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Bourgouin A, Keszti F, Schönfeld AA, Hackel T, Kozelka J, Hildreth J, Simon W, Schüller A, Kapsch RP, Renaud J. The probe-format graphite calorimeter, Aerrow, for absolute dosimetry in ultra-high pulse dose rate electron beams. Med Phys 2022; 49:6635-6645. [PMID: 35912973 DOI: 10.1002/mp.15899] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2022] [Revised: 07/02/2022] [Accepted: 07/22/2022] [Indexed: 11/12/2022] Open
Abstract
PURPOSE The purpose of this investigation is to evaluate the use of a probe-format graphite calorimeter, Aerrow, as an absolute and relative dosimeter of high-energy pulse dose rate (UHPDR) electron beams for in-water reference and depth-dose type measurements, respectively. METHODS In this paper, the calorimeter system is used to investigate the potential influence of dose per pulses delivered up to 5.6 Gy, the number of pulses delivered per measurement and its potential for relative measurement (depth dose curve measurement). The calorimeter system is directly compared against an Advanced Markus ion chamber. The finite element method was used to calculate heat transfer corrections along the percentage depth dose of a 20 MeV electron beam. Monte Carlo-calculated dose conversion factors necessary to calculate absorbed dose to-water at a point from the measured dose-to-graphite are also presented. RESULTS The comparison of Aerrow against a fully calibrated Advanced Markus chamber, corrected for the saturation effect, has shown consistent results in terms of dose to water determination. The measured reference depth is within 0.5 mm from the expected value from Monte Carlo simulation. The relative standard uncertainty estimated for Aerrow was 1.06%, which is larger compared to alanine dosimetry (McEwen et al. https://doi.org/10.1088/0026-1394/52/2/272) but has the advantage of being a real time detector. CONCLUSION In this investigation, it was demonstrated that the Aerrow probe-type graphite calorimeter can be used for relative and absolute dosimetry in water in an UHPDR electron beam. To the author's knowledge, this is the first reported use of an absorbed dose calorimeter for an in-water percentage depth dose curve measurement. The use of the Aerrow in quasi-adiabatic mode has greatly simplified the signal readout, compared to isothermal mode, as the resistance was directly measured with a high-stability Digital-Multimetre (DMM). This article is protected by copyright. All rights reserved.
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Affiliation(s)
- Alexandra Bourgouin
- Dosimetry for radiation therapy and diagnostic radiology, Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany
| | - Federico Keszti
- Medical Physics Unit, McGill University, Montreal, Quebec, Canada
| | | | - Thomas Hackel
- Dosimetry for radiation therapy and diagnostic radiology, Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany
| | - Jakub Kozelka
- Research, Sun Nuclear, A Mirion Medical Company, Melbourne, Florida, USA
| | - Jeff Hildreth
- Research, Sun Nuclear, A Mirion Medical Company, Melbourne, Florida, USA
| | - William Simon
- Research, Sun Nuclear, A Mirion Medical Company, Melbourne, Florida, USA
| | - Andreas Schüller
- Dosimetry for radiation therapy and diagnostic radiology, Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany
| | - Ralf-Peter Kapsch
- Dosimetry for radiation therapy and diagnostic radiology, Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany
| | - James Renaud
- Medical and Industrial Dosimetry, National Research Council of Canada (NRC), Ottawa, Ontario, Canada
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Nusrat BRM, Sarfehnia A, Renaud J. Monte Carlo optimization and experimental validation of a prototype ionization chamber for accurate magnetic resonance image guided radiation therapy (MRgRT) daily output constancy measurements in solid phantoms. Med Phys 2022; 49:5483-5490. [PMID: 35536047 DOI: 10.1002/mp.15695] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2021] [Revised: 03/21/2022] [Accepted: 04/25/2022] [Indexed: 11/09/2022] Open
Abstract
PURPOSE To optimize the design, develop and test a prototype ionization chamber for accurate daily output constancy measurements in solid phantoms in clinical MRgRT radiotherapy beams. Up to 4 % variations in response using commercial ionization chambers have been previously reported; the prototype ionization chamber developed here aims to minimize these variations. METHODS Monte Carlo simulations with the EGSnrc code system are used to optimize an ionization chamber design by increasing the thickness of a brass (high-density, non-ferromagnetic, easy-to-machine) wall until results consistent with no air gap are produced for simulations with a 1.5 T and 0.35 T magnetic field, with a 0.2 mm air gap and varying the placement of the chamber model within the air gap. Based on the results of these simulations, prototype ionization chambers are manufactured and tested in conventional linac beams and in a 7 MV Elekta Unity MR-linac. The chambers are rotated about their axes, both parallel and perpendicular to the 1.5 T magnetic field, through 360 degrees in a plastic phantom with measurements made at each cardinal angle. This reveals any variation in chamber response by varying the thickness of the air gap between the chamber and the phantom. RESULTS Monte Carlo simulations demonstrate that the optimal thickness of the chamber wall to mitigate the effect of an asymmetric air gap between the chamber and the plastic phantom is 1.1 mm of brass. With this thickness, the differences between simulations with and without an air gap and with asymmetric placement of the chamber within the air gap are less than 0.2 %. A prototype chamber constructed with a 1.1 mm brass wall thickness exhibits less than 0.3 % variation in response when rotated about its axis in the plastic phantom in a beam from an MR-linac, independent of whether its axis is parallel or perpendicular to the magnetic field. CONCLUSION The optimized ionization chamber design and validated prototype for accurate MR-linac daily output constancy measurements allows utilization of conventional phantoms and procedures in MRgRT systems. This can minimize disruption to clinical workflow for MR-linac QA measurements. This article is protected by copyright. All rights reserved.
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Affiliation(s)
| | - Arman Sarfehnia
- Department of Radiation Oncology, University of Toronto, Toronto, ON, M4N 3M5, Canada
| | - James Renaud
- NRC Metrology Research Centre, National Research Council of Canada, Ottawa, ON, K1A 0R6, Canada
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Côté B, Keszti F, Bancheri J, Sarfehnia A, Seuntjens J, Renaud J. Feasibility of operating a millimeter-scale graphite calorimeter for absolute dosimetry of small-field photon beams in the clinic. Med Phys 2021; 48:7476-7492. [PMID: 34549805 DOI: 10.1002/mp.15244] [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: 08/09/2020] [Revised: 07/06/2021] [Accepted: 08/28/2001] [Indexed: 11/11/2022] Open
Abstract
PURPOSE To characterize and build a cylindrically layered graphite calorimeter the size of a thimble ionization chamber for absolute dosimetry of small fields. This detector has been designed in a familiar probe format to facilitate integration into the clinical workflow. The feasibility of operating this absorbed dose calorimeter in quasi-adiabatic mode is assessed for high-energy accelerator-based photon beams. METHODS This detector, herein referred to as Aerrow MK7, is a miniaturized version of a previously validated aerogel-insulated graphite calorimeter known as Aerrow. The new model was designed and developed using numerical methods. Medium conversion factors from graphite to water, small-field output correction factors, and layer perturbation factors for this dosimeter were calculated using the EGSnrc Monte Carlo code system. A range of commercially available aerogel densities were studied for the insulating layers, and an optimal density was selected by minimizing the small-field output correction factors. Heat exchange within the detector was simulated using a five-body compartmental heat transfer model. In quasi-adiabatic mode, the sensitive volume (a 3 mm diameter cylindrical graphite core) experiences a temperature rise during irradiation on the order of 1.3 mK·Gy-1 . The absorbed dose is obtained by calculating the product of this temperature rise with the specific heat capacity of the graphite. The detector was irradiated with 6 MV ( % dd ( 10 ) x = 63.5%) and 10 MV ( % dd ( 10 ) x = 71.1%) flattening filter-free (FFF) photon beams for two field sizes, characterized by S clin dimensions of 2.16 and 11.0 cm. The dose readings were compared against a calibrated Exradin A1SL ionization chamber. All dose values are reported at d max in water. RESULTS The field output correction factors for this dosimeter design were computed for field sizes ranging from S clin = 0.54 to 11.0 cm. For all aerogel densities studied, these correction factors did not exceed 1.5%. The relative dose difference between the two dosimeters ranged between 0.3% and 0.7% for all beams and field sizes. The smallest field size experimentally investigated, S clin = 2.16 cm, which was irradiated with the 10 MV FFF beam, produced readings of 84.4 cGy (±1.3%) in the calorimeter and 84.5 cGy (±1.3%) in the ionization chamber. CONCLUSION The median relative difference in absorbed dose values between a calibrated A1SL ionization chamber and the proposed novel graphite calorimeter was 0.6%. This preliminary experimental validation demonstrates that Aerrow MK7 is capable of accurate and reproducible absorbed dose measurements in quasi-adiabatic mode.
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Affiliation(s)
- Benjamin Côté
- Medical Physics Unit, Gerald Bronfman Department of Oncology, McGill University, Montreal, Quebec, Canada
| | - Federico Keszti
- Medical Physics Unit, Gerald Bronfman Department of Oncology, McGill University, Montreal, Quebec, Canada
| | - Julien Bancheri
- Medical Physics Unit, Gerald Bronfman Department of Oncology, McGill University, Montreal, Quebec, Canada
| | - Arman Sarfehnia
- Medical Physics Unit, Gerald Bronfman Department of Oncology, McGill University, Montreal, Quebec, Canada.,Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada
| | - Jan Seuntjens
- Medical Physics Unit, Gerald Bronfman Department of Oncology, McGill University, Montreal, Quebec, Canada
| | - James Renaud
- Medical Physics Unit, Gerald Bronfman Department of Oncology, McGill University, Montreal, Quebec, Canada.,Metrology Research Centre, National Research Council Canada, Ottawa, Ontario, Canada
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Krauss A, Spindeldreier CK, Klüter S. Direct determination of [Formula: see text] for cylindrical ionization chambers in a 6 MV 0.35 T MR-linac. Phys Med Biol 2020; 65:235049. [PMID: 33300501 DOI: 10.1088/1361-6560/abab56] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
To ensure accurate reference dosimetry with ionization chambers in magnetic resonance linear accelerators (MR-linacs), the influence of the magnetic field on the response of the ionization chambers must be considered. The most direct method considering the influence of magnetic fields in dosimetry is to apply an appropriate absorbed-dose-to-water primary standard. At PTB, a new water calorimeter has been designed which is capable to determine Dw,Q in an MR-linac. The new device allows the direct calibration of ionization chambers in terms of absorbed dose to water for MR-linac irradiation conditions. Hence, the correction factors [Formula: see text] can be determined which replace the current radiation-quality dependent correction factors [Formula: see text] for dosimetry in the presence of magnetic fields. In cooperation with Heidelberg University Hospital,[Formula: see text] factors were measured at the 6 MV 0.35 T Viewray MR-linac for different cylindrical ionization chambers with sensitive volumes ranging from 0.015 cm3 to 0.65 cm3. The chambers were placed both perpendicular and parallel in respect to the magnetic field. Standard uncertainties of about 0.5% were achieved.
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Affiliation(s)
- A Krauss
- Department of Dosimetry for Radiation Therapy and Diagnostic Radiology, Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, D-38116, Braunschweig, Germany
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D'Souza M, Nusrat H, Iakovenko V, Keller B, Sahgal A, Renaud J, Sarfehnia A. Water calorimetry in MR‐linac: Direct measurement of absorbed dose and determination of chamber. Med Phys 2020; 47:6458-6469. [DOI: 10.1002/mp.14468] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2019] [Revised: 07/22/2020] [Accepted: 08/11/2020] [Indexed: 11/10/2022] Open
Affiliation(s)
- Mark D'Souza
- Department of Physics Ryerson University 350 Victoria St. Toronto ONM5B 2K3Canada
| | - Humza Nusrat
- Department of Radiation Oncology University of Toronto 2075 Bayview Ave. Toronto ONM4N 3M5Canada
| | - Viktor Iakovenko
- Department of Radiation Oncology University of Toronto 2075 Bayview Ave. Toronto ONM4N 3M5Canada
| | - Brian Keller
- Department of Radiation Oncology University of Toronto 2075 Bayview Ave. Toronto ONM4N 3M5Canada
| | - Arjun Sahgal
- Department of Radiation Oncology University of Toronto 2075 Bayview Ave. Toronto ONM4N 3M5Canada
| | - James Renaud
- Meterology Research Centre National Research Council Canada Montreal Rd. Ottawa ONK1A OR6Canada
- Medical Physics Unit McGill University 1001 Decarie Blvd. Montreal QCH4A 3J1Canada
| | - Arman Sarfehnia
- Department of Physics Ryerson University 350 Victoria St. Toronto ONM5B 2K3Canada
- Department of Radiation Oncology University of Toronto 2075 Bayview Ave. Toronto ONM4N 3M5Canada
- Department of Radiation Oncology McGill University 1001 Decarie Blvd. Montreal QCH4A 3J1Canada
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D'Souza M, Nusrat H, Renaud J, Peterson G, Sarfehnia A. First-stage validation of a portable imageable MR-compatible water calorimeter. Med Phys 2020; 47:5312-5323. [PMID: 32786081 DOI: 10.1002/mp.14448] [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: 02/04/2020] [Revised: 07/24/2020] [Accepted: 07/31/2020] [Indexed: 11/07/2022] Open
Abstract
PURPOSE The purpose of this study is to design a water calorimeter with three goals in mind: (a) To be fully magnetic resonance (MR)-compatible; (b) To be imaged using kV cone beam computed tomography (CBCT), MV portal imaging or MRI for accurate positioning; (c) To accommodate both vertical and horizontal beam incidence, as well as volumetric deliveries or Gamma Knife®. Following this, the calorimeter performance will be measured using an accelerator-based high-energy photon beam. METHODS A portable 4°C cooled stagnant water calorimeter was built using MR-compatible materials. The walls consist of layers of acrylic plastic, aerogel-based material acting as thermal insulation, as well as tubing for coolant to flow to keep the calorimeter temperature stable at 4°C. The lid contains additional pathways for coolant to flow through as well as two hydraulically driven stirrers. The water calorimeter was positioned in an Elekta Versa using kV CBCT imaging as well as orthogonal MV image pairs. Absolute absorbed dose to water was then determined under a 6 MV flattening filter-free (FFF) beam. This was compared against reference dosimetry results that were measured under identical conditions with an Exradin A1SL ionization chamber with a calibration coefficient directly traceable to the National Research Council Canada. RESULTS The dose to water determined with the calorimeter (n = 30) agreed with the A1SL ionization chamber reference dose measurements (n = 15) to within 0.25%. The uncertainty associated with the water calorimeter absorbed dose measurement was estimated to be 0.54% (k = 1). CONCLUSIONS An MR-compatible water calorimeter was successfully built and absolute absorbed dose to water under a conventional 6 MV FFF beam was determined successfully as a first-stage validation of the system.
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Affiliation(s)
- Mark D'Souza
- Department of Physics, Ryerson University, 350 Victoria St., Toronto, ON, M5B 2K3, Canada
| | - Humza Nusrat
- Department of Radiation Oncology, University of Toronto, 2075 Bayview Ave., Toronto, ON, M4N 3M5, Canada.,Department of Medical Physics, Sunnybrook Odette Cancer Centre, 2075 Bayview Ave, Toronto, ON, M4N 3M5, Canada
| | - James Renaud
- Metrology Research Centre, National Research Council Canada, Montreal Rd., Ottawa, ON, K1A 0R6, Canada.,Medical Physics Unit, McGill University, 1001 Decarie Blvd., Montreal, QC, H4A 3J1, Canada
| | - Gerard Peterson
- Department of Medical Physics, Sunnybrook Odette Cancer Centre, 2075 Bayview Ave, Toronto, ON, M4N 3M5, Canada
| | - Arman Sarfehnia
- Department of Physics, Ryerson University, 350 Victoria St., Toronto, ON, M5B 2K3, Canada.,Department of Radiation Oncology, University of Toronto, 2075 Bayview Ave., Toronto, ON, M4N 3M5, Canada.,Department of Medical Physics, Sunnybrook Odette Cancer Centre, 2075 Bayview Ave, Toronto, ON, M4N 3M5, Canada.,Medical Physics Unit, McGill University, 1001 Decarie Blvd., Montreal, QC, H4A 3J1, Canada
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Kurz C, Buizza G, Landry G, Kamp F, Rabe M, Paganelli C, Baroni G, Reiner M, Keall PJ, van den Berg CAT, Riboldi M. Medical physics challenges in clinical MR-guided radiotherapy. Radiat Oncol 2020; 15:93. [PMID: 32370788 PMCID: PMC7201982 DOI: 10.1186/s13014-020-01524-4] [Citation(s) in RCA: 79] [Impact Index Per Article: 19.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2019] [Accepted: 03/24/2020] [Indexed: 12/18/2022] Open
Abstract
The integration of magnetic resonance imaging (MRI) for guidance in external beam radiotherapy has faced significant research and development efforts in recent years. The current availability of linear accelerators with an embedded MRI unit, providing volumetric imaging at excellent soft tissue contrast, is expected to provide novel possibilities in the implementation of image-guided adaptive radiotherapy (IGART) protocols. This study reviews open medical physics issues in MR-guided radiotherapy (MRgRT) implementation, with a focus on current approaches and on the potential for innovation in IGART.Daily imaging in MRgRT provides the ability to visualize the static anatomy, to capture internal tumor motion and to extract quantitative image features for treatment verification and monitoring. Those capabilities enable the use of treatment adaptation, with potential benefits in terms of personalized medicine. The use of online MRI requires dedicated efforts to perform accurate dose measurements and calculations, due to the presence of magnetic fields. Likewise, MRgRT requires dedicated quality assurance (QA) protocols for safe clinical implementation.Reaction to anatomical changes in MRgRT, as visualized on daily images, demands for treatment adaptation concepts, with stringent requirements in terms of fast and accurate validation before the treatment fraction can be delivered. This entails specific challenges in terms of treatment workflow optimization, QA, and verification of the expected delivered dose while the patient is in treatment position. Those challenges require specialized medical physics developments towards the aim of fully exploiting MRI capabilities. Conversely, the use of MRgRT allows for higher confidence in tumor targeting and organs-at-risk (OAR) sparing.The systematic use of MRgRT brings the possibility of leveraging IGART methods for the optimization of tumor targeting and quantitative treatment verification. Although several challenges exist, the intrinsic benefits of MRgRT will provide a deeper understanding of dose delivery effects on an individual basis, with the potential for further treatment personalization.
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Affiliation(s)
- Christopher Kurz
- Department of Radiation Oncology, University Hospital, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany
- Department of Medical Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748, Garching, Germany
| | - Giulia Buizza
- Department of Electronics, Information and Bioengineering, Politecnico di Milano, P.za Leonardo da Vinci 32, 20133, Milano, Italy
| | - Guillaume Landry
- Department of Radiation Oncology, University Hospital, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany
- Department of Medical Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748, Garching, Germany
- German Cancer Consortium (DKTK), 81377, Munich, Germany
| | - Florian Kamp
- Department of Radiation Oncology, University Hospital, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany
| | - Moritz Rabe
- Department of Radiation Oncology, University Hospital, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany
| | - Chiara Paganelli
- Department of Electronics, Information and Bioengineering, Politecnico di Milano, P.za Leonardo da Vinci 32, 20133, Milano, Italy
| | - Guido Baroni
- Department of Electronics, Information and Bioengineering, Politecnico di Milano, P.za Leonardo da Vinci 32, 20133, Milano, Italy
- Bioengineering Unit, National Center of Oncological Hadrontherapy (CNAO), Strada Privata Campeggi 53, 27100, Pavia, Italy
| | - Michael Reiner
- Department of Radiation Oncology, University Hospital, LMU Munich, Marchioninistraße 15, 81377, Munich, Germany
| | - Paul J Keall
- ACRF Image X Institute, University of Sydney, Sydney, NSW, 2006, Australia
| | - Cornelis A T van den Berg
- Department of Radiotherapy, University Medical Centre Utrecht, PO box 85500, 3508 GA, Utrecht, The Netherlands
| | - Marco Riboldi
- Department of Medical Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748, Garching, Germany.
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