고에너지 광자선에서 액체전리함의 선량 특성 및 보정 계수 결정

Abstract

The Liquid ionization chamber has superior characteristics than air-filled ionization chamber due to small size, high signal-to-noise and perturbation effect. Especially in small field dosimetry, the liquid ionization cahmber may potentially become a proper tool as the reference dosimeter. Currently, however, the methodologies, and physical quantities, and the limitations for use in the clinical radiation dosimetry are not clearly described for the relative and reference dosimeter using the liquid ionization chamber. In the present work, we studied the dosimetric characteristics of the microLion chamber, which is a commercial liquid ionization chamber, and determined the correction factors for the absorbed dose to water measurement. In addition, we verified the experimental method to acquire the beam quality correction factor by Monte Carlo simulations. For the dosimetric characteristics of the micrioLion cahmber, the results were compared with those of semiflex, pinpoint and diode chambers and evaluated to be suitable for small fields. This study was performed using the 6MV photon beam from a Varian iX linac accelerator and a MP3 water phantom (PTW, Freiburg) was used for measurements of the dose rate and dose linearity dependency, spatial resolution and output factors. For the absorbed dose to water measurement, we determined the absorbed dose to water calibration factor, the beam quality correction factor and the influence quantities using a cobalt-60 gamma ray beam, a Clinac iX with 6 and 10 MV photon beams and an Oncor impression with 6 and 15 MV photon beams. For the ion recombination correction, the method presented in the present study was developed and compared with the existing theoretical and two-dose-rate methods. Finally, the EGSnrc Monte Carlo code was used to estimate the beam quality correction factor of the microLion chamber. Our results show that the readings of the chamber were proportional to the dose. The dose rate dependency showed a maximum difference of 5.0% from 100 MU/min to 600 MU/min. The spatial resolutions determined by comparing profiles for the field sizes of 0.5x0.5 cm2 to 10x10 cm2 agreed among the detectors except for the semiflex chamber to within 2 %. Outputs of the detectors showed good agreement (< 2 %) for all the detectors considered in this study. The N_(D,W,Q_0 ) factor of the microLion chamber acquired in the present study was 0.094342 ± 0.00158 Gy/nc. The beam quality correction factors (k_(Q,Q_0 )) calculated for the 6 MV and 10 MV photon beams for the Clinac iX were 1.0021±0.0116 and 1.0015±0.0114, respectively. For the 6 MV and 15 MV photon beams from the Oncor impression, the k_(Q,Q_0 ) factors were 1.0027±0.012 and 1.0022±0.0123, respectively. For ion recombination correction, our method showed a good result compared to the theoretical and the two-dose-rate methods. Finally, our Monte Carlo calculation results (1.0183±0.58%) agreed, within 0.56%, with experimental results (1.024±0.58%). The results from these two methods were in good agreements, i.e., within acceptable uncertainties. With these results, we believe that the microLion ionization chamber can be used to measure the absorbed dose to water not only in reference conditions, but also in small field or non-standard field condition. We also belive that the methodologies and physical quantities presented in the present study will be a good guide to use the liquid ionization chamber as reference dosimeter.