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Gebauer B, Baumann KS, Fuchs H, Georg D, Oborn BM, Looe HK, Lühr A. Proton dosimetry in a magnetic field: Measurement and calculation of magnetic field correction factors for a plane-parallel ionization chamber. Med Phys 2024; 51:2293-2305. [PMID: 37898105 DOI: 10.1002/mp.16797] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2023] [Revised: 08/31/2023] [Accepted: 09/04/2023] [Indexed: 10/30/2023] Open
Abstract
BACKGROUND The combination of magnetic resonance imaging and proton therapy offers the potential to improve cancer treatment. The magnetic field (MF)-dependent change in the dosage of ionization chambers in magnetic resonance imaging-integrated proton therapy (MRiPT) is considered by the correction factork B ⃗ , M , Q $k_{\vec{B},M,Q}$ , which needs to be determined experimentally or computed via Monte Carlo (MC) simulations. PURPOSE In this study,k B ⃗ , M , Q $k_{\vec{B},M,Q}$ was both measured and simulated with high accuracy for a plane-parallel ionization chamber at different clinical relevant proton energies and MF strengths. MATERIAL AND METHODS The dose-response of the Advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10 × $\times$ 10 cm2 $^2$ quasi mono-energetic fields, using 103.3, 128.4, 153.1, 223.1, and 252.7 MeV proton beams was measured in a water phantom placed in the MF of an electromagnet with MF strengths of 0.32, 0.5, and 1 T. The detector was positioned at a depth of 2 g/cm2 $^2$ in water, with chamber electrodes parallel to the MF lines and perpendicular to the proton beam incidence direction. The measurements were compared with TOPAS MC simulations utilizing COMSOL-calculated 0.32, 0.5, and 1 T MF maps of the electromagnet.k B ⃗ , M , Q $k_{\vec{B},M,Q}$ was calculated for the measurements for all energies and MF strengths based on the equation:k B ⃗ , M , Q = M Q M Q B ⃗ $k_{\vec{B},M,Q}=\frac{M_\mathrm{Q}}{M_\mathrm{Q}^{\vec{B}}}$ , whereM Q B ⃗ $M_\mathrm{Q}^{\vec{B}}$ andM Q $M_\mathrm{Q}$ were the temperature and air-pressure corrected detector readings with and without the MF, respectively. MC-based correction factors were determined ask B ⃗ , M , Q = D det D det B ⃗ $k_{\vec{B},M,Q}=\frac{D_\mathrm{det}}{D_\mathrm{det}^{\vec{B}}}$ , whereD det B ⃗ $D_\mathrm{det}^{\vec{B}}$ andD det $D_\mathrm{det}$ were the doses deposited in the air cavity of the ionization chamber model with and without the MF, respectively. Furthermore, MF effects on the chamber dosimetry are studied using MC simulations, examining the impact on the absorbed dose-to-water (D W $D_{W}$ ) and the shift in depth of the Bragg peak. RESULTS The detector showed a reduced dose-response for all measured energies and MF strengths, resulting in experimentally determinedk B ⃗ , M , Q $k_{\vec{B},M,Q}$ values larger than unity. For all energies and MF strengths examined,k B ⃗ , M , Q $k_{\vec{B},M,Q}$ ranged between 1.0065 and 1.0205. The dependence on the energy and the MF strength was found to be non-linear with a maximum at 1 T and 252.7 MeV. The MC simulatedk B ⃗ , M , Q $k_{\vec{B},M,Q}$ values agreed with the experimentally determined correction factors within their standard deviations with a maximum difference of 0.6%. The MC calculated impact onD W $D_{W}$ was smaller 0.2 %. CONCLUSION For the first time, measurements and simulations were compared for proton dosimetry within MFs using an Advanced Markus chamber. Good agreement ofk B ⃗ , M , Q $k_{\vec{B},M,Q}$ was found between experimentally determined and MC calculated values. The performed benchmarking of the MC code allows for calculatingk B ⃗ , M , Q $k_{\vec{B},M,Q}$ for various ionization chamber models, MF strengths and proton energies to generate the data needed for a proton dosimetry protocol within MFs and is, therefore, a step towards MRiPT.
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Affiliation(s)
- Benjamin Gebauer
- OncoRay National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
- Institute of Radiooncology-OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
| | - Kilian-Simon Baumann
- Department of Radiotherapy and Radiooncology, University Medical Center Giessen-Marburg, Marburg, Germany
- University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany
- Ion-Beam Therapy Center, Marburg, Germany
| | - Hermann Fuchs
- Department of Radiation Oncology, Medical University of Vienna, Wien, Austria
- MedAustron Iontherapy centre, Wiener Neustadt, Austria
| | - Dietmar Georg
- Department of Radiation Oncology, Medical University of Vienna, Wien, Austria
- MedAustron Iontherapy centre, Wiener Neustadt, Austria
| | - Brad M Oborn
- Institute of Radiooncology-OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, New South Wales, Australia
- Illawarra Cancer Care Centre, Wollongong, New South Wales, Australia
| | - Hui-Khee Looe
- Department for Radiotherapy and Radiooncology, Pius Hospital, Medical Campus Carl von Ossietzky University, Oldenburg, Germany
| | - Armin Lühr
- Department of Physics, TU Dortmund University, Dortmund, Germany
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Kim S, Yuan L, Kim S, Suh TS. Generation of tissues outside the field of view (FOV) of radiation therapy simulation imaging based on machine learning and patient body outline (PBO). Radiat Oncol 2024; 19:15. [PMID: 38273278 PMCID: PMC10811833 DOI: 10.1186/s13014-023-02384-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2023] [Accepted: 11/28/2023] [Indexed: 01/27/2024] Open
Abstract
BACKGROUND It is not unusual to see some parts of tissues are excluded in the field of view of CT simulation images. A typical mitigation is to avoid beams entering the missing body parts at the cost of sub-optimal planning. METHODS This study is to solve the problem by developing 3 methods, (1) deep learning (DL) mechanism for missing tissue generation, (2) using patient body outline (PBO) based on surface imaging, and (3) hybrid method combining DL and PBO. The DL model was built upon a Globally and Locally Consistent Image Completion to learn features by Convolutional Neural Networks-based inpainting, based on Generative Adversarial Network. The database used comprised 10,005 CT training slices of 322 lung cancer patients and 166 CT evaluation test slices of 15 patients. CT images were from the publicly available database of the Cancer Imaging Archive. Since existing data were used PBOs were acquired from the CT images. For evaluation, Structural Similarity Index Metric (SSIM), Root Mean Square Error (RMSE) and Peak signal-to-noise ratio (PSNR) were evaluated. For dosimetric validation, dynamic conformal arc plans were made with the ground truth images and images generated by the proposed method. Gamma analysis was conducted at relatively strict criteria of 1%/1 mm (dose difference/distance to agreement) and 2%/2 mm under three dose thresholds of 1%, 10% and 50% of the maximum dose in the plans made on the ground truth image sets. RESULTS The average SSIM in generation part only was 0.06 at epoch 100 but reached 0.86 at epoch 1500. Accordingly, the average SSIM in the whole image also improved from 0.86 to 0.97. At epoch 1500, the average values of RMSE and PSNR in the whole image were 7.4 and 30.9, respectively. Gamma analysis showed excellent agreement with the hybrid method (equal to or higher than 96.6% of the mean of pass rates for all scenarios). CONCLUSIONS It was first demonstrated that missing tissues in simulation imaging could be generated with high similarity, and dosimetric limitation could be overcome. The benefit of this study can be significantly enlarged when MR-only simulation is considered.
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Affiliation(s)
- Sunmi Kim
- Department of Biomedical Engineering and Research Institute of Biomedical Engineering, College of Medicine, The Catholic University of Korea, 222 Banpo-daero, Seocho-gu, Seoul, 06591, Republic of Korea
- Department of Radiation Oncology, Yonsei Cancer Center, Seoul, 03722, Republic of Korea
| | - Lulin Yuan
- Department of Radiation Oncology, School of Medicine, Virginia Commonwealth University, Richmond, VA, 23284, USA
| | - Siyong Kim
- Department of Radiation Oncology, School of Medicine, Virginia Commonwealth University, Richmond, VA, 23284, USA.
| | - Tae Suk Suh
- Department of Biomedical Engineering and Research Institute of Biomedical Engineering, College of Medicine, The Catholic University of Korea, 222 Banpo-daero, Seocho-gu, Seoul, 06591, Republic of Korea.
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Riis HL, Chick J, Dunlop A, Tilly D. The Quality Assurance of a 1.5 T MR-Linac. Semin Radiat Oncol 2024; 34:120-128. [PMID: 38105086 DOI: 10.1016/j.semradonc.2023.10.011] [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: 12/19/2023]
Abstract
The recent introduction of a commercial 1.5 T MR-linac system has considerably improved the image quality of the patient acquired in the treatment unit as well as enabling online adaptive radiation therapy (oART) treatment strategies. Quality Assurance (QA) of this new technology requires new methodology that allows for the high field MR in a linac environment. The presence of the magnetic field requires special attention to the phantoms, detectors, and tools to perform QA. Due to the design of the system, the integrated megavoltage imager (MVI) is essential for radiation beam calibrations and QA. Additionally, the alignment between the MR image system and the radiation isocenter must be checked. The MR-linac system has vendor-supplied phantoms for calibration and QA tests. However, users have developed their own routine QA systems to independently check that the machine is performing as required, as to ensure we are able to deliver the intended dose with sufficient certainty. The aim of this work is therefore to review the MR-linac specific QA procedures reported in the literature.
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Affiliation(s)
- Hans Lynggaard Riis
- Department of Oncology, Odense University Hospital, Odense, Denmark; Department of Clinical Research, University of Southern Denmark, Odense, Denmark.
| | - Joan Chick
- The Joint Department of Physics, The Royal Marsden Hospital and the Institute of Cancer Research, London, UK
| | - Alex Dunlop
- The Joint Department of Physics, The Royal Marsden Hospital and the Institute of Cancer Research, London, UK
| | - David Tilly
- Department of Immunology, Genetics and Pathology, Medical Radiation Physics, Uppsala University, Uppsala, Sweden; Medical Physics, Uppsala University Hospital, Uppsala, Sweden
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Andersen CE. Magnetic field influence on the light yield from fiber-coupled BCF-60 plastic scintillators of relevance for output factor dosimetry in MR-linacs. Biomed Phys Eng Express 2023; 10:015016. [PMID: 38064731 DOI: 10.1088/2057-1976/ad13aa] [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: 09/03/2023] [Accepted: 12/08/2023] [Indexed: 12/20/2023]
Abstract
Organic plastic scintillators are of interest for ionizing radiation dosimetry in megavoltage photon beams because plastic scintillators have a mass density very similar to that of water. This leads to insignificant perturbation of the electron fluence at the point of measurement in a water phantom. This feature is a benefit for dosimetry in strong magnetic fields (e.g., 1.5 T) as found in linacs with magnetic resonance imaging. The objective of this work was to quantify if the light yield per dose for the scintillating fiber BCF-60 material from Saint-Gobain Ceramics and Plastics Inc. is constant regardless of the magnetic flux density. This question is of importance for establishing traceable measurement in MR linacs using this detector type. Experiments were carried out using an accelerator combined with an electromagnet (max 0.7 T). Scintillator probes were read out using chromatic stem-removal techniques based on two optical channels or full spectral information. Reference dosimetry was carried out with PTW31010 and PTW31021 ionization chambers. TOPAS/GEANT4 was used for modelling. The light yield per dose for the BCF-60 was found to be strongly influenced by the magnitude of the magnetic field from about 1 mT to 0.7 T. The light yield per dose increased (1.3 ± 0.2)% (k = 1) from 1 mT to 10 mT and it increased (4.5 ± 0.9)% (k = 1) from 0 T to 0.7 T. Previous studies of the influence of magnetic fields on medical scintillator dosimetry have been unable to clearly identify if observed changes in scintillator response with magnetic field strength were related to changes in dose, stem signal removal, or scintillator light yield. In the current study of BCF-60, we see a clear change in light yield with magnetic field, and none of the other effects.
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Affiliation(s)
- Claus E Andersen
- DTU Health Tech, Technical University of Denmark, 4000 Roskilde, Denmark
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Episkopakis A, Margaroni V, Kanellopoulou S, Marinos N, Koutsouveli E, Karaiskos P, Pappas EP. Dose-response dependencies of OSL dosimeters in conventional linacs and 1.5T MR-linacs: an experimental and Monte Carlo study. Phys Med Biol 2023; 68:225002. [PMID: 37857285 DOI: 10.1088/1361-6560/ad051e] [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: 08/13/2023] [Accepted: 10/19/2023] [Indexed: 10/21/2023]
Abstract
Objective. This work focuses on the optically stimulated luminescence dosimetry (OSLD) dose-response characterization, with emphasis on 1.5T MR-Linacs.Approach. Throughout this study, the nanoDots OSLDs (Landauer, USA) were considered. In groups of three, the mean OSLD response was measured in a conventional linac and an MR-Linac under various irradiation conditions to investigate (i) dose-response linearity with and without the 1.5T magnetic field, (ii) signal fading rate and its dependencies, (iii) beam quality, detector orientation and dose rate dependencies in a conventional linac, (iii) potential MR imaging related effects on OSLD response and (iv) detector orientation dependence in an MR-Linac. Monte Carlo calculations were performed to further quantify angular dependence after rotating the detector around its central axis parallel to the magnetic field, and determine the magnetic field correction factors,kB,Q,for all cardinal detector orientations.Main results. OSLD dose-response supralinearity in an MR-Linac setting was found to agree within uncertainties with the corresponding one in a conventional linac, for the axial detector orientation investigated. Signal fading rate does not depend on irradiation conditions for the range of 3-30 d considered. OSLD angular (orientation) dependence is more pronounced under the presence of a magnetic field. OSLDs irradiated with and without real-time T2w MR imaging enabled during irradiation yielded the same response within uncertainties.kB,Qvalues were determined for all three cardinal orientations. Corrections needed reached up to 6.4%. However, if OSLDs are calibrated in the axial orientation and then irradiated in an MR-Linac placed again in the axial orientation (perpendicular to the magnetic field), then simulations suggest thatkB,Qcan be considered unity within uncertainties, irrespective of the incident beam angle.Significance. This work contributes towards OSLD dose-response characterization and relevant correction factors availability. OSLDs are suitable for QA checks in MR-based beam gating applications andin vivodosimetry in MR-Linacs.
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Affiliation(s)
- Anastasios Episkopakis
- Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, 75 Mikras Asias, 115 27 Athens, Greece
- Global Clinical Operations, Elekta Ltd., Fleming way, RH10 99RR Crawley, West Sussex, United Kingdom
| | - Vasiliki Margaroni
- Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, 75 Mikras Asias, 115 27 Athens, Greece
| | | | - Nikolas Marinos
- Global Clinical Operations, Elekta Ltd., Fleming way, RH10 99RR Crawley, West Sussex, United Kingdom
| | - Efi Koutsouveli
- Medical Physics Department, Hygeia Hospital, Kifissias Avenue & 4 Erythrou Stavrou, Marousi, 151 23 Athens, Greece
| | - Pantelis Karaiskos
- Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, 75 Mikras Asias, 115 27 Athens, Greece
| | - Eleftherios P Pappas
- Medical Physics Laboratory, Medical School, National and Kapodistrian University of Athens, 75 Mikras Asias, 115 27 Athens, Greece
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Fuchs H, Padilla-Cabal F, Georg D, Palmans H. MR-guided ion therapy: Detector response in magnetic fields during carbon ion irradiation. Med Phys 2023; 50:7167-7176. [PMID: 37434465 DOI: 10.1002/mp.16631] [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: 04/15/2023] [Revised: 06/14/2023] [Accepted: 06/17/2023] [Indexed: 07/13/2023] Open
Abstract
BACKGROUND Combining carbon ion therapy with on-bed MR imaging has the potential to bring particle therapy to a new level of precision. However, the introduction of magnetic fields brings challenges for dosimetry and quality assurance. For protons, a small, but significant change in detector response was shown in the presence of magnetic fields previously. For carbon ion beams, so far no such experiments have been performed. PURPOSE To investigate the influence of external magnetic fields on the response of air-filled ionization chambers. METHODS Four commercially available ionization chambers, three thimble type (Farmer, Semiflex, and PinPoint), and a plane parallel (Bragg peak) detector were investigated. Detectors were aligned in water such that their effective point of measurement was located at 2 cm depth. Irradiations were performed using10 × 10 cm 2 $10\times 10\nobreakspace \mathrm{cm}^2$ square fields for carbon ions of 186.1, 272.5, and 402.8 MeV/u employing magnetic field strengths of 0, 0.25, 0.5, and 1 T. In addition, the detector response for protons and carbon ions was compared taking into account the secondary electron spectra and employing protons of 252.7 MeV for comparison. RESULTS For all four detectors, a statistically significant change in detector response, dependent on the magnetic field strength, was found. The effect was more pronounced for higher energies. The highest effects were found at 0.5 T for the PinPoint detector with a change in detector response of 1.1%. The response of different detector types appeared to be related to the cavity diameter. For proton and carbon ion irradiation with similar secondary electron spectra, the change in detector response was larger for carbon ions compared to protons. CONCLUSION A small, but significant dependence of the detector response was found for carbon ion irradiation in a magnetic field. The effect was found to be larger for smaller cavity diameters and at medium magnetic field strengths. Changes in detector response were more pronounced for carbon ions compared to protons.
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Affiliation(s)
- Hermann Fuchs
- Division of Medical Radiation Physics, Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria
- MedAustron Ion Therapy Center, Wiener Neustadt, Wiener Neustadt, Austria
| | - Fatima Padilla-Cabal
- Division of Medical Radiation Physics, Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria
| | - Dietmar Georg
- Division of Medical Radiation Physics, Department of Radiation Oncology, Medical University of Vienna, Vienna, Austria
- MedAustron Ion Therapy Center, Wiener Neustadt, Wiener Neustadt, Austria
| | - Hugo Palmans
- MedAustron Ion Therapy Center, Wiener Neustadt, Wiener Neustadt, Austria
- National Physical Laboratory, Teddington, UK
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Maeyama T, Hayashi K, Watanabe Y, Ohara M, Nakagawa S. Development of a silicone-based radio-fluorogenic dosimeter using dihydrorhodamine 6G. Phys Med 2023; 114:102684. [PMID: 37778206 DOI: 10.1016/j.ejmp.2023.102684] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/22/2023] [Revised: 07/24/2023] [Accepted: 09/11/2023] [Indexed: 10/03/2023] Open
Abstract
A silicon-based three-dimensional dosimeter can be formed in a free shape without a container and deformed because of its flexibility. Several studies have focused on enhancing its radiological characteristics and assessing its applicability as a quality assurance tool for image-guided and adaptive radiation therapy, considering motion and deformation. Here, we applied a fluorescence probe (dihydrorhodamine 6G, DHR6G) to a silicon elastomer as a new radiosensitive compound that converts nonfluorescent into fluorescent dyes using irradiation, and its fluorescence intensity increases linearly with the absorbed dose. In this study, we demonstrated a cost-effective synthesis method and optimized the composition conditions. The results showed that the DHR6G-SE prepared from 2.2 × 10-3 wt% DHR6G, 0.024 wt% pyridine, and a silicone elastomer (SE) (SILPOT TM 184, base/curing agent = 10/1) exhibited a linear increase in fluorescence with radiation exposure within a dose range of 0-8 Gy and a highly stable sensitivity for as long as 64 h. To demonstrate its container-less characteristics, the possibility of dosimetry for low-energy X-rays using DHR6G-SE was investigated.
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Affiliation(s)
- Takuya Maeyama
- Department of Chemistry, School of Science, Kitasato University, 1-15-1 Kitasato, Minami, Sagamihara, Kanagawa 252-0373, Japan; RIKEN Nishina Center for Accelerator-Based Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
| | - Kiichiro Hayashi
- Department of Chemistry, School of Science, Kitasato University, 1-15-1 Kitasato, Minami, Sagamihara, Kanagawa 252-0373, Japan
| | - Yusuke Watanabe
- School of Allied Health Sciences, Kitasato University, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan
| | - Maki Ohara
- National Institutes for Quantum Science and Technology, 4-9-1 Anagawa, Inage-ku, Chiba-City, Chiba 263-8555, Japan
| | - Seiko Nakagawa
- Tokyo Metropolitan Industrial Technology Research Institute, 2-4-10 Aomi, Koto-ku, Tokyo 135-0064, Japan
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Alissa M, Zink K, Kapsch RP, Schoenfeld AA, Frick S, Czarnecki D. Experimental and Monte Carlo-based determination of magnetic field correction factors k B , Q $k_{B,Q}$ in high-energy photon fields for two ionization chambers. Med Phys 2023. [PMID: 36897832 DOI: 10.1002/mp.16345] [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: 09/22/2022] [Revised: 02/22/2023] [Accepted: 02/23/2023] [Indexed: 03/11/2023] Open
Abstract
BACKGROUND The integration of magnetic resonance tomography into clinical linear accelerators provides high-contrast, real-time imaging during treatment and facilitates online-adaptive workflows in radiation therapy treatments. The associated magnetic field also bends the trajectories of charged particles via the Lorentz force, which may alter the dose distribution in a patient or a phantom and affects the dose response of dosimetry detectors. PURPOSE To perform an experimental and Monte Carlo-based determination of correction factors k B , Q $k_{B,Q}$ , which correct the response of ion chambers in the presence of external magnetic fields in high-energy photon fields. METHODS The response variation of two different types of ion chambers (Sun Nuclear SNC125c and SNC600c) in strong external magnetic fields was investigated experimentally and by Monte Carlo simulations. The experimental data were acquired at the German National Metrology Institute, PTB, using a clinical linear accelerator with a nominal photon energy of 6 MV and an external electromagnet capable of generating magnetic flux densities of up to 1.5 T in opposite directions. The Monte Carlo simulation geometries corresponded to the experimental setup and additionally to the reference conditions of IAEA TRS-398. For the latter, the Monte Carlo simulations were performed with two different photon spectra: the 6 MV spectrum of the linear accelerator used for the experimental data acquisition and a 7 MV spectrum of a commercial MRI-linear accelerator. In each simulation geometry, three different orientations of the external magnetic field, the beam direction and the chamber orientation were investigated. RESULTS Good agreement was achieved between Monte Carlo simulations and measurements with the SNC125c and SNC600c ionization chambers, with a mean deviation of 0.3% and 0.6%, respectively. The magnitude of the correction factor k B , Q $k_{B,Q}$ strongly depends on the chamber volume and on the orientation of the chamber axis relative to the external magnetic field and the beam directions. It is greater for the SNC600c chamber with a volume of 0.6 cm3 than for the SNC125c chamber with a volume of 0.1 cm3 . When the magnetic field direction and the chamber axis coincide, and they are perpendicular to the beam direction, the ion chambers exhibit a calculated overresponse of less than 0.7(6)% (SNC600c) and 0.3(4)% (SNC125c) at 1.5 T and less than 0.3(0)% (SNC600c) and 0.1(3)% (SNC125c) for 0.35 T for nominal beam energies of 6 MV and 7 MV. This chamber orientation should be preferred, as k B , Q $k_{B,Q}$ may increase significantly in other chamber orientations. Due to the special geometry of the guard ring, no dead-volume effects have been observed in any orientation studied. The results show an intra-type variation of 0.17% and 0.07% standard uncertainty (k=1) for the SNC125c and SNC600c, respectively. CONCLUSION Magnetic field correction factors k B , Q $k_{B,Q}$ for two different ion chambers and for typical clinical photon beam qualities were presented and compared with the few data existing in the literature. The correction factors may be applied in clinical reference dosimetry for existing MRI-linear accelerators.
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Affiliation(s)
- Mohamad Alissa
- Institute for Medical Physics and Radiation Protection, University of Applied Sciences Giessen, Giessen, Germany
- Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Marburg, Germany
| | - Klemens Zink
- Institute for Medical Physics and Radiation Protection, University of Applied Sciences Giessen, Giessen, Germany
- Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Marburg, Germany
- Marburg Ionbeam Therapy Center (MIT), Marburg, Germany
| | | | | | - Stephan Frick
- German National Metrology Institute (PTB), Braunschweig, Germany
| | - Damian Czarnecki
- Institute for Medical Physics and Radiation Protection, University of Applied Sciences Giessen, Giessen, Germany
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Begg J, Jelen U, Moutrie Z, Oliver C, Holloway L, Brown R. ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators. Phys Eng Sci Med 2023; 46:1-17. [PMID: 36806156 PMCID: PMC10030536 DOI: 10.1007/s13246-023-01223-w] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 01/19/2023] [Indexed: 02/23/2023]
Abstract
Consistency and clear guidelines on dosimetry are essential for accurate and precise dosimetry, to ensure the best patient outcomes and to allow direct dose comparison across different centres. Magnetic Resonance Imaging Linac (MRI-linac) systems have recently been introduced to Australasian clinics. This report provides recommendations on reference dosimetry measurements for MRI-linacs on behalf of the Australiasian College of Physical Scientists and Engineers in Medicine (ACPSEM) MRI-linac working group. There are two configurations considered for MRI-linacs, perpendicular and parallel, referring to the relative direction of the magnetic field and radiation beam, with different impacts on dose deposition in a medium. These recommendations focus on ion chambers which are most commonly used in the clinic for reference dosimetry. Water phantoms must be MR safe or conditional and practical limitations on phantom set-up must be considered. Solid phantoms are not advised for reference dosimetry. For reference dosimetry, IAEA TRS-398 recommendations cannot be followed completely due to physical differences between conventional linac and MRI-linac systems. Manufacturers' advice on reference conditions should be followed. Beam quality specification of TPR20,10 is recommended. The configuration of the central axis of the ion chamber relative to the magnetic field and radiation beam impacts the chamber response and must be considered carefully. Recommended corrections to delivered dose are [Formula: see text], a correction for beam quality and [Formula: see text], for the impact of the magnetic field on dosimeter response in the magnetic field. Literature based values for [Formula: see text] are given. It is important to note that this is a developing field and these recommendations should be used together with a review of current literature.
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Affiliation(s)
- Jarrad Begg
- Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centre, Liverpool, NSW, 2170, Australia.
- Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia.
- South Western Sydney Clinical School, University of New South Wales, Liverpool, NSW, 2170, Australia.
| | - Urszula Jelen
- St Vincents Clinic, GenesisCare, Darlinghurst, NSW, 2010, Australia
| | - Zoe Moutrie
- Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centre, Liverpool, NSW, 2170, Australia
| | - Chris Oliver
- Primary Standards Dosimetry Laboratory, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, VIC, 3085, Australia
| | - Lois Holloway
- Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centre, Liverpool, NSW, 2170, Australia
- Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia
- South Western Sydney Clinical School, University of New South Wales, Liverpool, NSW, 2170, Australia
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia
- Institute of Medical Physics, University of Sydney, Camperdown, NSW, 2505, Australia
| | - Rhonda Brown
- Australian Clinical Dosimetry Service, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, VIC, 3085, Australia
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10
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Tanadini-Lang S, Budgell G, Bohoudi O, Corradini S, Cusumano D, Güngör G, Kerkmeijer LGW, Mahmood F, Nill S, Palacios MA, Reiner M, Thorwarth D, Wilke L, Wolthaus J. An ESTRO-ACROP guideline on quality assurance and medical physics commissioning of online MRI guided radiotherapy systems based on a consensus expert opinion. Radiother Oncol 2023; 181:109504. [PMID: 36736592 DOI: 10.1016/j.radonc.2023.109504] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2023] [Accepted: 01/25/2023] [Indexed: 02/05/2023]
Abstract
OBJECTIVE The goal of this consensus expert opinion was to define quality assurance (QA) tests for online magnetic resonance image (MRI) guided radiotherapy (oMRgRT) systems and to define the important medical physics aspects for installation and commissioning of an oMRgRT system. MATERIALS AND METHODS Ten medical physicists and two radiation oncologists experienced in oMRgRT participated in the survey. In the first round of the consensus expert opinion, ideas on QA and commissioning were collected. Only tests and aspects different from commissioning of a CT guided radiotherapy (RT) system were considered. In the following two rounds all twelve participants voted on the importance of the QA tests, their recommended frequency and their suitability for the two oMRgRT systems approved for clinical use as well as on the importance of the aspects to consider during medical physics commissioning. RESULTS Twenty-four QA tests were identified which are potentially important during commissioning and routine QA on oMRgRT systems compared to online CT guided RT systems. An additional eleven tasks and aspects related to construction, workflow development and training were collected. Consensus was found for most tests on their importance, their recommended frequency and their suitability for the two approved systems. In addition, eight aspects mostly related to the definition of workflows were also found to be important during commissioning. CONCLUSIONS A program for QA and commissioning of oMRgRT systems was developed to support medical physicists to prepare for safe handling of such systems.
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Affiliation(s)
- Stephanie Tanadini-Lang
- Department of Radiation Oncology, University Hospital Zurich, University of Zurich, 8091 Zurich, Switzerland.
| | - Geoff Budgell
- Christie Medical Physics and Engineering, The Christie NHS Foundation Trust, Wilmslow Road, Manchester iM20 4BX, UK
| | - Omar Bohoudi
- Amsterdam UMC, Vrije Universiteit Medical Centre, Dept. of Radiation Oncology, de Boelelaan 1117, 1081 HV Amsterdam, the Netherlands
| | - Stefanie Corradini
- Department of Radiation Oncology, University Hospital, LMU Munich, 81377 Munich, Germany
| | - Davide Cusumano
- Fondazione Policlinico Universitario Agostino Gemelli IRCCS, Rome, Italy; Mater Olbia Hospital, Olbia, SS, Italy
| | - Görkem Güngör
- Department of Medical Physics, Graduade School of Health Sciences, Istanbul Medipol University, Istanbul, Turkey
| | - Linda G W Kerkmeijer
- Department of Radiation Oncology, Radboud University Medical Center Nijmegen, the Netherlands
| | - Faisal Mahmood
- Laboratory of Radiation Physics, Department of Oncology, Odense University Hospital, Odense, Denmark; Department of Clinical Research, University of Southern Denmark, Odense, Denmark
| | - Simeon Nill
- The Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, UK
| | - Miguel A Palacios
- Amsterdam UMC, Vrije Universiteit Medical Centre, Dept. of Radiation Oncology, de Boelelaan 1117, 1081 HV Amsterdam, the Netherlands
| | - Michael Reiner
- Department of Radiation Oncology, University Hospital, LMU Munich, 81377 Munich, Germany
| | - Daniela Thorwarth
- Section for Biomedical Physics, Department of Radiation Oncology, University of Tübingen, Tübingen, Germany
| | - Lotte Wilke
- Department of Radiation Oncology, University Hospital Zurich, University of Zurich, 8091 Zurich, Switzerland
| | - Jochem Wolthaus
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands
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11
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Gayol A, Malano F, Ribo Montenovo C, Pérez P, Valente M. Dosimetry Effects Due to the Presence of Fe Nanoparticles for Potential Combination of Hyperthermic Cancer Treatment with MRI-Based Image-Guided Radiotherapy. Int J Mol Sci 2022; 24:ijms24010514. [PMID: 36613959 PMCID: PMC9820326 DOI: 10.3390/ijms24010514] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2022] [Revised: 12/03/2022] [Accepted: 12/06/2022] [Indexed: 12/29/2022] Open
Abstract
Nanoparticles have proven to be biocompatible and suitable for many biomedical applications. Currently, hyperthermia cancer treatments based on Fe nanoparticle infusion excited by alternating magnetic fields are commonly used. In addition to this, MRI-based image-guided radiotherapy represents, nowadays, one of the most promising accurate radiotherapy modalities. Hence, assessing the feasibility of combining both techniques requires preliminary characterization of the corresponding dosimetry effects. The present work reports on a theoretical and numerical simulation feasibility study aimed at pointing out preliminary dosimetry issues. Spatial dose distributions incorporating magnetic nanoparticles in MRI-based image-guided radiotherapy have been obtained by Monte Carlo simulation approaches accounting for all relevant radiation interaction properties as well as charged particles coupling with strong external magnetic fields, which are representative of typical MRI-LINAC devices. Two main effects have been evidenced: local dose enhancement (up to 60% at local level) within the infused volume, and non-negligible changes in the dose distribution at the interfaces between different tissues, developing to over 70% for low-density anatomical cavities. Moreover, cellular uptakes up to 10% have been modeled by means of considering different Fe nanoparticle concentrations. A theoretical temperature-dependent model for the thermal enhancement ratio (TER) has been used to account for radiosensitization due to hyperthermia. The outcomes demonstrated the reliability of the Monte Carlo approach in accounting for strong magnetic fields and mass distributions from patient-specific anatomy CT scans to assess dose distributions in MRI-based image-guided radiotherapy combined with magnetic nanoparticles, while the hyperthermic radiosensitization provides further and synergic contributions.
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Affiliation(s)
- Amiel Gayol
- Instituto de Física E. Gaviola (IFEG), CONICET & Facultad de Matemática, Astronomía, Física y Computación (FAMAF), Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina
- Laboratorio de Investigación e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X (LIIFAMIRx), Facultad de Matemática, Astronomía, Física y Computación (FAMAF), Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina
| | - Francisco Malano
- Centro de Excelencia de Física e Ingeniería en Salud (CFIS), Departamento de Ciencias Físicas, Universidad de La Frontera, Av. Salazar 01145, Casilla 54D, Temuco 4811230, Chile
- Correspondence: (F.M.); (M.V.)
| | - Clara Ribo Montenovo
- Laboratorio de Investigación e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X (LIIFAMIRx), Facultad de Matemática, Astronomía, Física y Computación (FAMAF), Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina
| | - Pedro Pérez
- Instituto de Física E. Gaviola (IFEG), CONICET & Facultad de Matemática, Astronomía, Física y Computación (FAMAF), Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina
- Laboratorio de Investigación e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X (LIIFAMIRx), Facultad de Matemática, Astronomía, Física y Computación (FAMAF), Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina
| | - Mauro Valente
- Instituto de Física E. Gaviola (IFEG), CONICET & Facultad de Matemática, Astronomía, Física y Computación (FAMAF), Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina
- Laboratorio de Investigación e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X (LIIFAMIRx), Facultad de Matemática, Astronomía, Física y Computación (FAMAF), Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina
- Centro de Excelencia de Física e Ingeniería en Salud (CFIS), Departamento de Ciencias Físicas, Universidad de La Frontera, Av. Salazar 01145, Casilla 54D, Temuco 4811230, Chile
- Correspondence: (F.M.); (M.V.)
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12
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Vedelago J, Karger CP, Jäkel O. A review on reference dosimetry in radiation therapy with proton and light ion beams: status and impact of new developments. RADIAT MEAS 2022. [DOI: 10.1016/j.radmeas.2022.106844] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/15/2022]
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13
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Navarro Campos J, de Pooter J, de Prez L, Jansen B. The impact of ion chamber components on kB,Qfor reference dosimetry in MRgRT. Phys Med Biol 2022; 67. [PMID: 35688138 DOI: 10.1088/1361-6560/ac77d0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2022] [Accepted: 06/10/2022] [Indexed: 01/05/2023]
Abstract
For reference dosimetry in MRgRT,kB,Qis used to correct for the impact of the magnetic field on the chamber calibration coefficient. It has been demonstrated that for accurate simulation ofkB,Qthe dead volume (DV) must be considered. This work goes one step further by analysing the contribution of secondary electrons generated in the various chamber components tokB,Q. The Farmer-type chamber PTW 30013 geometry was modelled for two different DVs. Monte Carlo simulations were performed for a60Co source and a 7 MV MRI-linac and the model was validated against measurements. Both parallel (α = 0° or 180°) and perpendicular (α = 90° or 270°) orientations of the chamber and the magnetic (B) field were considered, and severalB-field strengths between 0 T and 1.5 T. To study the dose contribution to the reduced volume (RV = cavity - DV) from the secondary electrons produced in certain components of the chamber the labelling of the particles was implemented in the PENELOPE user code PENMAIN. A separate model with each solid component of the chamber modelled as liquid water was used to investigate the impact of material choice onkB,Q. Results show that simulatedkB,Qvalues agree better with the measuredkB,Qwhen the DV is considered. It is demonstrated that small components of the chamber impactkB,Qconsiderably, since the contribution to the RV-dose from the bodies closer to the RV is higher than withoutB. Moreover, it is seen that the impact to the dose in the RV is reduced when the material of each component is modelled as liquid water. Therefore, chamber design and, to a lesser extent, choice of material affectkB,Q, and an accurate geometrical model of the chamber components and its further validation are important for correct calculations ofkB,Q.
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Affiliation(s)
| | | | - Leon de Prez
- VSL - NationalMetrology Institute, Delft, The Netherlands
| | - Bartel Jansen
- VSL - NationalMetrology Institute, Delft, The Netherlands
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14
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Begg J, Jelen U, Keall P, Liney G, Holloway L. Experimental characterisation of the magnetic field correction factor,kB⃗,for Roos chambers in a parallel MRI-linac. Phys Med Biol 2022; 67. [PMID: 35413694 DOI: 10.1088/1361-6560/ac66b8] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2021] [Accepted: 04/12/2022] [Indexed: 12/15/2022]
Abstract
Objective.Reference dosimetry on an MRI-linac requires a chamber specific magnetic field correction factor,kB⃗.This work aims to measure the correction factor for a parallel plate chamber on a parallel MRI-linac.Approach.kB⃗is defined as the ratio of the absorbed dose to water calibration coefficient in the presence of the magnetic field,ND,wB⃗relative to that under 0 T conditions,ND,w0T.kB⃗was measured via aND,wtransfer to a field chamber at each magnetic field strength from a chamber with knownND,wandkB⃗.This was achieved on the parallel MRI-linac by moving the measurement set-up between a high magnetic field strength region at the MRI-isocentre and a low magnetic field strength region at the end of the bore whilst maintaining consistent set-up and scatter conditions. Three PTW 34001 Roos chambers were investigated as well as a PTW 30013 Farmer used to validate methodology.Main Results.The beam quality used for the measurements ofkB⃗wasTPR20/10 = 0.632. ThekB⃗for the PTW Farmer chamber at 1 T on a parallel MRI-linac was 0.993 ± 0.013 (k = 1). The averagekB⃗factor measured for the three Roos chambers on a 1 T parallel MRI-linac was 0.999 ± 0.014 (k = 1).Significance.The results presented are the first measurements ofkB⃗for a Roos chamber on a parallel MRI-linac. The Roos chamber results demonstrate the potential for the chamber as a reference dosimeter in parallel MRI-linacs.
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Affiliation(s)
- Jarrad Begg
- Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centre, Liverpool, NSW, 2170, Australia.,Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia.,South Western Sydney Clinical School, University of New South Wales, Liverpool, NSW, 2170, Australia
| | - Urszula Jelen
- Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia
| | - Paul Keall
- Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia.,Image X Institute, Faculty of Medicine and Health, University of Sydney, Camperdown, NSW, 2005, Australia
| | - Gary Liney
- Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centre, Liverpool, NSW, 2170, Australia.,Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia.,South Western Sydney Clinical School, University of New South Wales, Liverpool, NSW, 2170, Australia.,Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia
| | - Lois Holloway
- Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centre, Liverpool, NSW, 2170, Australia.,Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia.,South Western Sydney Clinical School, University of New South Wales, Liverpool, NSW, 2170, Australia.,Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia.,Institute of Medical Physics, University of Sydney, Camperdown, NSW, 2005, Australia
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15
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Looe HK, Blum I, Schönfeld AB, Tekin T, Delfs B, Poppe B. Model-based machine learning for the recovery of lateral dose profiles of small photon fields in magnetic field. Phys Med Biol 2022; 67. [DOI: 10.1088/1361-6560/ac5bfa] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/22/2021] [Accepted: 03/09/2022] [Indexed: 11/12/2022]
Abstract
Abstract
Objective. To investigate the feasibility to train artificial neural networks (NN) to recover lateral dose profiles from detector measurements in a magnetic field. Approach. A novel framework based on a mathematical convolution model has been proposed to generate measurement-less training dataset. 2D dose deposition kernels and detector lateral fluence response functions of two air-filled ionization chambers and two diode-type detectors have been simulated without magnetic field and for magnetic field B = 0.35 and 1.5 T. Using these convolution kernels, training dataset consisting pairs of dose profiles
D
x
,
y
and signal profiles
M
x
,
y
were computed for a total of 108 2D photon fluence profiles
ψ
(
x
,
y
)
(80% training/20% validation). The NN were tested using three independent datasets, where the second test dataset has been obtained from simulations using realistic phase space files of clinical linear accelerator and the third test dataset was measured at a conventional linac equipped with electromagnets. Main results. The convolution kernels show magnetic field dependence due to the influence of the Lorentz force on the electron transport in the water phantom and detectors. The NN show good performance during training and validation with mean square error reaching a value of 1e-6 or smaller. The corresponding correlation coefficients R reached the value of 1 for all models indicating an excellent agreement between expected
D
x
,
y
and predicted
D
pred
x
,
y
.
The comparisons between
D
x
,
y
and
D
pred
x
,
y
using the three test datasets resulted in gamma indices (1 mm/1% global) <1 for all evaluated data points. Significance. Two verification approaches have been proposed to warrant the mathematical consistencies of the NN outputs. Besides offering a correction strategy not existed so far for relative dosimetry in a magnetic field, this work could help to raise awareness and to improve understanding on the distortion of detector’s signal profiles by a magnetic field.
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16
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MRI-LINAC dosimetry approach by Monte Carlo codes coupling charged particle radiation transport with strong magnetic fields. Radiat Phys Chem Oxf Engl 1993 2022. [DOI: 10.1016/j.radphyschem.2022.110171] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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17
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Tekin T, Blum I, Delfs B, Schönfeld AB, Poppe B, Looe HK. The magnetic field dependent displacement effect and its correction in reference and relative dosimetry. Phys Med Biol 2022; 67. [DOI: 10.1088/1361-6560/ac4a41] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/02/2021] [Accepted: 01/11/2022] [Indexed: 12/24/2022]
Abstract
Abstract
Objective. This study investigates the perturbation correction factors of air-filled ionization chambers regarding their depth and magnetic field dependence. Focus has been placed on the displacement or gradient correction factor
P
gr
.
Additionally, the shift of the effective point of measurement
P
eff
that can be applied to account for the gradient effect has been compared between the cases with and without magnetic field. Approach. The perturbation correction factors have been simulated by stepwise modifications of the models of three ionization chambers (Farmer 30013, Semiflex 3D 31021 and PinPoint 3D 31022, all from PTW Freiburg). A 10 cm × 10 cm 6 MV photon beam perpendicular to the chamber’s axis was used. A 1.5 T magnetic field was aligned parallel to the chamber’s axis. The correction factors were determined between 0.4 and 20 cm depth. The shift of
P
eff
from the chamber’s reference point
P
ref
,
Δ
z
,
was determined by minimizing the variation of the ratio between dose-to-water
D
w
z
r
e
f
+
Δ
z
and the dose-to-air
D
¯
a
i
r
z
r
e
f
along the depth. Main Results. The perturbation correction factors with and without magnetic field are depth dependent in the build-up region but can be considered as constant beyond the depth of dose maximum. Additionally, the correction factors are modified by the magnetic field.
P
gr
at the reference depth is found to be larger in 1.5 T magnetic field than in the magnetic field free case, where an increase of up to 1% is observed for the largest chamber (Farmer 30013). The magnitude of
Δ
z
for all chambers decreases by 40% in a 1.5 T magnetic field with the sign of
Δ
z
remains negative. Significance. In reference dosimetry, the change of
P
gr
in a magnetic field can be corrected by applying the magnetic field correction factor
k
Q
msr
B
when the chamber is positioned with its
P
ref
at the depth of measurement. However, due to the depth dependence of the perturbation factors, it is more convenient to apply the
Δ
z
-shift during chamber positioning in relative dosimetry.
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18
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Cervantes Y, Duane S, Bouchard H. Monte Carlo investigation of electron fluence perturbation in MRI-guided radiotherapy beams using six commercial radiation detectors. Phys Med Biol 2022; 67. [PMID: 35026745 DOI: 10.1088/1361-6560/ac4b36] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/17/2021] [Accepted: 01/13/2022] [Indexed: 12/30/2022]
Abstract
With the integration of treatments with MRI-linacs to the clinical workflow, the understanding and characterization of detector response in reference dosimetry in magnetic fields are required. The external magnetic field perturbs the electron fluence. The degree of perturbation depends on the irradiation conditions and on the detector type. The purpose of this study is to evaluate the magnetic field impact on the electron fluence spectra in several detectors to provide a deeper understanding of detector response in these conditions. Monte Carlo calculations of the electron fluence are performed in six detectors (solid-state: PTW60012 and PTW60019, ionization chambers: PTW30013, PTW31010, PTW31021, and PTW31022) in water and irradiated by a 7 MV FFF photon beam with a small and a reference field, at 0 and 1.5 T. Three chamber axis orientations are investigated: parallel or perpendicular (either the Lorentz force pointing towards the stem or the tip) to the magnetic field and always perpendicular to the photon beam. One orientation for the solid-state detector is studied: parallel to the photon beam and perpendicular to the magnetic field. Additionally, electron fluence spectra are calculated in modified detector geometries to identify the underlying physical mechanisms behind the fluence perturbations. The total electron fluence in the Farmer chamber varies up to 1.24% and 5.12% at 1.5 T, in the parallel and perpendicular orientation, respectively. The interplay between the gyration radius and the Farmer chamber cavity length significantly affects the electron fluence in the perpendicular orientation. For the small-cavity chambers, the maximal variation in total electron fluence is 0.19% in the parallel orientation for the reference field. Significant small-field effects occur in these chambers; the magnetic field reduces the total electron fluence (with respect to the no field case) between 9.86% and 14.50%, depending on the orientation. The magnetic field strongly impacted the solid-state detectors in both field sizes, probably due to the high-Z components and cavity density. The maximal reductions of total electron fluence are 15.06 ± 0.09% (silicon) and 16.00 ± 0.07% (microDiamond). This work provides insights into detector response in magnetic fields by illustrating the interplay between several factors causing dosimetric perturbation effects: (1) chamber and magnetic field orientation, (2) cavity size and shape, (3) extracameral components, (4) air gaps and their asymmetry, (5) electron energy. Low-energy electron trajectories are more susceptible to change in magnetic fields, and are associated with detector response perturbation. Detectors with higher density and high-Z extracameral components exhibit more significant perturbations in the presence of a magnetic field, regardless of field size.
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Affiliation(s)
- Yunuen Cervantes
- Département de physique, Université de Montréal, Complexe des sciences, 1375 Avenue Thérèse-Lavoie-Roux, Montréal, Québec H2V 0B3, Canada.,Centre de recherche du Centre hospitalier de l'Université de Montréal, 900 Rue Saint-Denis, Montréal, Québec, H2X 0A9, Canada
| | - Simon Duane
- National Physical Laboratory, Chemical, Medical and Environmental Science Department, Teddington, United Kingdom
| | - Hugo Bouchard
- Département de physique, Université de Montréal, Complexe des sciences, 1375 Avenue Thérèse-Lavoie-Roux, Montréal, Québec H2V 0B3, Canada.,Centre de recherche du Centre hospitalier de l'Université de Montréal, 900 Rue Saint-Denis, Montréal, Québec, H2X 0A9, Canada.,Département de radio-oncologie, Centre hospitalier de l'Université de Montréal (CHUM), 900 Rue Saint-Denis, Montréal, Québec, H2X 0A9, Canada
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19
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Cervantes Y, Duchaine J, Billas I, Duane S, Bouchard H. Monte Carlo calculation of detector perturbation and quality correction factors in a 1.5 T magnetic resonance guided radiation therapy small photon beams. Phys Med Biol 2021; 66. [PMID: 34700311 DOI: 10.1088/1361-6560/ac3344] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2021] [Accepted: 10/26/2021] [Indexed: 01/02/2023]
Abstract
Objective.With future advances in magnetic resonance imaging-guided radiation therapy, small photon beams are expected to be included regularly in clinical treatments. This study provides physical insights on detector dose-response to multiple megavoltage photon beam sizes coupled to magnetic fields and determines optimal orientations for measurements.Approach.Monte Carlo simulations determine small-cavity detector (solid-state: PTW60012 and PTW60019, ionization chambers: PTW31010, PTW31021, and PTW31022) dose-responses in water to an Elekta Unity 7 MV FFF photon beam. Investigations are performed for field widths between 0.25 and 10 cm in four detector axis orientations with respect to the 1.5 T magnetic field and the photon beam. The magnetic field effect on the overall perturbation factor (PMC) accounting for the extracameral components, atomic composition, and density is quantified in each orientation. The density (Pρ) and volume averaging (Pvol) perturbation factors and quality correction factors (kQB,QfB,f) accounting for the magnetic field are also calculated in each orientation.Main results.Results show thatPvolremains the most significant perturbation both with and without magnetic fields. In most cases, the magnetic field effect onPvolis 1% or less. The magnetic field effect onPρis more significant on ionization chambers than on solid-state detectors. This effect increases up to 1.564 ± 0.001 with decreasing field size for chambers. On the contrary, the magnetic field effect on the extracameral perturbation factor is higher on solid-state detectors than on ionization chambers. For chambers, the magnetic field effect onPMCis only significant for field widths <1 cm, while, for solid-state detectors, this effect exhibits different trends with orientation, indicating that the beam incident angle and geometry play a crucial role.Significance.Solid-state detectors' dose-response is strongly affected by the magnetic field in all orientations. The magnetic field impact on ionization chamber response increases with decreasing field size. In general, ionization chambers yieldkQB,QfB,fcloser to unity, especially in orientations where the chamber axis is parallel to the magnetic field.
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Affiliation(s)
- Yunuen Cervantes
- Département de physique, Université de Montréal, Complexe des sciences, 1375 Avenue Thérèse-Lavoie-Roux, Montréal, Québec H2V 0B3, Canada.,Centre de recherche du Centre hospitalier de l'Université de Montréal, 900 Rue Saint-Denis, Montréal, Québec, H2X 0A9, Canada
| | - Jasmine Duchaine
- Département de physique, Université de Montréal, Complexe des sciences, 1375 Avenue Thérèse-Lavoie-Roux, Montréal, Québec H2V 0B3, Canada.,Centre de recherche du Centre hospitalier de l'Université de Montréal, 900 Rue Saint-Denis, Montréal, Québec, H2X 0A9, Canada
| | - Ilias Billas
- National Physical Laboratory, Chemical, Medical and Environmental Science Department, Teddington, United Kingdom.,Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom
| | - Simon Duane
- National Physical Laboratory, Chemical, Medical and Environmental Science Department, Teddington, United Kingdom
| | - Hugo Bouchard
- Département de physique, Université de Montréal, Complexe des sciences, 1375 Avenue Thérèse-Lavoie-Roux, Montréal, Québec H2V 0B3, Canada.,Centre de recherche du Centre hospitalier de l'Université de Montréal, 900 Rue Saint-Denis, Montréal, Québec, H2X 0A9, Canada.,Département de radio-oncologie, Centre hospitalier de l'Université de Montréal (CHUM), 900 Rue Saint-Denis, Montréal, Québec, H2X 0A9, Canada
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20
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Subashi E, Lim SB, Gonzalez X, Tyagi N. Longitudinal assessment of quality assurance measurements in a 1.5T MR-linac: Part I-Linear accelerator. J Appl Clin Med Phys 2021; 22:190-201. [PMID: 34505349 PMCID: PMC8504604 DOI: 10.1002/acm2.13418] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2021] [Revised: 08/16/2021] [Accepted: 08/29/2021] [Indexed: 01/04/2023] Open
Abstract
Purpose To describe and report longitudinal quality assurance (QA) measurements for the mechanical and dosimetric performance of an Elekta Unity MR‐linac during the first year of clinical use in our institution. Materials and methods The mechanical and dosimetric performance of the MR‐linac was evaluated with daily, weekly, monthly, and annual QA testing. The measurements monitor the size of the radiation isocenter, the MR‐to‐MV isocenter concordance, MLC and jaw position, the accuracy and reproducibility of step‐and‐shoot delivery, radiation output and beam profile constancy, and patient‐specific QA for the first 50 treatments in our institution. Results from end‐to‐end QA using anthropomorphic phantoms are also included as a reference for baseline comparisons. Measurements were performed in water or water‐equivalent plastic using ion chambers of various sizes, an ion chamber array, MR‐compatible 2D/3D diode array, portal imager, MRI, and radiochromic film. Results The diameter of the radiation isocenter and the distance between the MR/MV isocenters was (μ ± σ) 0.39 ± 0.01 mm and 0.89 ± 0.05 mm, respectively. Trend analysis shows both measurements to be well within the tolerance of 1.0 mm. MLC and jaw positional accuracy was within 1.0 mm while the dosimetric performance of step‐and‐shoot delivery was within 2.0%, irrespective of gantry angle. Radiation output and beam profile constancy were within 2.0% and 1.0%, respectively. End‐to‐end testing performed with ion‐chamber and radiochromic film showed excellent agreement with treatment plan. Patient‐specific QA using a 3D diode array identified gantry angles with low‐pass rates allowing for improvements in plan quality after necessary adjustments. Conclusion The MR‐linac operates within the guidelines of current recommendations for linear accelerator performance, stability, and safety. The analysis of the data supports the recently published guidance in establishing clinically acceptable tolerance levels for relative and absolute measurements.
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Affiliation(s)
- Ergys Subashi
- Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, New York
| | - Seng Boh Lim
- Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, New York
| | | | - Neelam Tyagi
- Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, New York
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21
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Billas I, Bouchard H, Oelfke U, Duane S. Traceable reference dosimetry in MRI guided radiotherapy using alanine: calibration and magnetic field correction factors of ionisation chambers. Phys Med Biol 2021; 66. [PMID: 34049290 DOI: 10.1088/1361-6560/ac0680] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2020] [Accepted: 05/28/2021] [Indexed: 12/27/2022]
Abstract
Magnetic resonance imaging (MRI)-guided radiotherapy (RT) (MRIgRT) falls outside the scope of existing high energy photon therapy dosimetry protocols, because those protocols do not consider the effects of the magnetic field on detector response and on absorbed dose to water. The aim of this study is to evaluate and demonstrate the traceable measurement of absorbed dose in MRIgRT systems using alanine, made possible by the characterisation of alanine sensitivity to magnetic fields reported previously by Billaset al(2020Phys. Med. Biol.65115001), in a way which is compatible with existing standards and calibrations available for conventional RT. In this study, alanine is used to transfer absorbed dose to water to MRIgRT systems from a conventional linac. This offers an alternative route for the traceable measurement of absorbed dose to water, one which is independent of the transfer using ionisation chambers. The alanine dosimetry is analysed in combination with measurements with several Farmer-type chambers, PTW 30013 and IBA FC65-G, at six different centres and two different MRIgRT systems (Elekta Unity™ and ViewRay MRIdian™). The results are analysed in terms of the magnetic field correction factors, and in terms of the absorbed dose calibration coefficients for the chambers, determined at each centre. This approach to reference dosimetry in MRIgRT produces good consistency in the results, across the centres visited, at the level of 0.4% (standard deviation). Farmer-type ionisation chamber magnetic field correction factors were determined directly, by comparing calibrations in some MRIgRT systems with and without the magnetic field ramped up, and indirectly, by comparing calibrations in all the MRIgRT systems with calibrations in a conventional linac. Calibration coefficients in the MRIgRT systems were obtained with a standard uncertainty of 1.1% (Elekta Unity™) and 0.9% (ViewRay MRIdian™), for three different chamber orientations with respect to the magnetic field. The values obtained for the magnetic field correction factor in this investigation are consistent with those presented in the summary by de Pooteret al(2021Phys. Med. Biol.6605TR02), and would tend to support the adoption of a magnetic field correction factor which depends on the chamber type, PTW 30013 or IBA FC65-G.
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Affiliation(s)
- Ilias Billas
- National Physical Laboratory, Teddington, United Kingdom.,Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom
| | - Hugo Bouchard
- Université de Montréal, Département de Physique, Montréal, Canada and Centre Hospitalier de l'Université de Montréal, Montréal, Canada and Centre de recherche du CHUM, Montréal, Canada
| | - Uwe Oelfke
- Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom
| | - Simon Duane
- National Physical Laboratory, Teddington, United Kingdom
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22
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Fuchs H, Padilla‐Cabal F, Zimmermann L, Palmans H, Georg D. MR-guided proton therapy: Impact of magnetic fields on the detector response. Med Phys 2021; 48:2572-2579. [PMID: 33326614 PMCID: PMC8251909 DOI: 10.1002/mp.14660] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2020] [Revised: 12/03/2020] [Accepted: 12/03/2020] [Indexed: 11/24/2022] Open
Abstract
PURPOSE To investigate the response of detectors for proton dosimetry in the presence of magnetic fields. MATERIAL AND METHODS Four ionization chambers (ICs), two thimble-type and two plane-parallel-type, and a diamond detector were investigated. All detectors were irradiated with homogeneous single-energy-layer fields, using 252.7 MeV proton beams. A Farmer IC was additionally irradiated in the same geometrical configuration, but with a lower nominal energy of 97.4 MeV. The beams were subjected to magnetic field strengths of 0, 0.25, 0.5, 0.75, and 1 T produced by a research dipole magnet placed at the room's isocenter. Detectors were positioned at 2 cm water equivalent depth, with their stem perpendicular to both the magnetic field lines and the proton beam's central axis, in the direction of the Lorentz force. Normality and two sample statistical Student's t tests were performed to assess the influence of the magnetic field on the detectors' responses. RESULTS For all detectors, a small but significant magnetic field-dependent change of their response was found. Observed differences compared to the no magnetic field case ranged from +0.5% to -0.7%. The magnetic field dependence was found to be nonlinear and highest between 0.25 and 0.5 T for 252.7 MeV proton beams. A different variation of the Farmer chamber response with magnetic field strength was observed for irradiations using lower energy (97.4 MeV) protons. The largest magnetic field effects were observed for plane-parallel ionization chambers. CONCLUSION Small magnetic field-dependent changes in the detector response were identified, which should be corrected for dosimetric applications.
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Affiliation(s)
- Hermann Fuchs
- Division of Medical PhysicsDepartment of Radiation OncologyMedical University of Vienna1090ViennaAustria
- Division of Medical PhysicsMedAustron Ion Therapy Center2700Wiener NeustadtAustria
| | - Fatima Padilla‐Cabal
- Division of Medical PhysicsDepartment of Radiation OncologyMedical University of Vienna1090ViennaAustria
| | - Lukas Zimmermann
- Division of Medical PhysicsDepartment of Radiation OncologyMedical University of Vienna1090ViennaAustria
| | - Hugo Palmans
- Division of Medical PhysicsMedAustron Ion Therapy Center2700Wiener NeustadtAustria
- National Physical LaboratoryTW11 0LWTeddingtonUnited Kingdom
| | - Dietmar Georg
- Division of Medical PhysicsDepartment of Radiation OncologyMedical University of Vienna1090ViennaAustria
- Division of Medical PhysicsMedAustron Ion Therapy Center2700Wiener NeustadtAustria
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23
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Roberts DA, Sandin C, Vesanen PT, Lee H, Hanson IM, Nill S, Perik T, Lim SB, Vedam S, Yang J, Woodings SW, Wolthaus JWH, Keller B, Budgell G, Chen X, Li XA. Machine QA for the Elekta Unity system: A Report from the Elekta MR-linac consortium. Med Phys 2021; 48:e67-e85. [PMID: 33577091 PMCID: PMC8251771 DOI: 10.1002/mp.14764] [Citation(s) in RCA: 44] [Impact Index Per Article: 14.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2020] [Revised: 01/21/2021] [Accepted: 02/03/2021] [Indexed: 12/31/2022] Open
Abstract
Over the last few years, magnetic resonance image‐guided radiotherapy systems have been introduced into the clinic, allowing for daily online plan adaption. While quality assurance (QA) is similar to conventional radiotherapy systems, there is a need to introduce or modify measurement techniques. As yet, there is no consensus guidance on the QA equipment and test requirements for such systems. Therefore, this report provides an overview of QA equipment and techniques for mechanical, dosimetric, and imaging performance of such systems and recommendation of the QA procedures, particularly for a 1.5T MR‐linac device. An overview of the system design and considerations for QA measurements, particularly the effect of the machine geometry and magnetic field on the radiation beam measurements is given. The effect of the magnetic field on measurement equipment and methods is reviewed to provide a foundation for interpreting measurement results and devising appropriate methods. And lastly, a consensus overview of recommended QA, appropriate methods, and tolerances is provided based on conventional QA protocols. The aim of this consensus work was to provide a foundation for QA protocols, comparative studies of system performance, and for future development of QA protocols and measurement methods.
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Affiliation(s)
- David A Roberts
- Elekta Limited, Cornerstone, London Road, Crawley, RH10 9BL, United Kingdom
| | - Carlos Sandin
- Elekta Limited, Cornerstone, London Road, Crawley, RH10 9BL, United Kingdom
| | | | - Hannah Lee
- Allegheny Health Network Cancer Institute, Pennsylvania, USA
| | - Ian M Hanson
- The Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, UK
| | - Simeon Nill
- The Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, UK
| | - Thijs Perik
- Department of Radiation Oncology, The Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam, the Netherlands
| | - Seng Boh Lim
- Memorial Sloan Kettering Cancer Center, New York, USA
| | - Sastry Vedam
- Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Texas, USA
| | - Jinzhong Yang
- Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Texas, USA
| | - Simon W Woodings
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, the Netherlands
| | - Jochem W H Wolthaus
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, the Netherlands
| | - Brian Keller
- Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada
| | - Geoff Budgell
- Christie Medical Physics and Engineering, The Christie NHS Foundation Trust, Wilmslow Road, Manchester, United Kingdom
| | - Xinfeng Chen
- Department of Radiation Oncology, Froedtert Hospital and Medical College of Wisconsin, Milwaukee, USA
| | - X Allen Li
- Department of Radiation Oncology, Froedtert Hospital and Medical College of Wisconsin, Milwaukee, USA
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24
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Nedaie HA, Gholami S, Longo F, Banaee N, Hassani M, Sarfehnia A, Pang G. The effect of magnetic field on Linac based Stereotactic Radiosurgery dosimetric parameters. Biomed Phys Eng Express 2020; 7. [PMID: 35037902 DOI: 10.1088/2057-1976/abd2c4] [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: 05/27/2020] [Accepted: 12/11/2020] [Indexed: 11/11/2022]
Abstract
Objective: MR-linac machines are being developed for image-guided radiation therapy but the magnetic field of such machines could affect dose distributions. The purpose of this work was to evaluate the effect of a magnetic field on linac beam dosimetric parameters including penumbra for circular cones used in radiosurgery.Methods: Monte Carlo simulation was conducted for a linac machine with circular cones at 6 MV beam. A homogenous magnetic field of 1.5 T was applied transversely and parallel to the radiation beam. Percentage depth dose (PDD) and beam profiles in a water phantom with and without the magnetic field were calculated.Results: The results have shown that when the magnetic field is applied transversely, the PDDs in the water phantom differ in the buildup region and distant part of PDD curves. The beam profiles at three different depths are all significantly different from those without the magnetic field. The penumbra is greater when a magnetic field has been applied.Conclusion: Linear accelerator-based SRT and SRS use small circular cones. The beam penumbra for these cones can change in the presence of a magnetic field. The perturbation of dose distribution has been also observed in a patient plan due to the presence of a magnetic field. The results of this study show that dose distributions in the presence of a magnetic field must be considered for MR-guided radiotherapy treatments.
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Affiliation(s)
- H Ali Nedaie
- Radiation Oncology Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran.,Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran
| | - Somayeh Gholami
- Radiation Oncology Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran.,Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran
| | - Francesco Longo
- Department of Physics, University of Trieste and INFN Trieste, Italy
| | - Nooshin Banaee
- Department of Medical Radiation, Engineering Faculty, Central Tehran Branch, Islamic Azad University, Tehran, Iran
| | - Mohssen Hassani
- Radiation Oncology Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran
| | - Arman Sarfehnia
- Odette Cancer Centre, Department of Radiation Oncology, University of Toronto, 2075 Bayview Avenue, Toronto M4N 3M5, Canada
| | - G Pang
- Odette Cancer Centre, Department of Radiation Oncology, University of Toronto, 2075 Bayview Avenue, Toronto M4N 3M5, Canada
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25
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Pojtinger S, Nachbar M, Kapsch RP, Thorwarth D. Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy. Phys Imaging Radiat Oncol 2020; 16:95-98. [PMID: 33458350 PMCID: PMC7807647 DOI: 10.1016/j.phro.2020.10.005] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2020] [Revised: 10/07/2020] [Accepted: 10/07/2020] [Indexed: 01/09/2023] Open
Abstract
Correction factors for reference dosimetry in magnetic resonance (MR) imaging-guided radiation therapy (k B → , M , Q ) are often determined in setups that combine a conventional 6 MV linac with an electromagnet. This study investigated whether results based on these measurements were applicable for a 7 MV MR-linac using Monte Carlo simulations. For a Farmer-type ionization chamber,k B → , M , Q was assessed for different tissue-phantom ratios (TPR 20 , 10 ).k B → , M , Q differed by 0.0029 ( 43 ) betweenTPR 20 , 10 = 0.6790 ( 23 ) (6 MV linac) andTPR 20 , 10 = 0.7028 ( 14 ) (7 MV MR-linac) at 1.5 T . The agreement was best in an orientation in which the secondary electrons were deflected to the stem of the ionization chamber.
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Affiliation(s)
- Stefan Pojtinger
- Physikalisch-Technische Bundesanstalt, Braunschweig, Germany
- Section for Biomedical Physics, Department of Radiation Oncology, University of Tübingen, Germany
| | - Marcel Nachbar
- Section for Biomedical Physics, Department of Radiation Oncology, University of Tübingen, Germany
| | | | - Daniela Thorwarth
- Section for Biomedical Physics, Department of Radiation Oncology, University of Tübingen, Germany
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