Article

Development of a guarded liquid ionization chamber for clinical dosimetry

Medical Physics Unit, Montreal General Hospital, Montreal, Canada.
Physics in Medicine and Biology (Impact Factor: 2.92). 07/2007; 52(11):3089-104. DOI: 10.1088/0031-9155/52/11/011
Source: PubMed

ABSTRACT Liquid ionization chambers are considered superior to air-filled chambers in terms of size, energy dependence and perturbation effects. We constructed and tested a liquid ionization chamber for clinical dosimetry, the GLIC-03, with a sensitive volume of approximately 2 mm3. We also examined two methods to correct for general ion recombination in pulsed photon beams: that of Johansson et al, which modifies Boag's theory for recombination in gases, and an empirical method relating recombination to dose per pulse. The second method can be used even in cases where the first method is not applicable. The response of the GLIC-03 showed a stable, linear and reproducible decrease of 1% over 10 h. The liquid-filled GLIC-03 had a 1.1 +/- 0.4% energy dependence while that of the air-filled GLIC-03 was 2.1 +/- 0.3% between the 6 and 18 MV beams from a Clinac 21EX. The two methods for recombination correction agreed within 0.2% for measurements at 18 MV, 700 V, 100 MU min(-1). Measurements with the GLIC-03 in Solid Water in the build-up region of an 18 MV beam agreed with extrapolation chamber measurements within 1.4%, indicating that the GLIC-03 causes minimal perturbation.

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Available from: J. P. Seuntjens, Feb 18, 2014
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    • "Liquid-filled ionisation chambers (LICs) are used in radiotherapy for dosimetry and quality assurance: several prototypes and commercial devices have been presented in the last few years (Wickman; Wickman and Nyström, 1992; Wickman et al., 1998; Martens et al., 2001; Eberle et al., 2003; Pardo et al., 2005; Stewart et al., 2007; González-Castaño et al., 2011; Brualla-Gonzaíez et al., 2012; Poppe et al., 2013). LICs present a water-equivalent response, and the high density of the ionisation medium (when compared to air) allows the building of very small volume detectors, which makes these devices especially suitable for the verification of small and/or high dose gradient fields as those present in intensity-modulated radiotherapy or radiosurgery techniques . "
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    ABSTRACT: Analytical extension of Greening and Boag theories to multiple charge carriers. Detailed numerical study of process of volume recombination in LICs. Recombination in pulsed beams is independent of number and mobilities of carriers. Multiple charge carriers have a significant effect in continuous beams. a b s t r a c t Liquid-filled ionisation chambers (LICs) are used in radiotherapy for dosimetry and quality assurance. Volume recombination can be quite important in LICs for moderate dose rates, causing non-linearities in the dose rate response of these detectors, and needs to be corrected for. This effect is usually described with Greening and Boag models for continuous and pulsed radiation respectively. Such models assume that the charge is carried by two different species, positive and negative ions, each of those species with a given mobility. However, LICs operating in non-ultrapure mode can contain different types of electronegative impurities with different mobilities, thus increasing the number of different charge carriers. If this is the case, Greening and Boag models can be no longer valid and need to be reformulated. In this work we present a theoretical and numerical study of volume recombination in parallel-plate LICs with multiple charge carrier species, extending Boag and Greening models. Results from a recent publication that reported three different mobilities in an isooctane-filled LIC have been used to study the effect of extra carrier species on recombination. We have found that in pulsed beams the inclusion of extra mobilities does not affect volume recombination much, a behaviour that was expected because Boag formula for charge collection efficiency does not depend on the mobilities of the charge carriers if the Debye relationship between mobilities and recombination constant holds. This is not the case in continuous radiation, where the presence of extra charge carrier species significantly affects the amount of volume recombination.
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    • "Liquid-filled ionization chambers (Wickman and Nyström, 1992) are currently used in clinic routine to determine dose distributions (relative dosimetry) delivered by medical 20 accelerators. Several prototypes (Pardo et al , 2005; Stewart et al , 2007; González- Castaño et al , 2011; Brualla-González et al , 2012) and commercial devices (Martens et al , 2001; Poppe et al , 2013) have been introduced in the last few years. Nonpolar liquid-filled detectors are suitable for the penumbra and small field measurements (Brualla-González et al , 2012; Poppe et al , 2013) due to their small sensitive volume 25 (∼mm 3 ). "
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    ABSTRACT: The use of liquid ionization chambers in radiotherapy has grown during the past few years. While for air ionization chambers the kTP correction for air mass density due to pressure and temperature variations is well known, less work has been done on the case of liquid ionization chambers, where there is still the need to take into account the influence of temperature in the free ion yield. We have measured the PTW microLion isooctane-filled ionization chamber temperature dependence in a ∼ ±10 °C interval around the standard 20 °C room temperature for three operation voltages, including the manufacturer recommended voltage, and two beam qualities, (60)Co and 50 kV x-rays. Within the measured temperature range, the microLion signal exhibits a positive linear dependence, which is around 0.24% K(-1) at 800 V with (60)Co irradiation. This effect is of the same order of magnitude as the T dependence found in air ionization chambers, but its nature is completely different and its sign opposite to that of an air chamber. Onsager theory has been used to model the results and is consistent with this linear behaviour. However, some inconsistencies in the modelling of the 50 kV x-ray results have been found that are attributed to the failure of Onsager's isolated pair assumption for such radiation quality.
    Physics in Medicine and Biology 05/2014; 59(11):2705-2712. DOI:10.1088/0031-9155/59/11/2705 · 2.92 Impact Factor
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    • "These chambers employed either isooctane or tetramethylsilane and had guarded or unguarded designs. The performance of these chambers was comparable to that of diode detectors when carrying out relative dosimetry measurements [9] [10]. "
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    ABSTRACT: Radiosurgery and intensity modulated radiation therapy (IMRT) treatments are based on the delivery of narrow and/or irregularly shaped megavoltage photon beams. This kind of beams present both lack of charged particle equilibrium and steep dose gradients. Quality assurance (QA) measurements involved in these techniques must therefore be carried out with a dosimeter featuring high small volume. In order to obtain a good signal to noise ratio, a relatively dense material is needed as active medium. Non-polar organic liquids were proposed as active mediums with both good tissue equivalence and showing high signal to noise ratio. In this work, a liquid-filled ionization chamber is presented. Some results acquired with this detector in relative dosimetry are studied and compared with results obtained with unshielded diode. Medium-term stability measurements were also carried out and its results are shown. The liquid-filled ionization chamber presented here shows its ability to perform profile measurements and penumbrae determination with excellent accuracy. The chamber features a proper signal stability over the period studied.
    Physica Medica 04/2011; 27(2):89-96. DOI:10.1016/j.ejmp.2010.04.002 · 1.85 Impact Factor
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