ESTRO 2024 - Abstract Book

S3175

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Figure 1 Mass density obtained from CT versus infill density in 3D-printing

Figure 2 presents a comparison of calculated and measured dose with the scintillator as a reference detector, often described as a water-equivalent detector with no field size dependence of its signal, even for fields smaller than 1 cm. In open fields of 10×10 cm² and 2×2 cm², doses measured with the scintillator align with those calculated in the TPS within 5.9 %. All detectors agree with the scintillator within 6.9 % in said fields. The higher variability between the detectors in the 2×2 cm² fields is especially due to the achievable positioning accuracy in the isocenter and the identification of the active volume of the larger measurement chambers. The smaller scintillator can be positioned in a more precise way. In case of 2×2 cm² offset-fields, where the detector is not in the direct beam, larger differences emerge. It should be noted that all absolute dose values of the offset fields (TPS and measured) are below 0.2 Gy, while for all other plans it is at least 1.35 Gy. While the studied diodes and ion chambers deviate from the TPS calculation by up to 55.5 %, the scintillator yields results within 11.4 % to the calculation. This also holds true for VMAT 2, which used an irradiation technique designed to spare the spinal cord and which incorporated numerous offset fields.