ESTRO 2024 - Abstract Book

S3333

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

2409

Digital Poster

Investigation of center-of-axis offsets in ion chamber-based profile measurements in 1.5T MR-Linacs

Anastasios Episkopakis 1,2 , Vasiliki Margaroni 1 , Pantelis Karaiskos 1 , Nikolas Marinos 2 , Eleftherios P Pappas 1

1 National and Kapodistrian University of Athens, Medical School, Athens, Greece. 2 Elekta Ltd, Global Clinical Operations, Crawley, United Kingdom

Purpose/Objective:

In 1.5T MR-Linacs, the precise set-up of measuring devices is challenging due to the presence of the strong magnetic field. The Lorentz force affects the trajectories of secondary electrons that interact with the ionization chamber’s (IC’s) cavity [1], potentially affecting the IC’s Effective Point of Measurement (EPOM) [2]. Therefore, shifts may be needed to account for these changes. The goal of this study is to investigate the IC’s EPOM by evaluating systematic center-of axis (CAX) offsets in beam profile measurements in the presence of 1.5T magnetic field.

Material/Methods:

Beam profiles were collected from 3 different MR-Linac Unity systems (Elekta Ltd, UK), using the same type of equipment and implementing the same setup and analysis procedures. The MR compatible PTW 3D water tank was used following the standardized set-up procedure. The PTW Semiflex 3D 31021 (SF-3D) IC and the PTW 60019 Microdiamond (MD) (PTW, Germany) detectors were used throughout this study. Both detectors were positioned at the isocenter (SAD=143.5cm) at a depth of 10cm inside the water tank. For the SF-3D, two orientations were considered; (i) antiparallel to the magnetic field and perpendicular to the beam axis (hereinafter, “parallel set-up”) and (ii) perpendicular to the magnetic field and parallel to the beam axis (hereinafter “vertical set-up”) (Figure 1). The MD detector was positioned according to the vertical set-up only. Gantry angle was always at 0 o . Profile scans of the 10x10cm 2 field size in the in-line and cross-line directions were performed. Data were acquired using the PTW Mephysto software V4.5 and were analyzed with the Data Analyze tool v4.5 (PTW, Germany), according to the Elekta2020 protocol. The CAX offsets were calculated based on the penumbra inflection points. In addition, computational dosimetry using the EGSnrc V2019 Monte Carlo (MC) package was used to further evaluate the experimental results. Both SF-3D and MD detectors were simulated according to blueprints made available by the manufacturer. In the case of SF-3D, MC simulations were performed with and without considering the dead volume (DV) (hereinafter, “complete model” and “no DV model”, respectively) [3,4]. MC-based CAX offsets were also determined based on the inflection points, in accordance to the experimental measurements. Figure 1: The irradiation set-ups considered. (a) Parallel set-up; the detector is antiparallel to the magnetic field and perpendicular to the beam axis. (b) Vertical set-up; the detector is perpendicular to the magnetic field and parallel to the beam axis. Color legend; red: water phantom, black: air, purple: electrode. Yellow within the chamber’s cavity represents its calculated dead volume, i.e., air not included in the scoring volume. Abbreviations: B : magnetic field; Φ : primary photon fluence at gantry angle 0 o ; F L : Lorentz force for an electron with velocity parallel to Φ .