Quality assurance of scanned proton beams at different gantry angles using an ionization chamber array in a rotational phantom

• Published in: Physica medica 2022-12, Vol.104, p.67-74
• Main Authors: Decabooter, Esther, Roijen, Erik, Martens, Jonathan, Unipan, Mirko, Bosmans, Geert, Vilches-Freixas, Gloria
• Format: Article
• Language: English
• Subjects: Dosimetry; Ionisation chamber array; Proton therapy; Proton Therapy - standards; Protons; Quality Assurance, Health Care; Rotational phantom
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2022.10.014

•Development non gantry mounted ‘set-and-forget’ QA procedure for 9 beam parameters.•Freedom to measure at all gantry angles.•The sensitivity is comparable to the scintillator detector.•Measurement time is reduced by 40%.•QA results over 12 months at our pencil beam scanning proton centre. To implement a single set-up monthly QA procedure for 9 different beam parameters at different gantry angles and evaluate its clinical implementation over a 12 month period. We developed a QA procedure using an array detector (PTW Octavius 1500XDR) embedded in a rotational unit (PTW Octavius 4D) at our proton facility. With a single set-up we can monitor field central axis position, field symmetry, field size, flatness, penumbrae, output, spot size, spot position and range at different gantry angles (AAPM TG 224). The set-up is irradiated with homogenous 2D fields with dynamic aperture and spot patterns at five gantry angles. A modular top is used to check the range consistency. Absolute γ analysis were performed to compare measured dose distributions to calculated dose. All other parameters are directly extracted from the measurements. Additionally, the sensitivity of the set-up to small changes in beam parameters were compared to the Lynx detector (IBA). Over a 12 month period, output, symmetry, and flatness were within ± 2 %; FWHM, spot positions, penumbra widths, and central axis fields were within ± 1 mm. Range differences were all within 1/2 of the energy spacing (±0.6 MeV) relative to baseline. Most (2 %, 2 mm) γ-analysis showed agreement scores higher than 90 %. The sensitivity is comparable to the Lynx detector and measurement time is reduced by 40 %. The time-efficient monthly QA procedure that we developed can accurately be used to measure a large range of beam parameters at different gantry angles, within the TG 224 AAPM recommendations.