ESTRO 2024 - Abstract Book

S3402

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

60 Co spectrum (beam quality Q

0 ). The change in the chamber’s dose response attributed to the different beam quality

(between Q 0 and Q) is accounted for by the beam quality correction factor k Q .

To determine the k Q of the Semiflex 3D chamber for the reference dosimetry at ZAP-X, the ratio of the absorbed dose in a 0.2 x 0.2 x 0.2 mm³ water voxel and the absorbed dose in the sensitive air volume of the chamber D w /D IK is calculated for both the 60 Co spectrum (Q 0 ) as well as the spectrum of the 3 MV photon beam as provided by ZAP-X (Q), where k Q =(D w ⁄D IK ) Q ⁄(D w ⁄D IK ) Q0 . To study its depth-dependence, k Q was simulated as a function of measurement depth between 0.4 mm and 50 mm. The chamber was positioned with its reference point at the indicated measurement depth. For further investigations, detailed simulations were performed to determine the individual perturbation correction factors P wall , P dens , P cel , P stem and P vol by modifying the chamber’s model stepwise in the simulations as performed in Tekin et al. . Each perturbation factor was computed as the ratio of the deposited energies between two subsequent steps. The perturbation correction factors are shown in Figure 1 (left axis). At small depths < 15 mm, all perturbation factors, except for the P fl ⋅ S w,air , show depth-dependent behavior. In particular, both the volume-averaging P vol and density P dens perturbation correction factors, and consequently the gradient perturbation correction factor P gr = P vol ∙ P dens increase strongly with decreasing measurement depth. At the depth of dose maximum (7 mm), P gr amounts to 1.017. The beam quality correction factors, as the product of the individual perturbation correction factors, f Q , normalized to that under the calibration conditions simulated using the 60 Co quality ( f Q0 ), are presented on the right axis. For the depth of 7 mm, where strong depth dependence of the perturbation correction factors is observed, k Q is larger than unity with a value of 1.009(2). At larger depths (> 15 mm), the k Q can be considered as constant with a value of 0.994(2). Results: