Session Item

ICRU report 36 as well as the report on microdosimetry of the European Radiation Dosimetry Group deal with the theoretical and experimental methods of microdosimetry. This work reports on our progress using a gas-filled detector to validate microdosimetric equipment as well as Monte Carlo (MC) tools to simulate the pulse height spectra.

Measurements were performed with a mini TEPC (tissue-equivalent proportional counter), described by De Nardo et al (2004), in a 62.4 MeV single-spot proton beam with a FWHM of approximately 2.5 cm. The beam was delivered with a reduced particle rate of 4 MHz over the entire spot to reduce pile-up and saturation effects. It has a cylindrical sensitive volume (SV) of 1 mm³ and was filled with propane gas at 430 mbar. The voltage was varied to find the optimal gas gain. A python script was developed to convert the measured pulse height spectra into microdosimetric spectra, with appropriate corrections for the linearity of the electronics and the calibration using the proton edge technique published by Bianchi et al (2021).

A Monte Carlo (MC) simulation using the GATE/Geant4 toolkit was performed, utilizing a simple geometry and our facility’s beam characteristics and nozzle design, see Elia et al (2020). The detector geometry was a gas-filled sphere with three surrounding layers of G4WATER. Maximum step sizes and production cuts were selected for each region, decreasing for each layer closer to the scoring geometry. The TEPCActor source code was modified to calculate the correct mean chord length for a cylindrical SV instead of the default spherical SV.

The increase of the gas gain with increasing voltages was demonstrated for a range of 580 V to 780 V. Several spectra along the whole depth dose curve of a 62.4 MeV proton beam were obtained.

Figure 1 shows the dose distribution in lineal energy for the plateau and dose fall-off region. The solid lines represent the measured spectra, while the dashed lines represent the MC simulated spectra. While the overall shape of the simulated and measured spectra agree well, the positions of the edge region at the rightmost part of the spectra and peak show deviations, which are more pronounced for the dose fall-off spectra.

Figure 1:  Microdosimetric spectra obtained in the plateau and distal dose fall-off of a 62.4 MeV proton beam (solid lines) and their corresponding MC simulated spectra (dashed lines).