Determination of the beam quality correction factor kQmsr${k}_{{Q}_{{\mathrm{msr}}}}$ for the PTW Semiflex 3D ionization chamber for the reference dosimetry at ZAP‐X
Katrin Saße, Karina Albers, Daniela Eulenstein, Georg Weidlich, Björn Poppe, Hui Khee LooeAbstract
Purpose
The self‐shielding radiosurgery system ZAP‐X consists of a 3 MV linear accelerator and eight round collimators. For this system, it is a common practice to perform the reference dosimetry using the largest 25 mm diameter collimator at a source‐to‐axis distance (SAD) of 45 cm with the PTW Semiflex3D chamber placed at a measurement depth of 7 mm in water. Existing dosimetry protocols do not provide correction for these measurement conditions. Therefore, Monte Carlo simulations were performed to quantify the associated beam quality correction factor .
Methods
The of the Semiflex3D chamber was computed from the ratio of the absorbed doses in a water voxel and in the sensitive air volume of the chamber simulated using a 60Co spectrum as the calibration beam quality (Qref) and the spectrum of the ZAP‐X 3 MV photon beam (Qmsr). was computed as a function of measurement depth from 4 to 50 mm. Furthermore, detailed simulations were performed to determine the individual chamber's perturbation correction factors by modifying the chamber's model step‐wise.
Results
All perturbation correction factors, except Sw,air ⋅Pfl, show depth‐dependent behavior up to a depth of 15 mm. In particular, the volume‐averaging Pvol and density Pdens perturbation correction factors and, consequently, the resulting gradient perturbation correction factor Pgr = Pvol∙Pdens increase with decreasing measurement depth. Therefore, is larger than unity, amounting to at 7 mm measurement depth. At larger depths (> 15 mm), the can be considered as constant.
Conclusion
At small measurement depths, was found to be depth‐dependent with values larger than unity due to the gradient‐related perturbation factors. Therefore, the uncertainty related to the chamber's positioning can be reduced by performing the reference dosimetry at ZAP‐X at depths larger than 15 mm, where can be regarded as depth independent.