Measuring ion recombination factor for a pencil beam scanning synchrotron-based proton therapy system

Abstract

One of the well-known dosimetry standards that guides dosimetry practise is the International Atomic Energy Agency and its Technical Series Report 398 (TRS 398) in which, the TRS 398 provides a systematic framework of quantifying absorbed dose to water using an air-filled ionisation chamber. The TRS 398 does so by using a number of scaling and correction faction to translate the absorbed dose at a reference condition. One of this correction factors is the recombination correction factor and is used to account for charge losses due to ion recombination. This experiment will study the ion recombination correction factor in proton beams. TRS 398 recommends that the two-voltage-method (TVM) be used to determine the recombination correction factor by taking a measurement at a voltage to be investigated and at a lower voltage. While this method is simple to put into practice, some degree of approximations exists in this method. The actual form of the recombination correction factor is dependent on the temporal structure of the beam incident on the ionisation chamber. This experiment will investigate the ion recombination within ionisation chambers and determine if the accuracy of the ion recombination correction factor using the TVM is sufficient for clinical use. The experiment was conducted on a Hitachi ProBeat proton therapy system in National Cancer Centre Singapore (NCCS). A total of 3 different energy level and MUs were investigated: 70.2 MeV, 150.2 MeV and 220.7 MeV and at 6 MU, 50 MU and 200 MU. The field size was 10x10 cm2 and the spot spacing was 2.5 mm. The ionisation chambers to be investigated were placed at 2 cm water depth using plastic water.