Properties of liquid ionization chambers at LDR brachytherapy dose rates

Johansson, Bengt; Wickman, G.; Holmström, Thord

doi:10.1088/0031-9155/40/4/006

Cited by 11 publications

(18 citation statements)

References 18 publications

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“…Thus, at low voltages the current increases fast with respect to the electric field. This flattening in the current rise has been observed at similar field strengths also by Johansson et al (1995).…”

Section: Signal Responsesupporting

confidence: 79%

First tests of a liquid ionization chamber to monitor intensity modulated radiation beams

Eberle

Engler²,

Hartmann

et al. 2003

Phys. Med. Biol.

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First measurements with a prototype ionization chamber are described to be applied in online monitoring of modulated fields in radiation therapy. The liquids isooctane, isononane (TMP) and tetramethylsilane (TMS) are used in a high purity grade in order to realize high current signals for electronic read-out in parallel at frequencies exceeding 10 Hz. Signals of more than a factor 4 with respect to isooctane, analysis grade, are obtained. With an electrode structure of 400 pads, a uniformity in efficiency within 1.2% has been measured. The penumbra of a multileaf collimator could be resolved. Theoretical examination verifies that the free electrons in the liquids cause higher signals when the measured currents are compared with expectation for ion transport only.

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Section: Signal Responsesupporting

confidence: 79%

First tests of a liquid ionization chamber to monitor intensity modulated radiation beams

Eberle

Engler²,

Hartmann

et al. 2003

Phys. Med. Biol.

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show abstract

“…The ionization currents given in this report are always corrected for the actual leakage current. In a previous paper 9 it was found that a polarization voltage of 300 V was close to op- timum for low dose-rate measurements; this voltage has therefore been used throughout the experiments.…”

Section: B Polarizing Voltage and Current Measurementsmentioning

confidence: 99%

“…The current results are as good as those observed when guarded planeparallel LICs with a 1 mm liquid layer thickness of TMS or isooctane were tested. 9 This indicates that neither the use of a TMS-isooctane mixture nor the omission of the guard electrode significantly deteriorates the long-term calibration stability of an LIC.…”

Section: A Long-term Stabilitymentioning

confidence: 99%

“…It has recently been shown 9 that ionization chambers using isooctane or tetramethylsilane ͑TMS͒ as sensitive medium show very good properties with respect to spatial resolution and reproducibility at measurements in low dose-rate ͑LDR͒ brachytherapy radiation fields. Liquid ionization chambers ͑LIC͒ having a liquid layer thickness of 1 mm and operating at a polarizing voltage of 300 V provide a volume sensitivity of approximately 300 times that of an air ioniza-tion chamber.…”

Section: Introductionmentioning

confidence: 99%

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Liquid ionization chambers for absorbed dose measurements in water at low dose rates and intermediate photon energies

et al. 1998

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Two new liquid ionization chamber (LIC) designs, consisting of cylindrical and plane-parallel configurations, are presented. They are designed to be suitable for high-precision measurements of absorbed dose-to-water at dose rates and photon energies typical for LDR intermediate photon energy brachytherapy sources. The chambers have a sensitive liquid layer thickness of 1 mm and sensitive volumes of 7 mm3 (plane-parallel) and 20 mm3 (cylindrical). The liquids used as sensitive media in the chambers are either isooctane (C8H18), tetramethylsilane (Si(CH3)4) or mixtures of these two liquids in the approximate proportions 2 to 1. A chamber filled with such a liquid mixture and with a polarizing voltage of 300 V, provides a volume sensitivity of about 10(-9)C Gy(-1) mm(-3) for absorbed dose measurements in water in an x-ray radiation field with an effective photon energy of 120 keV. In the interval 30 to 140 keV, the relative change in sensitivity is less than +/- 2.5%. The leakage current of the chambers is low and stable, which implies that absorbed dose measurements can be done with good reproducibility at dose-rates as low as 50 microGy min-1 (sigma < 3%). The long-term calibration stability was tested for a set of five chambers over a period of more than 1 year. No systematic change in their sensitivity could be observed. The general recombination at a polarizing voltage of 300 V is less than 2% for dose-rates up to about 100 mGy min-1. The temperature dependence at room temperature is 0.5% per degree C. The response is almost independent of the direction of the radiation for the plane-parallel LIC.

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“…However, in dose measurements with LICs attention must be paid to chamber leakage current and general recombination. 11,13,14…”

Section: Introductionmentioning

confidence: 99%

Assessment of the relative dose distribution around an line source using a liquid ionization chamber

et al. 1999

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The relative absorbed dose distribution in water around an 192Ir line source of 50 mm length and 0.3 mm diam has been measured using a liquid ionization chamber (LIC). The sensitive volume of the chamber is a cylinder with 3.0 mm diam and a thickness of 1.0 mm. The sensitive medium in the LIC consists of a mixture of two dielectric liquids, tetramethylsilane and isooctane. The mixture has been optimized so that the LIC provides an almost energy-independent response for the radiation qualities present at different distances from the source in water. The measurements were carried out at distances of 2.5-50.0 mm along the source bisector and at distances of 0.0 to +/-40.0 mm along the source axis. The results were compared with measurements made with LiF chips in Solid Water and with calculated data based on an analytical solution to the Sievert integral as well as with Monte Carlo calculations of absolute dose rate in water. Considering the uncertainties involved, the dose distribution measured by the LIC is in reasonably good agreement with the theoretical data as well as the results held with LiF. In comparison with the Monte Carlo calculations the discrepancies range from 0.1% to 18% with the largest differences at points close to the source. This work demonstrates the ability of the LIC for mapping the dose distribution around a low-dose-rate brachytherapy source emitting photons of intermediate energies.

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Properties of liquid ionization chambers at LDR brachytherapy dose rates

Cited by 11 publications

References 18 publications

First tests of a liquid ionization chamber to monitor intensity modulated radiation beams

First tests of a liquid ionization chamber to monitor intensity modulated radiation beams

Liquid ionization chambers for absorbed dose measurements in water at low dose rates and intermediate photon energies

Assessment of the relative dose distribution around an line source using a liquid ionization chamber

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