ESTRO 2024 - Abstract Book

S3194

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Material/Methods:

MC calculations were performed using the egs_chamber user-code incorporated in the EGSnrc V2019 software package. The nanoDots TM (Landauer Inc., USA) and the myOSLchip TM (RadPro International GmbH, Germany) commercially available dosimeters were considered in this study. They comprise active volumes made of Al 2 O 3 (disk of 5 mm in diameter and 0.2 mm thick, density ρ=1.41 g/cm 3 [4]) and BeO (square 4.65x4.65 mm 2 and 0.5 mm thick, density ρ=2.85 g/cm 3 ), respectively. Both dosimeters were modelled in-detail (Figure 1), including their housing cases and air gaps, according to blueprints provided by the corresponding manufacturers. Phase space files for the 10x10 cm 2 field size of a 1.5T/7MV Unity MR-Linac system (ELEKTA Ltd, UK) were provided by the vendor and used as a source model throughout this study. OSL dosimeters were simulated at the depth of 5 cm inside a 30x30x19 cm 3 phantom made of RW3 (PTW, Germany), with the OSL’s active volume fixed at the linac’s isocenter. The magnetic field was always perpendicular to the beam and parallel to the treatment couch. In all calculations, gantry angle was fixed at 0 o , and k B,Q values for the 10x10cm 2 field were determined for the three nominal cardinal orientations: (i) coronal, (ii) sagittal, (iii) axial, as defined in Figure 1. Calculations were repeated with the OSL dosimeters rotated by +90 o and -90 o around the y-axis, i.e., the only allowed axis of rotation for the MR-Linac’s gantry. Computational steps and runs necessary for MC-based k B,Q determination have been described in a previous study for ionization chambers [5].

Figure 1. The two commercially available OSL dosimeters modelled in EGSnrc. The three nominal cardinal orientations with respect to the magnetic field, B, are defined and referred to as “Coronal”, “Sagittal” and “Axial”. The yellow arrow indicates the positive rotational direction applied to the nominal cardinal orientations for the k B,Q values given in Table 1. Color legend: magenta: active material, blue: housing case, red: air, light blue: polyester film cover.

Results:

k B,Q values for both detectors and all orientations considered are presented in Table 1 and were found to strongly vary with orientation. Corrections needed range from 0.3% up to 6.4% (Table 1). Given that the gantry may rotate around the y-axis, dosimetry for a treatment plan comprising several beam angles would require mixed corrections under the sagittal and coronal setups. However, if the OSL dosimeters are placed in the axial orientation, k B,Q values are equal to unity within uncertainties, irrespective to the incident beam angle.