Journal Article

General ion recombination effect in a liquid ionization chamber in high-dose-rate pulsed photon and electron beams

Hideyuki Takei, Tomonori Isobe, Nozomi Kitamura, Yutaro Mori, Tetsuya Tomita, Daisuke Kobayashi, Satoshi Kamizawa, Tomoharu Sato, Hideyuki Sakurai and Takeji Sakae

in Journal of Radiation Research

Volume 59, issue 3, pages 282-285
Published in print May 2018 | ISSN: 0449-3060
Published online January 2018 | e-ISSN: 1349-9157 | DOI: https://dx.doi.org/10.1093/jrr/rrx088
General ion recombination effect in a liquid ionization chamber in high-dose-rate pulsed photon and electron beams

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  • Clinical Genetics
  • Molecular Biology and Genetics
  • Epidemiology
  • Radiology
  • Nuclear Chemistry, Photochemistry, and Radiation

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Abstract

Liquid ionization chambers (LICs) are highly sensitive to dose irradiation and have small perturbations because of their liquid-filled sensitive volume. They require a sensitive volume much smaller than conventional air-filled chambers. However, it has been reported that the collection efficiency has dependencies on the dose per pulse and the pulse repetition frequency of a pulsed beam. The purpose of this study was to evaluate in detail the dependency of the ion collection efficiency on the pulse repetition frequency. A microLion (PTW, Freiburg, Germany) LIC was exposed to photon and electron beams from a TrueBeam (Varian Medical Systems, Palo Alto, USA) linear accelerator. The pulse repetition frequency was varied, but the dose per pulse was fixed. A theoretical evaluation of the collection efficiency was performed based on Boag’s theory. Linear correlations were observed between the frequency and the relative collection for all energies of the photon and electron beams. The decrease in the collected charge was within 1% for all the flattened photon and electron beams, and they were 1.1 and 1.8% for the 6 and 10 MV flattening filter-free photon beams, respectively. The theoretical ion collection efficiency was 0.990 for a 10 MV flattened photon beam with a dose rate of 3 Gy·min−1. It is suggested that the collected charge decreased because of the short time intervals of the beam pulse compared with the ion collection time. Thus, it is important to correctly choose the pulse repetition frequency, particularly when flattening filter-free mode is used for absolute dose measurements.

Keywords: liquid ionization chamber; dosimetry; photon therapy; electron therapy; ion recombination

Journal Article.  2237 words.  Illustrated.

Subjects: Clinical Genetics ; Molecular Biology and Genetics ; Epidemiology ; Radiology ; Nuclear Chemistry, Photochemistry, and Radiation

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