SU-E-T-172: Ion Recombination Correction Factor in Medium Dose Rate for Advanced Markus Ionization Chamber.

Abstract

To determine the ion recombination correction factor using different methods in medium dose per pulse electron beams for Advance Markus ionization chamber (AMC) Methods: Two different methods were applied to obtain ion recombination correction factor. Conventional Boag's two voltage analysis (TVA) recommended in IAEA TRS-398 dosimetry protocol and recently modified Boag's TVA expressions which consider the effect of free electron contribution on ks value. High energy electron beams of a dedicated linear accelerator, NOCAC7 (NRT, Italy), for intraoperative radiation therapy were applied. Dose per pulse in medium range of 4 to 41 mGy/pulse was investigated. Radiochromic films (ISP Gafchromic EBT2, USA) have been exploited as reference dose rate independent method. Dose measurement was performed by AMC in a small water phantom (IBA Dosimetry, Germany). At reference conditions (9 MeV, 100 mm applicator, SSD 80 cm (27 mGy/pulse)), ks values derived from IAEA TRS- 398 dosimetry protocols deviates 0.3% from reference value. From 4-35 mGy/pulse TVA yielded acceptable result (in average 1.0% deviation). For dose rate more that 35 mGy/pulse, conventional two voltage analysis (TVA) might be applied with caution (4.2% deviation at 41 mGy/pulse). In this range first model in the new modified Boag's expressions yielded acceptable accordance (average deviation 1.3%) in comparison with dose rate independent reference dosimeter (EBT2, Gafchromic film). The deviation in ks value among Boag's modified expressions increased when the dose rate raised. Authors concluded that AMC could be employed for absorbed dose determination of medium dose per pulse electron beams. The accuracy of TVA which is recommended currently in dosimetry protocol IAEA-TRS-398 is dependent on the range of applied dose rate. It is proper to be used for dose rate less than 35 mGy/pulse. First expression in Boag's modified formulas is proper for ks determination in high dose rate values (> 35 mGy/pulse).