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Cylindrical ionization chamber response in static and dynamic 6 and 15 MV photon beams
RISE Research Institutes of Sweden, Materials and Production, Product Realisation Methodology. University of Gothenburg, Sweden.
University of Gothenburg, Sweden; Sahlgrenska University Hospital, Sweden.
Sahlgrenska University Hospital, Sweden.
University of Gothenburg, Sweden; Sahlgrenska University Hospital, Sweden.
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2023 (English)In: Biomedical Engineering & Physics Express, E-ISSN 2057-1976, Vol. 9, no 2, article id 025004Article in journal (Refereed) Published
Abstract [en]

Purpose. To investigate the response of the CC13 ionization chamber under non-reference photon beam conditions, focusing on penumbra and build-up regions of static fields and on dynamic intensity-modulated beams. Methods. Measurements were performed in 6 MV 100 × 100, 20 × 100, and 20 × 20 mm2 static fields. Monte Carlo calculations were performed for the static fields and for 6 and 15 MV dynamic beam sequences using a Varian multi-leaf collimator. The chamber was modelled using EGSnrc egs_chamber software. Conversion factors were calculated by relating the absorbed dose to air in the chamber air cavity to the absorbed dose to water. Correction and point-dose correction factors were calculated to quantify the conversion factor variations. Results. The correction factors for positions on the beam central axis and at the penumbra centre were 0.98-1.02 for all static fields and depths investigated. The largest corrections were obtained for chamber positions beyond penumbra centre in the off-axis direction. Point-dose correction factors were 0.54-0.71 at 100 mm depth and their magnitude increased with decreasing field size and measurement depth. Factors of 0.99-1.03 were obtained inside and near the integrated penumbra of the dynamic field at 100 mm depth, and of 0.92-0.94 beyond the integrated penumbra centre. The variations in the ionization chamber response across the integrated dynamic penumbra qualitatively followed the behaviour across penumbra of static fields. Conclusions. Without corrections, the CC13 chamber was of limited usefulness for profile measurements in 20-mm-wide fields. However, measurements in dynamic small irregular beam openings resembling the conditions of pre-treatment patient quality assurance were feasible. Uncorrected ionization chamber response could be applied for dose verification at 100 mm depth inside and close to large gradients of dynamically accumulating high- and low-dose regions assuming 3% tolerance between measured and calculated doses. © 2023 The Author(s).

Place, publisher, year, edition, pages
Institute of Physics , 2023. Vol. 9, no 2, article id 025004
Keywords [en]
Ionization chamber dosimetry, Monte Carlo simulations, non-reference conditions, radiotherapy, VMAT, Intelligent systems, Ionization chambers, Monte Carlo methods, Particle beams, Patient treatment, Photoelectrons, Photoionization, Quality assurance, Absorbed dose, Conversion factor, Correction factors, Monte Carlo's simulation, Non-reference condition, Photon beams, Reference condition, Static fields, Photons, water, human, Monte Carlo method, procedures, radiometry, software, Humans
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:ri:diva-63977DOI: 10.1088/2057-1976/acb553Scopus ID: 2-s2.0-85147143449OAI: oai:DiVA.org:ri-63977DiVA, id: diva2:1737457
Note

Correspondence Address: Andersson, P.; RISE and Sahlgrenska Academy, Sweden; Funding details: Strålsäkerhetsmyndigheten, SSM; Funding text 1: Financial support of King Gustav the V Jubilee Clinic Foundation, The Swedish Radiation Safety Authority and The Healthcare Committee, Region Västra Götaland are greatly acknowledged.

Available from: 2023-02-16 Created: 2023-02-16 Last updated: 2023-02-16Bibliographically approved

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