jsrt_70.9.877] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The uncertainty of the beam quality conversion factor (k(Q,Q0)) of standard dosimetry of absorbed dose to water in external beam radiotherapy 12 (JSMP12) is determined by combining the uncertainty of each beam quality conversion factor calculated for each type of ionization chamber. However, there is no guarantee that ionization chambers of the same type have the same structure and thickness, so there may be individual variations. We evaluated the uncertainty of k(Q,Q0) for JSMP12 using an ionization chamber dosimeter and linear accelerator without a specific device or technique in consideration of the individual variation of ionization chambers and in clinical radiation field. The cross calibration formula was modified and the beam quality conversion factor for the experimental values [(k(Q,Q0))field] determined using the modified formula. It's uncertainty was calculated to be 1.9%. The differences between (k(Q,Q0))field of experimental values and k(Q,Q0) for Japan Society of Medical Physics 12 (JSMP12) were 0.73% and 0.88% for 6- and 10-MV photon beams, respectively, remaining within ± 1.9%. This showed k(Q,Q0) for JSMP12 to be consistent with (k(Q,Q0))field of experimental values within the estimated uncertainty range. Although inter-individual differences may be generated, even when the same type of ionized chamber is used, k(Q,Q0) for JSMP12 appears to be consistent within the estimated uncertainty range of (k(Q,Q0)field.
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Krauss A, Kapsch RP. Experimental determination ofkQfactors for cylindrical ionization chambers in 10 cm × 10 cm and 3 cm × 3 cm photon beams from 4 MV to 25 MV. Phys Med Biol 2014; 59:4227-46. [DOI: 10.1088/0031-9155/59/15/4227] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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17
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Thomas SJ, Aspradakis MM, Byrne JP, Chalmers G, Duane S, Rogers J, Thomas RAS, Tudor GSJ, Twyman N. Reference dosimetry on TomoTherapy: an addendum to the 1990 UK MV dosimetry code of practice. Phys Med Biol 2014; 59:1339-52. [DOI: 10.1088/0031-9155/59/6/1339] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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18
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Marsh MB, Peters C, Rawluk N, Schreiner LJ. Investigation of photon shielding property changes in curing high density concrete. HEALTH PHYSICS 2013; 105:318-325. [PMID: 23982607 DOI: 10.1097/hp.0b013e31829651d3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
High density concrete is usually used for radiation shielding around radiotherapy treatment rooms. Because the concrete is specified differently at the design, construction, and verification stages, the relationship between the intended performance and the actual performance of the shielding material might not be entirely clear. In this study, cylindrical samples of high density shielding concrete were taken as each section of a new radiotherapy bunker was poured. The shielding performance of each sample [measured by beam attenuation and tenth-value layers (TVL)] was evaluated for 15 MV and 6 MV x-ray beams and for the 1.25 MeV monoenergetic gamma beam from a Co source. Transmission curves to 3 TVL were mapped for a representative sample. The samples were also imaged and analyzed using Co Cone Beam Computed Tomography (CoCBCT). Results indicate no significant change in the TVL of high density concrete samples as they cure. The minor fluctuations in shielding properties observed are explained by the heterogeneous structure of the samples as indicated in the CoCBCT images.
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Affiliation(s)
- Matthew B Marsh
- Department of Physics, Queen's University, Kingston, Ontario, Canada
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19
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Ali ESM, McEwen MR, Rogers DWO. Detailed high-accuracy megavoltage transmission measurements: a sensitive experimental benchmark of EGSnrc. Med Phys 2012; 39:5990-6003. [PMID: 23039637 DOI: 10.1118/1.4745561] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
PURPOSE There are three goals for this study: (a) to perform detailed megavoltage transmission measurements in order to identify the factors that affect the measurement accuracy, (b) to use the measured data as a benchmark for the EGSnrc system in order to identify the computational limiting factors, and (c) to provide data for others to benchmark Monte Carlo codes. METHODS Transmission measurements are performed at the National Research Council Canada on a research linac whose incident electron parameters are independently known. Automated transmission measurements are made on-axis, down to a transmission value of ∼1.7%, for eight beams between 10 MV (the lowest stable MV beam on the linac) and 30 MV, using fully stopping Be, Al, and Pb bremsstrahlung targets and no fattening filters. To diversify energy differentiation, data are acquired for each beam using low-Z and high-Z attenuators (C and Pb) and Farmer chambers with low-Z and high-Z buildup caps. Experimental corrections are applied for beam drifts (2%), polarity (2.5% typical maximum, 6% extreme), ion recombination (0.2%), leakage (0.3%), and room scatter (0.8%)-the values in parentheses are the largest corrections applied. The experimental setup and the detectors are modeled using EGSnrc, with the newly added photonuclear attenuation included (up to a 5.6% effect). A detailed sensitivity analysis is carried out for the measured and calculated transmission data. RESULTS The developed experimental protocol allows for transmission measurements with 0.4% uncertainty on the smallest signals. Suggestions for accurate transmission measurements are provided. Measurements and EGSnrc calculations agree typically within 0.2% for the sensitivity of the transmission values to the detector details, to the bremsstrahlung target material, and to the incident electron energy. Direct comparison of the measured and calculated transmission data shows agreement better than 2% for C (3.4% for the 10 MV beam) and typically better than 1% for Pb. The differences can be explained by acceptable photon cross section changes of ≤0.4%. CONCLUSIONS Accurate transmission measurements require accounting for a number of influence quantities which, if ignored, can collectively introduce errors larger than 10%. Accurate transmission calculations require the use of the most accurate data and physics options available in EGSnrc, particularly the more accurate bremsstrahlung angular sampling option and the newly added modeling of photonuclear attenuation. Comparison between measurements and calculations implies that EGSnrc is accurate within 0.2% for relative ion chamber response calculations. Photon cross section uncertainties are the ultimate limiting factor for the accuracy of the calculated transmission data (Monte Carlo or analytical).
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Affiliation(s)
- E S M Ali
- Department of Physics, Carleton University, Ottawa, Ontario, Canada.
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20
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Lye JE, Butler DJ, Ramanathan G, Franich RD. Spectral differences in 6 MV beams with matched PDDs and the effect on chamber response. Phys Med Biol 2012; 57:7599-614. [DOI: 10.1088/0031-9155/57/22/7599] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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21
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Palmans H. Determination of the beam quality index of high-energy photon beams under nonstandard reference conditions. Med Phys 2012; 39:5513-9. [DOI: 10.1118/1.4745565] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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22
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Muir BR, McEwen MR, Rogers DWO. Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams. Med Phys 2012; 39:1618-31. [DOI: 10.1118/1.3687864] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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23
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Muir BR, McEwen MR, Rogers DWO. Measured and Monte Carlo calculatedkQfactors: Accuracy and comparison. Med Phys 2011; 38:4600-9. [DOI: 10.1118/1.3600697] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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24
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Georg D, Knöös T, McClean B. Current status and future perspective of flattening filter free photon beams. Med Phys 2011; 38:1280-93. [PMID: 21520840 DOI: 10.1118/1.3554643] [Citation(s) in RCA: 208] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
PURPOSE Flattening filters (FFs) have been considered as an integral part of the treatment head of a medical accelerator for more than 50 years. The reasons for the longstanding use are, however, historical ones. Advanced treatment techniques, such as stereotactic radiotherapy or intensity modulated radiotherapy have stimulated the interest in operating linear accelerators in a flattening filter free (FFF) mode. The current manuscript reviews treatment head physics of FFF beams, describes their characteristics and the resulting potential advantages in their medical use, and closes with an outlook. METHODS A number of dosimetric benefits have been determined for FFF beams, which range from increased dose rate and dose per pulse to favorable output ratio in-air variation with field size, reduced energy variation across the beam, and reduced leakage and out-of-field dose, respectively. Finally, the softer photon spectrum of unflattened beams has implications on imaging strategies and radiation protection. RESULTS The dosimetric characteristics of FFF beams have an effect on treatment delivery, patient comfort, dose calculation accuracy, beam matching, absorbed dose determination, treatment planning, machine specific quality assurance, imaging, and radiation protection. When considering conventional C-arm linacs in a FFF mode, more studies are needed to specify and quantify the clinical advantages, especially with respect to treatment plan quality and quality assurance. CONCLUSIONS New treatment units are already on the market that operate without a FF or can be operated in a dedicated clinical FFF mode. Due to the convincing arguments of removing the FF, it is expected that more vendors will offer dedicated treatment units for advanced photon beam therapy in the near future. Several aspects related to standardization, dosimetry, treatment planning, and optimization need to be addressed in more detail in order to facilitate the clinical implementation of unflattened beams.
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Affiliation(s)
- Dietmar Georg
- Department of Radiotherapy, Division of Medical Radiation Physics, Medical University of Vienna/AKH Vienna, A-1090 Vienna, Austria.
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25
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Muir BR, Rogers DWO. Monte Carlo calculations of kQ, the beam quality conversion factor. Med Phys 2010; 37:5939-50. [DOI: 10.1118/1.3495537] [Citation(s) in RCA: 96] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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26
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McEwen MR. Measurement of ionization chamber absorbed dose kQ factors in megavoltage photon beams. Med Phys 2010; 37:2179-93. [DOI: 10.1118/1.3375895] [Citation(s) in RCA: 106] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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27
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Abdel-Rahman W, Evans MDC, Serré L, McCaffrey JP, Podgorsak EB, Seuntjens JP. Clinic based transfer of the ND,wC60o calibration coefficient using a linear accelerator. Med Phys 2009; 36:929-38. [DOI: 10.1118/1.3075822] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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28
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Lacroix F, Archambault L, Gingras L, Guillot M, Beddar AS, Beaulieu L. Clinical prototype of a plastic water-equivalent scintillating fiber dosimeter array for QA applicationsa). Med Phys 2008; 35:3682-90. [DOI: 10.1118/1.2953564] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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29
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Castro P, García-Vicente F, Mínguez C, Floriano A, Sevillano D, Pérez L, Torres JJ. Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration. J Appl Clin Med Phys 2008; 9:70-86. [PMID: 18449162 PMCID: PMC5721533 DOI: 10.1120/jacmp.v9i1.2676] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2007] [Revised: 06/24/2007] [Accepted: 08/21/2007] [Indexed: 11/23/2022] Open
Abstract
To achieve a good clinical outcome in radiotherapy treatment, a certain accuracy in the dose delivered to the patient is required. Therefore, it is necessary to keep the uncertainty in each of the steps of the process inside some acceptable values, which implies as low a global uncertainty as possible. The work reported here focused on the uncertainty evaluation of absorbed dose to water in the routine calibration for clinical beams in the range of energies used in external‐beam radiotherapy. With this aim, we considered various uncertainty components (corrected electrometer reading, calibration factor, beam quality correction factor, and reference conditions) associated with beam calibration. Results show a typical uncertainty in the determination of absorbed dose to water during beam calibration of approximately 1.3% for photon beams and 1.5% for electron beams (k=1 in both cases) when the ND,w formalism is used and kQ,Q0 is calculated theoretically. These values may vary depending on the uncertainty provided by the standards laboratory for calibration factor, which is shown in the work. For primary standards based on clinical linear accelerator beam energies, the uncertainty in this step of the process could be placed close to 1.0%. We also discuss the possibility of an uncertainty reduction with the adoption of the absorbed dose to water formalism as compared with the air kerma formalism. PACS numbers: 87.53.Dq, 87.53.Hv
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Affiliation(s)
- Pablo Castro
- Servicio de Oncología Radioterápica, Departamento de Radiofísica, Hospital Universitario "La Princesa", Madrid, Spain
| | - Feliciano García-Vicente
- Servicio de Oncología Radioterápica, Departamento de Radiofísica, Hospital Universitario "La Princesa", Madrid, Spain
| | - Cristina Mínguez
- Servicio de Oncología Radioterápica, Departamento de Radiofísica, Hospital Universitario "La Princesa", Madrid, Spain
| | - Alejandro Floriano
- Servicio de Oncología Radioterápica, Departamento de Radiofísica, Hospital Universitario "La Princesa", Madrid, Spain
| | - David Sevillano
- Servicio de Oncología Radioterápica, Departamento de Radiofísica, Hospital Universitario "La Princesa", Madrid, Spain
| | - Leopoldo Pérez
- Servicio de Oncología Radioterápica, Departamento de Radiofísica, Hospital Universitario "La Princesa", Madrid, Spain
| | - Juan J Torres
- Servicio de Oncología Radioterápica, Departamento de Radiofísica, Hospital Universitario "La Princesa", Madrid, Spain
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Krauss A, Kapsch RP. Calorimetric determination ofkQfactors for NE 2561 and NE 2571 ionization chambers in 5 cm × 5 cm and 10 cm × 10 cm radiotherapy beams of 8 MV and 16 MV photons. Phys Med Biol 2007; 52:6243-59. [DOI: 10.1088/0031-9155/52/20/011] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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31
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Bruggmoser G, Saum R, Schmachtenberg A, Schmid F, Schüle E. Determination of the recombination correction factorkSfor some specific plane-parallel and cylindrical ionization chambers in pulsed photon and electron beams. Phys Med Biol 2006; 52:N35-50. [PMID: 17202615 DOI: 10.1088/0031-9155/52/2/n01] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
It has been shown from an evaluation of the inverse reading of the dosemeter (1/M) against the inverse of the polarizing voltage (1/V), obtained with a number of commercially available ionization chambers, using dose per pulse values between 0.16 and 5 mGy, that a linear relationship between the recombination correction factor kS and dose per pulse (DPP) can be found. At dose per pulse values above 1 mGy the method of a general equation with coefficients dependent on the chamber type gives more accurate results than the Boag method. This method was already proposed by Burns and McEwen (1998, Phys. Med. Biol. 43 2033) and avoids comprehensive and time-consuming measurements of Jaffé plots which are a prerequisite for the application of the multi-voltage analysis (MVA) or the two-voltage analysis (TVA). We evaluated and verified the response of ionization chambers on the recombination effect in pulsed accelerator beams for both photons and electrons. Our main conclusions are: (1) The correction factor k(S) depends only on the DPP and the chamber type. There is no influence of radiation type and energy. (2) For all the chambers investigated there is a linear relationship between kS and DPP up to 5 mGy/pulse, and for two chambers we could show linearity up to 40 mGy/pulse. (3) A general formalism, such as that of Boag, characterizes chambers exclusively by the distance of the electrodes and gives a trend for the correction factor, and therefore (4) a general formalism has to reflect the influence of the chamber construction on the recombination by the introduction of chamber-type dependent coefficients.
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Affiliation(s)
- G Bruggmoser
- Univ.-Klinikum Freiburg, Klinik f Strahlenheilkunde, FB Med. Physik, Freiburg, Germany.
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32
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Tailor RC, Hanson WF, Wells N, Ibbott GS. Consistency of absorbed dose to water measurements using 21 ion-chamber models following the AAPM TG51 and TG21 calibration protocols. Med Phys 2006; 33:1818-28. [PMID: 16872089 DOI: 10.1118/1.2199598] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
In 1999, the AAPM introduced a reference dosimetry protocol, known as TG51, based on an absorbed dose standard. This replaced the previous protocol, known as TG21, which was based on an air kerma standard. A significant body of literature has emerged discussing the improved accuracy and robustness of the absorbed dose standard, and quantifying the changes in baseline dosimetry with the introduction of the absorbed dose protocol. A significant component playing a role in the overall accuracy of beam output determination is the variability due to the use of different dosimeters. This issue, not adequately addressed in the past, is the focus of the present study. This work provides a comparison of absorbed dose determinations using 21 different makes and models of ion chambers for low- and high-energy photon and electron beams. The study included 13 models of cylindrical ion chambers and eight models of plane-parallel chambers. A high degree of precision (<0.25%) resulted from measurements with all chambers in a single setting, a sufficient number of repeat readings, and the use of high quality ion chambers as external monitors. Cylindrical chambers in photon beams show an improvement in chamber-to-chamber consistency with TG51. For electron dosimetry with plane-parallel chambers, the parameters Ngas and the product ND,w x k(ecal) were each determined in two ways, based on (i) an ADCL calibration, and (ii) a cross comparison with an ADCL-calibrated cylindrical chamber in a high-energy electron beam. Plane-parallel chamber results, therefore, are presented for both methods of chamber calibration. Our electron results with technique (i) show that plane-parallel chambers, as a group, overestimate the beam output relative to cylindrical chambers by 1%-2% with either protocol. Technique (ii), by definition, normalizes the plane-parallel results to the cylindrical results. In all cases, the maximum spread in output from the various cylindrical chambers is <2% implying a standard deviation of less than 0.5%. For plane-parallel chambers, the maximum spread is somewhat larger, up to 3%. A few chambers have been identified as outliers.
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Affiliation(s)
- Ramesh C Tailor
- MD Anderson Cancer Center, University of Texas, Houston, Texas 77030, USA
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Buckley LA, Rogers DWO. Wall correction factors, Pwall, for parallel-plate ionization chambers. Med Phys 2006; 33:1788-96. [PMID: 16872086 DOI: 10.1118/1.2199988] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, Pwall,, for parallel-plate ionization chambers in photon and electron beams. A set of Pwall values, computed at the reference depth in water, is presented for several commonly used parallel-plate chambers. These values differ from the standard assumption of unity used by dosimetry protocols by up to 1.7% for clinical electron beams. Calculations also show that Pwall is strongly dependent on the depth of measurement and can vary by as much as 6% for a 6 MeV beam in moving from a depth of dref to a depth of R50. In photon beams, where there is limited information available regarding Pwall for parallel-plate chambers, CSnrc calculations show Pwall values of up to 2.4% at the reference depth over a range of photon energies. The Pwall values for photon beams are in good agreement with previous estimates of the wall correction but have much lower statistical uncertainties and cover a wider range of photon beam energies.
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Affiliation(s)
- Lesley A Buckley
- Ottawa-Carleton Institute of Physics, Carleton University, Ottawa, K1S 5B6 Canada
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34
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Medin J, Ross CK, Klassen NV, Palmans H, Grusell E, Grindborg JE. Experimental determination of beam quality factors,kQ, for two types of Farmer chamber in a 10 MV photon and a 175 MeV proton beam. Phys Med Biol 2006; 51:1503-21. [PMID: 16510959 DOI: 10.1088/0031-9155/51/6/010] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Absorbed doses determined with a sealed water calorimeter operated at 4 degrees C are compared with the results obtained using ionization chambers and the IAEA TRS-398 code of practice in a 10 MV photon beam (TPR(20,10) = 0.734) and a 175 MeV proton beam (at a depth corresponding to the residual range, R(res) = 14.7 cm). Three NE 2571 and two FC65-G ionization chambers were calibrated in terms of absorbed-dose-to-water in (60)Co at the Swedish secondary standard dosimetry laboratory, directly traceable to the BIPM. In the photon beam quality, calorimetry was found to agree with ionometry within 0.3%, confirming the k(Q) values tabulated in TRS-398. In contrast, a 1.8% deviation was found in the proton beam at 6 g cm(-2) depth, suggesting that the TRS-398 tabulated k(Q) values for these two ionization chamber types are too high. Assuming no perturbation effect in the proton beam for the ionization chambers, a value for (w(air)/e)(Q) of 33.6 J C(-1) +/- 1.7% (k = 1) can be derived from these measurements. An analytical evaluation of the effect from non-elastic nuclear interactions in the ionization chamber wall indicates a perturbation effect of 0.6%. Including this estimated result in the proton beam would increase the determined (w(air)/e)(Q) value by the same amount.
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Affiliation(s)
- Joakim Medin
- Department of Medical Radiation Physics, Lund University, Malmö University Hospital, SE-205 02 Malmö, Sweden.
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35
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Abstract
The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, Pwall, for thimble ionization chambers in photon and electron beams. CSnrc calculated values of Pwall give closer agreement with previous experimental results than do the values from the standard formalism used in current dosimetry protocols. A set of Pwall values, computed at the reference depth in water, is presented for several commonly used thimble chambers. These values differ from the commonly used values by up to 0.8% for megavoltage photon beams, particularly for nominal beam energies below 6 MV. The sleeve effect, which is not currently taken into account by the TG-51 dosimetry protocol, is computed to be up to 0.3% and is in some cases larger than the Pwal1 correction itself. In electron beams, where dosimetry protocols assume a wall correction of unity, CSnrc calculations show Pwall values of up to 0.6% at the reference depth, depending on the wall material. Pwall is shown to be sensitive to the depth of measurement, varying by 2.5% for a graphite-walled cylindrical Farmer-like chamber between a depth of 0.5 cm and R50 in a 6 MeV electron beam.
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Affiliation(s)
- Lesley A Buckley
- Ottawa-Carleton Institute of Physics, Carleton University, Ottawa, K1S 5B6, Canada
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36
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Kirov AS, Piao JZ, Mathur NK, Miller TR, Devic S, Trichter S, Zaider M, Soares CG, LoSasso T. The three-dimensional scintillation dosimetry method: test for a106Ru eye plaque applicator. Phys Med Biol 2005; 50:3063-81. [PMID: 15972981 DOI: 10.1088/0031-9155/50/13/007] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Abstract
The need for fast, accurate and high resolution dosimetric quality assurance in radiation therapy has been outpacing the development of new and improved 2D and 3D dosimetry techniques. This paper summarizes the efforts to create a novel and potentially very fast, 3D dosimetry method based on the observation of scintillation light from an irradiated liquid scintillator volume serving simultaneously as a phantom material and as a dose detector medium. The method, named three-dimensional scintillation dosimetry (3DSD), uses visible light images of the liquid scintillator volume at multiple angles and applies a tomographic algorithm to a series of these images to reconstruct the scintillation light emission density in each voxel of the volume. It is based on the hypothesis that with careful design and data processing, one can achieve acceptable proportionality between the local light emission density and the locally absorbed dose. The method is applied to a Ru-106 eye plaque immersed in a 16.4 cm3 liquid scintillator volume and the reconstructed 3D dose map is compared along selected profiles and planes with radiochromic film and diode measurements. The comparison indicates that the 3DSD method agrees, within 25% for most points or within approximately 2 mm distance to agreement, with the relative radiochromic film and diode dose distributions in a small (approximately 4.5 mm high and approximately 12 mm diameter) volume in the unobstructed, high gradient dose region outside the edge of the plaque. For a comparison, the reproducibility of the radiochromic film results for our measurements ranges from 10 to 15% within this volume. At present, the 3DSD method is not accurate close to the edge of the plaque, and further than approximately 10 mm (<10% central axis depth dose) from the plaque surface. Improvement strategies, considered important to provide a more accurate quick check of the dose profiles in 3D for brachytherapy applicators, are discussed.
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Affiliation(s)
- A S Kirov
- Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY, USA.
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37
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An experimental investigation of the electron energy dependence of the EPR alanine dosimetry system. RADIAT MEAS 2005. [DOI: 10.1016/j.radmeas.2004.05.010] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
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38
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Huq MS, Andreo P. Advances in the determination of absorbed dose to water in clinical high-energy photon and electron beams using ionization chambers. Phys Med Biol 2004; 49:R49-104. [PMID: 15005158 DOI: 10.1088/0031-9155/49/4/r01] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
During the last two decades, absorbed dose to water in clinical photon and electron beams was determined using dosimetry protocols and codes of practice based on radiation metrology standards of air kerma. It is now recommended that clinical reference dosimetry be based on standards of absorbed dose to water. Newer protocols for the dosimetry of radiotherapy beams, based on the use of an ionization chamber calibrated in terms of absorbed dose to water, N(D,w), in a standards laboratory's reference quality beam, have been published by several national or regional scientific societies and international organizations. Since the publication of these protocols multiple theoretical and experimental dosimetry comparisons between the various N(D,w) based recommendations, and between the N(D,w) and the former air kerma (NK) based protocols, have been published. This paper provides a comprehensive review of the dosimetry protocols based on these standards and of the intercomparisons of the different protocols published in the literature, discussing the reasons for the observed discrepancies between them. A summary of the various types of standards of absorbed dose to water, together with an analysis of the uncertainties along the various steps of the dosimetry chain for the two types of formalism, is also included. It is emphasized that the NK-N(D,air) and N(D,w) formalisms have very similar uncertainty when the same criteria are used for both procedures. Arguments are provided in support of the recommendation for a change in reference dosimetry based on standards of absorbed dose to water.
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Affiliation(s)
- M Saiful Huq
- Department of Radiation Oncology, Kimmel Cancer Center of Jefferson Medical College, Thomas Jefferson University, Philadelphia, PA 19107, USA
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Zeng GG, McEwen MR, Rogers DW, Klassen NV. An experimental and Monte Carlo investigation of the energy dependence of alanine/EPR dosimetry: I. Clinical x-ray beams. Phys Med Biol 2004; 49:257-70. [PMID: 15083670 DOI: 10.1088/0031-9155/49/2/006] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The energy dependence of alanine/EPR dosimetry, in terms of absorbed dose-to-water for clinical 6, 10, 25 MV x-rays and 60Co gamma-rays was investigated by measurements and Monte Carlo (MC) calculations. The dose rates were traceable to the NRC primary standard for absorbed dose, a sealed water calorimetry. The electron paramagnetic resonance (EPR) spectra of irradiated pellets were measured using a Bruker EMX 081 EPR spectrometer. The DOSRZnrc Monte Carlo code of the EGSnrc system was used to simulate the experimental conditions with BEAM code calculated input spectra of x-rays and gamma-rays. Within the experimental uncertainty of 0.5%, the alanine EPR response to absorbed dose-to-water for x-rays was not dependent on beam quality from 6 MV to 25 MV, but on average, it was about 0.6% lower than its response to 60Co gamma-rays. Combining experimental data with Monte Carlo calculations, it is found that the alanine/EPR response per unit absorbed dose-to-alanine is the same for clinical x-rays and 60Co gamma-rays within the uncertainty of 0.6%. Monte Carlo simulations showed that neither the presence of PMMA holder nor varying the dosimeter thickness between 1 mm and 5 mm has significant effect of the energy dependence of alanine/EPR dosimetry within the calculation uncertainty of 0.3%.
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Affiliation(s)
- G G Zeng
- National Research Council of Canada, Ottawa, K1A 0R6, Canada.
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40
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Lopez Medina A, Teijeiro A, Salvador F, Medal D, Vazquez J, Salgado M, Carrion MC. Comparison between TG-51 and TRS-398: electron contamination effect on photon beam-quality specification. Phys Med Biol 2003; 49:17-32. [PMID: 14971770 DOI: 10.1088/0031-9155/49/1/002] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Two dosimetry protocols based on absorbed dose to water have recently been implemented: TG-51 and TRS-398. These protocols use different beam-quality indices: %dd(10)x and TPR20,10. The effect of electron contamination in measurements of %dd(10)x has been proposed as a disadvantage of the TG-51. For actual measurements of %dd(10)x in five clinical beams (Primus 6-18 MV, SL-75/5 6 MV, SL-18 6-15 MV) a purging magnet was employed to remove the electron contamination. Also, %dd(10)x was measured in the different ways described in TG-51 for high-energy beams: with a lead foil at 50 cm from the phantom surface, at 30 cm, and for open beam. Moreover, TPR20,10 was determined. Also, periodic quality-control measurements were used for comparing both quality indices and variation over time, but D20,10 was used instead of TPR20,10 and measurements in open beam for the %dd(10)x determination. Considering both protocols, S(w,air) and kQ were calculated in order to compare the results with the experimental data. Significant differences (0.3% for kQ) were only found for the two high-energy beams, but when the electron contamination is underestimated by TG-51, the difference in kQ is lower. Differences in the other cases and variations over time were less than 0.1%.
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Affiliation(s)
- Antonio Lopez Medina
- Department of Medical Physics, Instituto Galego de Medicina Tecnica, Hospital do Meixoeiro, 36200 Vigo, Spain.
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41
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Kalach NI, Rogers DWO. Which accelerator photon beams are "clinic-like" for reference dosimetry purposes? Med Phys 2003; 30:1546-55. [PMID: 12906172 DOI: 10.1118/1.1573205] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
Previous work has demonstrated that, for photon beam dosimetry, TPR10(20) is not an ideal beam quality specifier for all bremsstrahlung beams, especially for lightly filtered beams in some standards laboratories. This paper addresses the following questions: Is TPR10(20) an adequate beam quality specifier for all modern clinical therapy accelerators? When can nonclinical beams in standards laboratories be used to calibrate ion chambers or measure kQ factors as a function of TPR10(20)? Based on detailed Monte Carlo simulations of Varian, Siemens, Elekta, and GE (Saturn) accelerators one can conclude that TPR10(20) is an adequate beam quality specifier for all these machines in the sense that for a given value of TPR10(20), the value of stopping-power ratios is the same. It is shown that, for the heavily filtered beams used in standards laboratories, TPR10(20) is an adequate beam quality specifier. It is also demonstrated that, for a larger range of bremsstrahlung beams than previously, %dd(10)x is a good beam quality specifier for all clinical beams as well as the lightly and heavily filtered beams in some standards laboratories. A criterion, based on the measured values of TPR10(20) and %dd(10)x for the beam, is proposed for determining whether a nonclinical beam is well specified by TPR10(20). Agreement between calculations for specific accelerators and measured beam quality specifiers is shown to be good, but agreement with published data for a variety of clinical accelerators is not as good. Possible reasons for the discrepancy are discussed.
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Affiliation(s)
- N I Kalach
- National Research Council of Canada, Ottawa K1A OR6, Canada.
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42
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Bergstrand ES, Shortt KR, Ross CK, Hole EO. An investigation of the photon energy dependence of the EPR alanine dosimetry system. Phys Med Biol 2003; 48:1753-71. [PMID: 12870581 DOI: 10.1088/0031-9155/48/12/306] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
The electron paramagnetic resonance (EPR) alanine dosimetry system is based on EPR measurements of radicals formed in alanine by ionizing radiation. The system has been studied to determine its energy dependence for photons in the 10-30 MV region relative to those of 60Co and to find out if the system would be suitable for dosimetry comparisons. The irradiations were carried out at the National Research Council, Ottawa, Canada and the doses ranged from 8 to 54 Gy. The EPR measurements were performed at the University of Oslo, Norway. The ratio of the slope of the alanine reading versus dose-to-water curve for a certain linac photon beam quality and the corresponding slope for a reference 60Co gamma-radiation gives an experimental measure of the relative dose-to-water response of the EPR alanine dosimetry system. For calculating the linear regression coefficients of these alanine reading versus dose curves, the method of weighted least squares was used. This method is assumed to produce more accurate regression coefficients when applied to EPR dosimetry than the common method of standard least squares. The overall uncertainty on the ratio of slopes was between 0.5 and 0.6% for all three linac energies. The relative response for all the linac beams compared to cobalt was less than unity: by about 0.5% for the 20 and 30 MV points but by more than 1% for the 10 MV point. The given standard uncertainties negate concluding that there is any significant internal variation in the measured response as a function of beam quality between the three linac energies. Thus, we calculated the average dose response for all three energies and found that the alanine response is 0.8% (+/-0.5%) lower for high energy x-rays than for 60Co gamma-rays. This result indicates a small energy dependence in the alanine response for the high-energy photons relative to 60Co which may be significant. This result is specific to our dosimetry system (alanine with 20% polyethylene binder pressed into a particular shape) including its waterproofing sleeve of PMMA (2 mm thick); however, we expect that this result may apply to other similar detectors.
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43
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Scheithauer M, Schwedas M, Wiezorek T, Keller A, Wendt TG, Harder D. [Improvement of the accuracy of the Laplace transform method for the determination of radiotherapy spectra of clinical linear accelerators]. Z Med Phys 2003; 13:22-9. [PMID: 12723520 DOI: 10.1078/0939-3889-00131] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
The present study focused on the reconstruction of the bremsstrahlung spectrum of a clinical linear accelerator from the measured transmission curve, with the aim of improving the accuracy of this method. The essence of the method was the analytic inverse Laplace transform of a parameter function fitted to the measured transmission curve. We tested known fitting functions, however they resulted in considerable fitting inaccuracy, leading to inaccuracies of the bremsstrahlung spectrum. In order to minimise the fitting errors, we employed a linear combination of n equations with 2n-1 parameters. The fitting errors are now considerably smaller. The measurement of the transmission function requires that the energy-dependent detector response is taken into account. We analysed the underlying physical context and developed a function that corrects for the energy-dependent detector response. The factors of this function were experimentally determined or calculated from tabulated values.
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Affiliation(s)
- Marcel Scheithauer
- Klinik für Radiologie, Abt. Strahlentherapie, Universitätsklinikum Jena.
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44
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Ślosarek K. Techniki dynamiczne generujące zróżnicowany rozkład dawki promieniowania w radioterapii. Rep Pract Oncol Radiother 2003. [DOI: 10.1016/s1507-1367(01)70484-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022] Open
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45
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Palm A, Mattsson O. Experimental determination of beam quality conversion factors k(Q) in clinical photon beams using ferrous sulphate (Fricke) dosimetry. Med Phys 2002; 29:2756-62. [PMID: 12512708 DOI: 10.1118/1.1521941] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
The implementation of protocols based on absorbed dose to water standards requires beam quality conversion factors, k(Q). Calculated values of k(Q) are available for ionization chambers used for reference dosimetry. Ideally, k(Q) should be experimentally determined at the same beam qualities as that of the user. In this work we measure k(Q) factors in clinical photon beams and compare them with calculated and measured values. Beam quality conversion factors are determined for clinical photon beams of nominal energies 4 MV, 6 MV, 15 MV, and 25 MV, for commonly used cylindrical ionization chambers. Twelve chambers of eight different types are used. For three of them, no experimental data have previously been available. The experimental procedure is based on measurements with ionization chambers and Fricke dosimetry in the reference beam (60Co gamma radiation) and in clinical linear accelerator beams. The k(Q) values determined in this work generally agree within 0.5% with previously reported experimental values both when %dd(10)x and TPR2010 are used for beam quality specification. The agreement with calculated data is generally within 0.5%, except for the 15 MV beam. For this beam the measured values are usually between 0.5% and 1% lower than the data taken from the TG-51 protocol or the TRS-398 code of practice. For three NE2571 chambers and three NE2581 chambers, the maximum observed deviation of individual k(Q) values is 0.2% and 0.4%, respectively.
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Affiliation(s)
- Asa Palm
- Department of Radiation Physics, Sahlgrenska University Hospital, SE413 45 Göteborg, Sweden.
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46
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Thierens HPWMAH. Beam quality of high-energy photon beams at the Ghent University linear accelerator. Phys Med Biol 2002. [DOI: 10.1088/0031-9155/47/17/101] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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47
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Araki F, Kubo HD. Comparison of high-energy photon and electron dosimetry for various dosimetry protocols. Med Phys 2002; 29:857-68. [PMID: 12033582 DOI: 10.1118/1.1470208] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
The American Association of Physicists in Medicine Task Group 51 (TG-51) and the International Atomic Energy Agency (IAEA) published a new high-energy photon and electron dosimetry protocol, in 1999 and 2000, respectively. These protocols are based on the use of an ion chamber having an absorbed-dose to water calibration factor with a 60Co beam. These are different from the predecessors, the TG-21 and IAEA TRS-277 protocols, which require a 60Co exposure or air-kerma calibration factor. The purpose of this work is to present the dose comparison between various dosimetry protocols and the AAPM TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. The absorbed-dose to water calculated according to the Japanese Association of Radiological Physics (JARP), International Atomic Energy Agency Technical Report Series No. 277 (IAEA TRS-277) and No. 398 (IAEA TRS-398) protocols is compared to that calculated using the TG-51 protocol. For various Farmer-type chambers in photon beams, TG-51 is found to predict 0.6-2.1% higher dose than JARP. Similarly, TG-51 is found to be higher by 0.7-1.7% than TRS-277. For electron beams TG-51 is higher than JARP by 1.5-3.8% and TRS-277 by 0.2-1.9%. The reasons for these differences are presented in terms of the cavity-gas calibration factor, Ngas, and a dose conversion factor, Fw, which converts the absorbed-dose to air in the chamber to the absorbed-dose to water. The ratio of cavity-gas calibration factors based on absorbed-dose to water calibration factors, N60Co(D,w), in TG-51 and cavity-gas calibration factors which are equivalent to absorbed-dose to air chamber factors, N(D,air), based on the IAEA TRS-381 protocol is 1.008 on average. However, the estimated uncertainty of the ratio between the two cavity-gas calibration factors is 0.9% (1 s.d.) and consequently, the observed difference of 0.8% is not significant. The absorbed-dose to water and exposure or air-kerma calibration factors are based on standards traceable to the National Institute of Standards and Technology (NIST). In contrast, the absorbed-dose to water determined with TRS-398 is in good agreement with TG-51 within about 0.5% for photon and electron beams.
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Affiliation(s)
- Fujio Araki
- Department of Radiation Oncology, University of California Davis Cancer Center, Sacramento 95817, USA.
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48
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Stewart KJ, Seuntjens JP. Comparing calibration methods of electron beams using plane-parallel chambers with absorbed-dose to water based protocols. Med Phys 2002; 29:284-9. [PMID: 11929010 DOI: 10.1118/1.1449876] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
Abstract
Recent absorbed-dose-based protocols allow for two methods of calibrating electron beams using plane-parallel chambers, one using the N(Co)D,w for a plane-parallel chamber, and the other relying on cross-calibration of the plane-parallel chamber in a high-energy electron beam against a cylindrical chamber which has an N(Co)D,w factor. The second method is recommended as it avoids problems associated with the Pwall correction factors at 60Co for plane-parallel chambers which are used in the determination of the beam quality conversion factors. In this article we investigate the consistency of these two methods for the PTW Roos, Scanditronics NACP02, and PTW Markus chambers. We processed our data using both the AAPM TG-51 and the IAEA TRS-398 protocols. Wall correction factors in 60Co beams and absorbed-dose beam quality conversion factors for 20 MeV electrons were derived for these chambers by cross-calibration against a cylindrical ionization chamber. Systematic differences of up to 1.6% were found between our values of Pwall and those from the Monte Carlo calculations underlying AAPM TG-51, and up to 0.6% when comparing with the IAEA TRS-398 protocol. The differences in Pwall translate directly into differences in the beam quality conversion factors in the respective protocols. The relatively large spread in the experimental data of Pwall, and consequently the absorbed-dose beam quality conversion factor, confirms the importance of the cross-calibration technique when using plane-parallel chambers for calibrating clinical electron beams. We confirmed that for well-guarded plane-parallel chambers, the fluence perturbation correction factor at d(max) is not significantly different from the value at d(ref). For the PTW Markus chamber the variation in the latter factor is consistent with published fits relating it to average energy at depth.
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Affiliation(s)
- K J Stewart
- Department of Medical Physics, McGill University Health Centre, Montreal General Hospital, Canada.
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49
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Nutbrown RF, Duane S, Shipley DR, Thomas RAS. Evaluation of factors to convert absorbed dose calibrations from graphite to water for the NPL high-energy photon calibration service. Phys Med Biol 2002; 47:441-54. [PMID: 11858210 DOI: 10.1088/0031-9155/47/3/306] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
The National Physical Laboratory (NPL) provides a high-energy photon calibration service using 4-19 MV x-rays and 60Co gamma-radiation for secondary standard dosemeters in terms of absorbed dose to water. The primary standard used for this service is a graphite calorimeter and so absorbed dose calibrations must be converted from graphite to water. The conversion factors currently in use were determined prior to the launch of this service in 1988. Since then, it has been found that the differences in inherent filtration between the NPL LINAC and typical clinical machines are large enough to affect absorbed dose calibrations and, since 1992, calibrations have been performed in heavily filtered qualities. The conversion factors for heavily filtered qualities were determined by interpolation and extrapolation of lightly filtered results as a function of tissue phantom ratio 20,10 (TPR20,10). This paper aims to evaluate these factors for all mega-voltage photon energies provided by the NPL LINAC for both lightly and heavily filtered qualities and for 60Co y-radiation in two ways. The first method involves the use of the photon fluence-scaling theorem. This states that if two blocks of different material are irradiated by the same photon beam, and if all dimensions are scaled in the inverse ratio of the electron densities of the two media, then, assuming that all photon interactions occur by Compton scatter the photon attenuation and scatter factors at corresponding scaled points of measurement in the phantom will be identical. The second method involves making in-phantom measurements of chamber response at a constant target-chamber distance. Monte Carlo techniques are then used to determine the corresponding dose to the medium in order to determine the chamber calibration factor directly. Values of the ratio of absorbed dose calibration factors in water and in graphite determined in these two ways agree with each other to within 0.2% (1sigma uncertainty). The best fit to both sets of results agrees with values determined in previous work to within 0.3% (1sigma uncertainty). It is found that the conversion factor is not sensitive to beam filtration.
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Affiliation(s)
- R F Nutbrown
- Centre for Ionising Radiation Metrology, National Physical Laboratory, Teddington, Middlesex, UK
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50
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Palmans H, Nafaa L, De JJ, Gillis S, Hoornaert MT, Martens C, Piessens M, Thierens H, Van der Plaetsen A, Vynckier S. Absorbed dose to water based dosimetry versus air kerma based dosimetry for high-energy photon beams: an experimental study. Phys Med Biol 2002; 47:421-40. [PMID: 11848121 DOI: 10.1088/0031-9155/47/3/305] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
In recent years, a change has been proposed from air kerma based reference dosimetry to absorbed dose based reference dosimetry for all radiotherapy beams of ionizing radiation. In this paper, a dosimetry study is presented in which absorbed dose based dosimetry using recently developed formalisms was compared with air kerma based dosimetry using older formalisms. Three ionization chambers of each of three different types were calibrated in terms of absorbed dose to water and air kerma and sent to five hospitals. There, reference dosimetry with all the chambers was performed in a total of eight high-energy clinical photon beams. The selected chamber types were the NE2571, the PTW-30004 and the Wellhöfer-FC65G (previously Wellhöfer-IC70). Having a graphite wall, they exhibit a stable volume and the presence of an aluminium electrode ensures the robustness of these chambers. The data were analysed with the most important recommendations for clinical dosimetry: IAEA TRS-398, AAPM TG-51, IAEA TRS-277, NCS report-2 (presently recommended in Belgium) and AAPM TG-21. The necessary conversion factors were taken from those protocols, or calculated using the data in the different protocols if data for a chamber type are lacking. Polarity corrections were within 0.1% for all chambers in all beams. Recombination corrections were consistent with theoretical predictions, did not vary within a chamber type and only slightly between different chamber types. The maximum chamber-to-chamber variations of the dose obtained with the different formalisms within the same chamber type were between 0.2% and 0.6% for the NE2571, between 0.2% and 0.6% for the PTW-30004 and 0.1% and 0.3% for the Wellhöfer-FC65G for the different beams. The absorbed dose results for the NE2571 and Wellhöfer-FC65G chambers were in good agreement for all beams and all formalisms. The PTW-30004 chambers gave a small but systematically higher result compared to the result for the NE2571 chambers (on the average 0.1% for IAEA TRS-277, 0.3% for NCS report-2 and AAPM TG-21 and 0.4% for IAEA TRS-398 and AAPM TG-51). Within the air kerma based protocols, the results obtained with the TG-21 protocol were 0.4-0.8% higher mainly due to the differences in the data used. Both absorbed dose to water based formalisms resulted in consistent values within 0.3%. The change from old to new formalisms is discussed together with the traceability of calibration factors obtained at the primary absorbed dose and air kerma standards in the reference beams (60Co). For the particular situation in Belgium (calibrations at the Laboratory for Standard Dosimetry of Ghent) the change amounts to 0.1-0.6%. This is similar to the magnitude of the change determined in other countries.
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Affiliation(s)
- Hugo Palmans
- Subatomic and Radiation Physics Department, Ghent University, Gent, Belgium.
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