A comparison of measured and treatment planning system out-of-field dose for a 1.5 T MR linac
Powers, Marcus, and Baines, John (2023) A comparison of measured and treatment planning system out-of-field dose for a 1.5 T MR linac. Physics in Medicine and Biology, 68 (20). 20NT01.
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Abstract
Objective: Dose due to the electron streaming effect (ESE) is a significant contribution to out-of-field dose on the Elekta Unity MR-Linac. The aim of this work is to provide a systematic comparison of calculated and measured streaming dose for this system.
Approach: Beams 1.0 × 1.0 cm2 to 5.0 × 5.0 cm2, gantry 90.0°, 1000 MU, were incident on an in-house phantom. At the beam entrance and exit surfaces of the phantom, ESE was generated in the Y-direction (IEC 61217). EBT3 film, orientated within the X-Z plane and at 14.0 mm depth in a solid water block, was used to determine ESE dose 5.0 cm beyond the phantom. The experimental arrangement was simulated in the Monaco v5.4 treatment planning system (TPS), utilising a CT phantom dataset with differing relative electron densities (RED) for the surrounding air. Horizontal (Xdirection) and vertical (Zdirection) film dose profiles were compared to the corresponding TPS profiles.
Main results: For each field, the maximum ESE dose was observed at the beam exit, the magnitude of which decreases with decreasing field size. For the 5.0 × 5.0 cm2field, the exit and entry ESE doses were 19.6% and 7.0% of the Dmax dose to water, respectively. Across horizontal profiles, differences (simulated-measured) were reduced with smaller fields and lower RED. The maximum absolute profile difference was 1.7% of the Dmax dose to water for optimal RED and isocentre location. In vertical profiles an offset consistent with the Lorentz force was observed relative to the X-Y isoplane.
Significance: For the fields investigated, maximum absolute differences (simulated-measured) ≤ 5.2% occurred in peak regions of ESE, at the beam entrance and exit from the phantom. Generally, there is good agreement between Monaco simulated and measured ESE. Simulated out-of-field dose is sensitive to the RED assigned to air structures and unforced RED optimises out-of-field dose calculation accuracy.
Item ID: | 80835 |
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Item Type: | Article (Research - C1) |
ISSN: | 1361-6560 |
Keywords: | electron streaming effect, elekta unity, monaco, out-of-field dose, spiralling contaminant electrons |
Copyright Information: | © 2023 The Author(s). Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. |
Date Deposited: | 19 Feb 2024 06:18 |
FoR Codes: | 40 ENGINEERING > 4003 Biomedical engineering > 400304 Biomedical imaging @ 100% |
SEO Codes: | 20 HEALTH > 2001 Clinical health > 200101 Diagnosis of human diseases and conditions @ 100% |
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