Effects of ion recombination on high dose rate electron using TRS-398, AAPM TG-51, and modified methods Ratna Sari Dewi (a*), Supriyanto Ardjo Pawiro (a), Dwi Seno Kuncoro (a)
a) Department of Physics, Faculty of Mathematics and Natural Science, Universitas Indonesia, Depok, West Java Indonesia
*ratna.sari14[at]ui.ac.id
Abstract
Aims: Precise dosimetry is essential for ensuring the effectiveness and safety of radiation therapy, particularly in High Dose Rate Electron (HDRE) beams, which exhibit a higher ionization density compared to conventional electron beams. At elevated dose rates, significant charge recombination effects may impose limitations on certain ionization chambers, potentially affecting measurement accuracy. This study aims to evaluate ion recombination (Ks) and polarity effects (Kpol) to determine the most appropriate dosimetry measurement protocol for clinical applications.
Materials & methods: Measurement were performed on LINAC VersaHD, Elekta using ionization chamber IBA PPC40, FC65-P and CC13 at 6 MeV and 10 MeV in HDRE mode. The chambers were positioned at Z placement and a 100 cm SSD. Ks and Kpol were evaluated at voltages of 100 to 400 V using methods from TRS-398, AAPM TG-51, and Modified methods. The modified methods involves a Zplacement shifting correction = 1.39 mm (muir) for IBA PPC40.
Results: Ks values decreased by approximately 0.1% to 0.2% as the voltage increased from 200 V to 400 V. The PPC40 exhibited the lowest Ks value of 1.0099 at 400 V, while the FC65-P exhibited the highest Ks value of 1.0735 at 200 V, exceeding TG 51 and TRS 398 recommendation (Ks<1.05). All Kpol values remained within the acceptable limits, with deviations of less than 0.4% from the ideal chamber readings.
Conclusion: The results show that PPC40 with all three methods at a higher voltage (400 V) than the recommended (300 V) effectively reduces recombination effects, enhancing charge collection efficiency, lowering Ks values, and maintaining Kpol value at acceptable limits.
