Cylindrical ionization chamber response in static and dynamic 6 and 15 MV photon beams

• Published in: Biomedical physics & engineering express 2023-01, Vol.9 (2), p.25004
• Main Authors: Andersson, P, Swanpalmer, J, Palm, Å, Båth, M, Chakarova, R
• Format: Article
• Language: English
• Subjects: Absorbed dose; Conversion factor; Correction factors; human; Humans; Intelligent systems; Ionization chamber dosimetry; Ionization chambers; Monte Carlo; Monte Carlo Method; Monte Carlo methods; Monte Carlo simulations; Monte Carlo's simulation; Non-reference condition; non-reference conditions; Particle beams; Patient treatment; Photoelectrons; Photoionization; Photon beams; Photons; procedures; Quality assurance; Radiologi och bildbehandling; Radiology and Medical Imaging; radiometry; Radiometry - methods; radiotherapy; Reference condition; simulations; Software; Static fields; VMAT; Water
• ISSN: 2057-1976; 2057-1976
• DOI: 10.1088/2057-1976/acb553

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. . Measurements were performed in 6 MV 100 × 100, 20 × 100, and 20 × 20 mm 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. . 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. . 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.