A Study of Characteristics of MicroLion Liquid Ionization Chamber for 6 MV Photon Beam

6 MV 광자빔에 대한 MicroLion 액체이온함의 특성 연구

  • Choi, Sang-Hyoun (Department of Radiation Oncology, College of Medicine, Inha University) ;
  • Huh, Hyun-Do (Department of Radiation Oncology, College of Medicine, Inha University) ;
  • Kim, Seong-Hoon (Department of Radiation Oncology, College of Medicine, Hanyang University) ;
  • Ji, Young-Hoon (Research Center for Radiotherapy, Korea Institute of Radiological & Medical Sciences) ;
  • Kim, Kum-Bae (Research Center for Radiotherapy, Korea Institute of Radiological & Medical Sciences) ;
  • Kim, Woo-Chul (Department of Radiation Oncology, College of Medicine, Inha University) ;
  • Kim, Hun-Jeong (Department of Radiation Oncology, College of Medicine, Inha University) ;
  • Shin, Dong-Oh (Department of Radiation Oncology, College of Medicine, Kyung Hee University) ;
  • Kim, Chan-Hyeong (Department of Nuclear technology, Hanyang University)
  • 최상현 (인하대학교 의과대학 방사선종양학교실) ;
  • 허현도 (인하대학교 의과대학 방사선종양학교실) ;
  • 김성훈 (한양대학교 의과대학 방사선종양학교실) ;
  • 지영훈 (한국원자력의학원 방사선치료연구센터) ;
  • 김금배 (한국원자력의학원 방사선치료연구센터) ;
  • 김우철 (인하대학교 의과대학 방사선종양학교실) ;
  • 김헌정 (인하대학교 의과대학 방사선종양학교실) ;
  • 신동오 (경희대학교 의과대학 방사선종양학교실) ;
  • 김찬형 (한양대학교 대학원 원자력공학과)
  • Received : 2011.11.28
  • Accepted : 2011.12.12
  • Published : 2011.12.30

Abstract

Recently PTW developed a MicroLion liquid ionization chamber which is water_equivalent and has a small sensitive volume of $0.002cm^3$. The aim of this work is to investigate such dosimetric characteristics as dose linearity, dose rate dependency, spatial resolution, and output factors of the chamber for the external radiotherapy photon beam. The results were compared to those of Semiflex chamber, Pinpoint chamber and Diode chamber with the sensitive volumes of $0.125cm^3$, $0.03cm^3$ and $0.0025cm^3$, respectively and evaluated to be suitable for small fields. This study was performed in the 6MV photon energy from a Varian 2300 C/D linac accelerator and the MP3 water phantom (PTW, Freiburg) was used. Penumbras in the varios field sizes ranged from $0.5{\times}0.5cm^2$ to $10{\times}10cm^2$ were used to evaluate the spatial resolution. Output factors were measured in the field sizes of $0.5{\times}0.5$ to $40{\times}40cm^2$. Readings of the chamber was linearly proportional to dose. Dose rate dependency was measured from 100 MU/min to 600 MU/min, showed a maximum difference of 5.0%, and outputs decreased with dose rates. The spatial resolutions determined with comparing profiles for the field sizes of $0.5{\times}0.5cm^2$ to $10{\times}10cm^2$ agreed between every detector except the Semiflex chamber to within 2%. Outputs of detectors were compared to that of Semiflex chamber and showed good agreements within 2% for every chamber. This study shows that MicroLion chamber characterized by a high signal-to-noise ratio and water equivalence could be suitable for the small field dosimetry.

최근 PTW사에서는 물등가물질로 구성되어 있고, 측정체적(sensitive volume, $0.002cm^3$)이 매우 작은 MicroLion 액체이온함을 내놓았다. 본 연구의 목적은 외부방사선 치료용 광자빔에 대해 MicroLion 액체이온함의 선량선형성, 선량률의존성, 공간분해능, 그리고 출력인수와 같은 선량측정학적 특성을 조사하는 것이다. 이 결과를 Semiflex 이온함($0.125cm^3$), Pinpoint 이온함($0.015cm^3$), 다이오드 검출기($0.0025mm^3$)의 결과와 비교분석하여 소조사면 측정에 적절한지를 평가하고 자 하였다. Varian clinac 2300 C/D의 6 MV 광자빔에서 측정하였으며, MP3 물팬톰(PTW, Freiburg)을 이용하였다. 공간분해능은 반음영(penumbra)을 측정하여 평가하였으며, $0.5{\times}0.5cm^2$에서 $10{\times}10cm^2$까지 다양한 조사면에 대하여 측정하였다. 출력인수는 $0.5{\times}0.5cm^2$에서 $40{\times}40cm^2$에 대하여 측정하였다. 선량에 따른 MicroLion 액체이온함의 측정값은 선형적인 비례성을 보였다. 그러나 선량률은 100 MU/min와 600 MU/min에 의한 측정값의 차이가 최대 5%의 차이를 보였으며, 선량율이 커질수록 출력선량이 작아지는 결과를 보였다. 공간분해능의 경우 조사면 $0.5{\times}0.5cm^2$에서 $10{\times}10cm^2$까지의 측방선량분포 비교에서 Semiflex 이온함을 제외한 다른 모든 검출기들의 경우 2% 이내에서 일치하였다. 출력선량은 $2{\times}2cm^2$에서 $10{\times}10cm^2$ 조사면에서 Semiflex 검출기 대비 모든 검출기가 2% 이내에서 잘 일치하였다. 연구 결과 MicroLion 액체이온함은 물등가물질로 이루어져 있으며, 감응 면적이 매우 작기 때문에 소조사면에서 매우 유용할 것으로 사료된다.

Keywords

References

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