ESTRO 2024 - Abstract Book

S3372

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

In figure 1, f Q factors are shown for two exemplary plane-parallel ionization chambers as a function of proton energy for the two positioning approaches. It can be seen that differences are present especially for low energies whereas for high energies differences are negligible. The largest difference of 1.2% can be seen for the PPC-05 chamber at 60 MeV.

Figure 1: Monte Carlo calculated f Q factors for two exemplary ionization chambers as a function of proton energy for both positioning approaches.

In table 1, differences in f Q for 60 MeV protons for both positioning approaches are shown for all ionization chambers investigated in this study along with the differences in absorbed dose-to-water between the depth of the reference point and the effective point of measurement. The results prove that the change in f Q due to the positioning of the ionization chamber is due the dose gradient at the measurement depth.

Table 1: Difference in f Q factors between both positioning approaches for 60 MeV protons and the difference in dose between the depth of the reference point and the effective point of measurement.

Ionization chamber model

Difference in f Q in %

Difference in dose in %

PTW Roos

0.38

0.37

PTW Markus

-0.46 -0.47 0.75 1.19 0.28

-0.49 -0.50 0.74 1.17 0.30

PTW Advanced Markus

IBA NACP-02 IBA PPC-05 IBA PPC-40

Conclusion:

The positioning of plane-parallel ionization chambers in clinical proton beams has an influence on f Q and hence k Q factors. The impact is driven by the dose gradient at the measurement depth and is up to 1.2% while it depends on the water-equivalent thickness of the entrance window. It is important to point out that k Q factors presented in the IAEA TRS-398 CoP are only valid if the chamber is positioned with its reference point at the measurement depth. If the chamber is positioned with its effective point of measurement k Q factors have to be corrected by the dose gradient.

Keywords: Particle therapy, ionization chambers, dosimetry