Calculation of peak to valley dose ratio due to multi-slice X-ray beams using ionization chamber and Geant4 simulation code

Studies carried out with synchrotron radiation have shown that micro-beam radiation therapy (MRT) has unique advantages in the treatment of cancerous tumors. In this method, the determination of dose distribution and calculation of peak to valley dose ratio (PVDR) are considered as the most important steps in treatment planning. The PVDR is a criterion to evaluate the destruction of cancer cells and protection of normal cells in the tissues surrounding a tumor.   Using a multi-slit collimator, planar sliced beams were generated in an X-ray generator in order to determine dose distribution in a multilayer phantom made of plexiglass. An ionization chamber was used to measure absorbed dose. Given the large size of the sensitive area of the chamber in comparison with the narrow beams, a mono-slit collimator made of tungsten with a slit of 0.3×7.5 mm2 in its center was placed in front of the ionization chamber. Furthermore, by using Geant4 computer code, a model, including X-ray source, multi-slit collimator, phantom, mono-slit collimator, and detector, was designed to compare experimental and simulation results.

Findings: The investigation of dose distribution in the phantom with both methods indicated the presence of peaks and valleys. Given the low intensity of X-ray beam generated by the X-ray generator, and limited exposure time, the experimental errors were considerable. When using 1 mm (Air)+0.5 mm (W) collimator, PVDRs were obtained as 8.7 and 10.5 for ionization chamber and simulation, respectively, in the depth of 8 mm of the phantom. On the other hand, with a 1 mm (Air)+1 mm (W) collimator, the values obtained for this parameter were 11.1 and 13.3 for ionization chamber and simulation, respectively.

Based on the results, a multi-slit collimator made of tungsten could produce multi-slice X-ray. The estimated dose distribution using the Geant4 code was more accurate than the one obtained through ionization chamber, which can be due to the possibility of using a detector in much smaller dimensions in the Geant4 code