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Khan AU, Das IJ, Yadav P. Computational and experimental small field dosimetry using a commercial plastic scintillator detector for the 0.35 T MR-linac. Phys Med 2024; 123:103403. [PMID: 38870643 DOI: 10.1016/j.ejmp.2024.103403] [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: 12/18/2023] [Revised: 05/08/2024] [Accepted: 06/05/2024] [Indexed: 06/15/2024] Open
Abstract
PURPOSE Although plastic scintillator detectors (PSDs) are considered ideal dosimeters for small field dosimetry in conventional linear accelerators (linacs), the impact of the magnetic field strength on the response of the PSD must be investigated. METHODS A linac Monte Carlo (MC) head model for a low-field MR-linac was validated for small field dosimetry and utilized to calculate field output factors (OFs). The MC-calculated OFs were compared with the treatment planning system (TPS)-calculated OFs and measured OFs using a Blue Physics (BP) Model 10 commercial PSD and a synthetic diamond detector. The field-specific correction factors, [Formula: see text] , were calculated for the PSD in the presence of a 0.35 T and magnetic field. The impact of the source focal spot size and initial electron energy on the MC-calculated OFs was investigated. RESULTS Good agreement to within 2 % was found between the MC-calculated OFs and BP PSD OFs except for the 0.415 × 0.415 cm2 field size. The BP PSD [Formula: see text] correction factors were calculated to be within 1 % of unity. For field sizes ≥1.66 × 1.66 cm2, the MC-calculated OFs were relatively insensitive to the focal spot size and initial electron energy to within 2.5 %. However, for smaller field sizes, the MC-calculated OFs were found to differ up to 9.50 % and 7.00 % when the focal spot size and initial electron energy was varied, respectively. CONCLUSIONS The BP PSD was deemed suitable for small field dosimetry in MR-linacs without requiring any [Formula: see text] correction factors.
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Affiliation(s)
- Ahtesham Ullah Khan
- Department of Radiation Oncology, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA.
| | - Indra J Das
- Department of Radiation Oncology, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA
| | - Poonam Yadav
- Department of Radiation Oncology, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA.
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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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