ESTRO 2024 - Abstract Book

S3289

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

keeping the number of Monitor Units per arc from the original patient plan. The NIPAM polymer gel content was 89% deionized water, 5% gelatine, 3% n-isopropyl acrylamide, 3% N’-methylenebisacrylamide, and 10 mM of the antioxidant tetrakis(hydroxymethyl) phosphonium chloride [3]. The gel was prepared under normal oxygen levels inside a fume hood and the solutions were stirred continuously throughout the mixing procedure. An “End to End” test was then undertaken following the same treatment path as the patient. The procedure was as follows: The gel phantom was first fixated in a vacuum bag, then CT-simulated, and positioned at the treatment machine using CBCT imaging for setup verification. The HyperArc plan was delivered (Varian True Beam), and Magnetic Resonance Imaging (MRI) (GE Signa Architect 3 T) using T2 weighted imaging with a 16 multiple spin-echo sequence was used for readout of the irradiation induced polymerization in the gel. The image-processing program Hero version 2023.1.0 (Hero Imaging AB, Umeå, Sweden) was used for data analysis. Owing to the difference in spatial resolution of the CT and MRI images, i.e., 1x1x1 mm 3 and 1x1x2 mm 3 respectively, the dose distributions were linearly interpolated and resampled to 2x2x2 mm 3 . Background subtraction was carried out using a volume in the gel phantom which received as low dose as possible. The dose distribution was normalized to a homogeneous dose region. The methods for background subtraction and normalization are described in previous work [4]. The pass rate for different gamma criteria within the 50 % dose volume (VOI 50 ), using the dose distribution from Eclipse TPS as the reference dose map and the dose distribution from NIPAM gel dosimeter as the measured dose, was investigated.

Results:

The 3D absorbed dose volumes generated from the TPS and measured using gel dosimetry could be visualized and compared in optional ways such as in Figure 1 a) and b). An overlay of the two volumes is presented in c), which is also rotated to show were the dotted white line cut through the dose max of both targets. The relative dose profiles along the dotted line were generally in very good agreement (d). However, differences between calculations and