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

ABSTRACT

Recently PTW developed a MicroLion liquid ionization chamber which is water_equivalent and has a small sensitive volume of 0.002 cm3. 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 0.125 cm3, 0.03 cm3 and 0.0025 cm3, 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 0.5x0.5 cm2 to 10x10 cm2 were used to evaluate the spatial resolution. Output factors were measured in the field sizes of 0.5x0.5 to 40x40 cm2. 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 0.5x0.5 cm2 to 10x10 cm2 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.