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Patterson E, Stokes P, Cutajar D, Rosenfeld A, Baines J, Metcalfe P, Powers M. High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac. Phys Eng Sci Med 2023; 46:787-800. [PMID: 36988905 DOI: 10.1007/s13246-023-01251-6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2022] [Accepted: 03/21/2023] [Indexed: 03/30/2023]
Abstract
The magnetic field of a transverse MR-linac alters electron trajectories as the photon beam transits through materials, causing lower doses at flat entry surfaces and increased doses at flat beam-exiting surfaces. This study investigated the response of a MOSFET detector, known as the MOSkin™, for high-resolution surface and near-surface percentage depth dose measurements on an Elekta Unity. Simulations with Geant4 and the Monaco treatment planning system (TPS), and EBT-3 film measurements, were also performed for comparison. Measured MOSkin™ entry surface doses, relative to Dmax, were (9.9 ± 0.2)%, (10.1 ± 0.3)%, (11.3 ± 0.6)%, (12.9 ± 1.0)%, and (13.4 ± 1.0)% for 1 × 1 cm2, 3 × 3 cm2, 5 × 5 cm2, 10 × 10 cm2, and 22 × 22 cm2 fields, respectively. For the investigated fields, the maximum percent differences of Geant4, TPS, and film doses extrapolated and interpolated to a depth suitable for skin dose assessment at the beam entry, relative to MOSkin™ measurements at an equivalent depth were 1.0%, 2.8%, and 14.3%, respectively, and at a WED of 199.67 mm at the beam exit, 3.2%, 3.7% and 5.7%, respectively. The largest measured increase in exit dose, due to the electron return effect, was 15.4% for the 10 × 10 cm2 field size using the MOSkin™ and 17.9% for the 22 × 22 cm2 field size, using Geant4 calculations. The results presented in the study validate the suitability of the MOSkin™ detector for transverse MR-linac surface dosimetry.
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Affiliation(s)
- E Patterson
- Centre of Medical and Radiation Physics, University of Wollongong, Wollongong, NSW, Australia.
| | - P Stokes
- Townsville Cancer Centre, Townsville Hospital and Health Service, Townsville, QLD, Australia
| | - D Cutajar
- Centre of Medical and Radiation Physics, University of Wollongong, Wollongong, NSW, Australia
| | - A Rosenfeld
- Centre of Medical and Radiation Physics, University of Wollongong, Wollongong, NSW, Australia
- Illawarra Health Medical Research Institute, University of Wollongong, Wollongong, NSW, Australia
| | - J Baines
- Townsville Cancer Centre, Townsville Hospital and Health Service, Townsville, QLD, Australia
- College of Science and Engineering, James Cook University, Townsville, QLD, Australia
| | - P Metcalfe
- Centre of Medical and Radiation Physics, University of Wollongong, Wollongong, NSW, Australia
- Illawarra Health Medical Research Institute, University of Wollongong, Wollongong, NSW, Australia
- Ingham Institute for Applied Medical Research, Liverpool, NSW, Australia
| | - M Powers
- Townsville Cancer Centre, Townsville Hospital and Health Service, Townsville, QLD, Australia
- College of Science and Engineering, James Cook University, Townsville, QLD, Australia
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Madden L, Roberts N, Jelen U, Dong B, Holloway L, Metcalfe P, Rosenfeld A, Li E. In-line MRI-LINAC depth dose measurements using an in-house plastic scintillation dosimeter. Biomed Phys Eng Express 2021; 7. [PMID: 33530066 DOI: 10.1088/2057-1976/abe295] [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: 10/08/2020] [Accepted: 02/02/2021] [Indexed: 11/12/2022]
Abstract
Plastic scintillation dosimeters (PSDs) have many properties that make them desirable for relative dosimetry with MRI-LINACs. An in-house PSD, Farmer ionisation chamber and Gafchromic EBT3 film were used to measure central axis percentage depth dose distributions (PDDs) at the Australian MRI-LINAC Mean errors were calculated between each detector's responses, where the in-house PSD was on average within 0.7% of the Farmer chamber and 1.4% of film, while the Farmer chamber and film were on average within 1.1% of each other. However, the PSD systematically over-estimated the dose as depth increased, approaching a maximum overestimation of the order of 3.5% for the smallest field size measured. This trend was statistically insignificant for all other field sizes measured; further investigation is required to determine the source of this effect. The calculated values of mean absolute error are comparable to the those of trusted dosimeters reported in the literature. These mean absolute errors, and the ubiquity of desirable dosimetric qualities inherent to PSDs suggest that PSDs in general are accurate for relative dosimetry with the MRI-LINAC. Further investigation is required into the source of the reported systematic trends dependent on field-size and depth of measurement.
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Affiliation(s)
- Levi Madden
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.,Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia
| | - Natalia Roberts
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.,Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia
| | - Urszula Jelen
- GenesisCare St Vincent's Clinic, Darlinghurst, NSW 2010, Australia
| | - Bin Dong
- Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia
| | - Lois Holloway
- Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia.,Liverpool Cancer Therapy Centre, Liverpool, NSW 2170, Australia.,Macauthur Cancer Therapy Clinic, Campbelltown, NSW 2560, Australia
| | - Peter Metcalfe
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.,Illawarra Medical and Health Research Institute, University of Wollongong, NSW 2522, Australia
| | - Anatoly Rosenfeld
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.,Illawarra Medical and Health Research Institute, University of Wollongong, NSW 2522, Australia
| | - Enbang Li
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia
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Madden L, Archer J, Li E, Jelen U, Dong B, Holloway L, Rosenfeld A. MRI-LINAC beam profile measurements using a plastic scintillation dosimeter. Phys Med 2020; 73:111-116. [PMID: 32361155 DOI: 10.1016/j.ejmp.2020.04.016] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/21/2020] [Revised: 04/14/2020] [Accepted: 04/16/2020] [Indexed: 10/24/2022] Open
Abstract
Plastic scintillation dosimeters (PSDs) possess many desirable qualities for dosimetry with LINACs. These qualities are expected to make PSDs effective for MRI-LINAC dosimetry, however little research has been conducted investigating their dosimetric performance with MRI-LINACs. In this work, an in-house PSD was used to measure 8 beam profiles with an in-line MRI-LINAC, compared with film measurements. One dimensional global gamma indices (γ) and corresponding γ pass rates were calculated to compare PSD and film profiles for the 1%/1 mm, 2%/2 mm and 3%/3 mm criterion. The mean global pass rates were 85.8%, 97.5% and 99.4% for the 1%/1 mm, 2%/2 mm and 3%/3 mm criteria, respectively. The majority of the γ failures occurred in the penumbral regions. Penumbra widths were measured to be slightly narrower with the PSD compared to film, however, the uncertainties in the measured penumbra widths brought the PSD and film penumbra widths into agreement. Differences in dose were calculated between the PSD and film, and remained within 2.2% global agreement for the central regions and 1.5% global agreement for out of field regions. These values for range of agreement were similar to the those reported in the literature for other dosimeters which are trusted for relative MRI-LINAC dosimetry.
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Affiliation(s)
- Levi Madden
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia; Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia
| | - James Archer
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia
| | - Enbang Li
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia.
| | - Urszula Jelen
- Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia
| | - Bin Dong
- Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia
| | - Lois Holloway
- Ingham Institute for Applied Medical Research, Liverpool, NSW 2170, Australia
| | - Anatoly Rosenfeld
- Centre for Medical Radiation Physics, University of Wollongong, NSW 2522, Australia; Illawarra Medical and Health Research Institute, University of Wollongong, NSW 2522, Australia
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Trachsel MA, Pojtinger S, Meier M, Schrader M, Kapsch RP, Kottler C. Chemical radiation dosimetry in magnetic fields: characterization of a Fricke-type chemical detector in 6 MV photon beams and magnetic fields up to 1.42 T. ACTA ACUST UNITED AC 2020; 65:065005. [DOI: 10.1088/1361-6560/ab7360] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Roberts NF, Patterson E, Jelen U, Causer T, Holloway L, Liney G, Lerch M, Rosenfeld AB, Cutajar D, Oborn BM, Metcalfe P. Experimental characterization of magnetically focused electron contamination at the surface of a high-field inline MRI-linac. Med Phys 2019; 46:5780-5789. [PMID: 31633212 DOI: 10.1002/mp.13847] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2019] [Revised: 08/26/2019] [Accepted: 09/23/2019] [Indexed: 11/08/2022] Open
Abstract
PURPOSE The fringe field of the Australian MRI-linac causes contaminant electrons to be focused along the central axis resulting in a high surface dose. This work aims to characterize this effect using Gafchromic film and high-resolution detectors, MOSkinTM and microDiamond. The secondary aim is to investigate the influence of the inline magnetic field on the relative dose response of these detectors. METHODS The Australian MRI-linac has the unique feature that the linac is mounted on rails allowing for measurements to be performed at different magnetic field strengths while maintaining a constant source-to-surface distance (SSD). Percentage depth doses (PDD) were collected at SSD 1.82 m in a solid water phantom positioned in a low magnetic field region and then at isocenter of the MRI where the magnetic field is 1 T. Measurements for a range of field sizes were taken with the MOSkinTM , microDiamond, and Gafchromic® EBT3 film. The detectors' relative responses at 1 T were compared to the near 0 T PDD beyond the region of electron contamination, that is, 20 mm depth. The near surface measurements inside the MRI bore were compared among the different detectors. RESULTS Skin dose in the MRI, as measured with the MOSkinTM , was 104.5% for 2.1 × 1.9 cm2 , 185.6% for 6.1 × 5.8 cm2 , 369.1% for 11.8 × 11.5 cm2 , and 711.1% for 23.5 × 23 cm2 . The detector measurements beyond the electron contamination region showed agreement between the relative response at 1 T and near 0 T. Film was in agreement with both detectors in this region further demonstrating their relative response is unaffected by the magnetic field. CONCLUSIONS Experimental characterization of the high electron contamination at the surface was performed for a range of field sizes. The relative response of MOSkinTM and microDiamond detectors, beyond the electron contamination region, were confirmed to be unaffected by the 1-T inline magnetic field.
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Affiliation(s)
- Natalia F Roberts
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia.,Centre for Oncology Education and Research Translation, Wollongong, NSW, 2522, Australia.,Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia
| | - Elizabeth Patterson
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia
| | - Urszula Jelen
- Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia
| | - Trent Causer
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia.,Illawarra Cancer Care Centre, Wollongong Hospital, Wollongong, NSW, 2500, Australia
| | - Lois Holloway
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia.,Centre for Oncology Education and Research Translation, Wollongong, NSW, 2522, Australia.,Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia.,Department of Medical Physics, Liverpool and Macarthur Cancer Care Centres, Liverpool, NSW, 2170, Australia.,South Western Sydney Clinical School, University of New South Wales, Liverpool, NSW, Australia.,Institute of Medical Physics, University of Sydney, Camperdown, NSW, 2505, Australia
| | - Gary Liney
- Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia
| | - Michael Lerch
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia
| | - Anatoly B Rosenfeld
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia
| | - Dean Cutajar
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia
| | - Bradley M Oborn
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia.,Illawarra Cancer Care Centre, Wollongong Hospital, Wollongong, NSW, 2500, Australia
| | - Peter Metcalfe
- Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2522, Australia.,Ingham Institute for Applied Medical Research, Liverpool, NSW, 2170, Australia
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Matsuoka T, Araki F, Ohno T. Perturbation effect of parallel-plate ionization chambers on buildup dose measurements in transverse magnetic fields. Phys Med 2019; 59:112-116. [DOI: 10.1016/j.ejmp.2019.03.011] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/10/2018] [Revised: 02/16/2019] [Accepted: 03/12/2019] [Indexed: 10/27/2022] Open
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