Development of a guarded liquid ionization chamber for clinical dosimetry.

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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IOP PUBLISHING
PHYSICS IN MEDICINE AND BIOLOGY
Phys. Med. Biol. 52 (2007) 3089–3104
doi:10.1088/0031-9155/52/11/011
Development of a guarded liquid ionization chamber
for clinical dosimetry
K J Stewart1, A Elliott2and J P Seuntjens
Medical Physics Unit, Montreal General Hospital, Montreal, H3G IA4, Canada
E-mail: kristin.stewart@scf.sk.ca and jseuntjens@medphys.mcgill.ca
Received 11 December 2006, in final form 20 March 2007
Published 10 May 2007
Online at stacks.iop.org/PMB/52/3089
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
correctforgeneralionrecombinationinpulsedphotonbeams: thatofJohansson
etal,whichmodifiesBoag’stheoryforrecombinationingases,andanempirical
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
10h. Theliquid-filledGLIC-03hada1.1±0.4%energydependencewhilethat
oftheair-filledGLIC-03was2.1±0.3%betweenthe6and18MVbeamsfrom
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 WaterTMin 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.
1. Introduction
Air-filled ionization chambers are the most commonly used instruments for clinical radiation
dose measurements.Their ease of use, exceptional long-term stability and well-studied
characteristics make this the detector of choice for many dosimetry applications. However,
there are some characteristics of air-filled chambers that limit their accuracy for certain
measurements. First of all, because air is a low-density medium, the amount of ionization
producedbyradiationinanair-filledchamberislow. Thisbecomesaproblemforconstructing
chambers with very small volumes and therefore limits the size (and thereby the spatial
1Present address: Allan Blair Cancer Centre, Regina, SK, S4T 7T1, Canada.
2Present address: OnCURE Medical Corporation, Jacksonville, FL 32216, USA.
0031-9155/07/113089+16$30.00© 2007 IOP Publishing LtdPrinted in the UK3089

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3090 K J Stewart et al
resolution) of air-filled chambers. When measuring in a medium of higher density, such as
water, the introduction of a low-density cavity also perturbs the electron fluence within the
medium; thus perturbation corrections are required to determine the dose in the absence of the
cavity. Finally,becausetheratioofthemeanrestrictedcollisionmassstoppingpowerwater-to-
airvariesbyseveralpercentwithelectronenergyovertherangeofenergiesusedclinically, the
conversionofionizationresponsetoabsorbeddosetowaterisenergydependent. Theseeffects
become particularly important in regions requiring high resolution, in areas where charged
particle equilibrium is not present, and in regions where the average electron energy varies or
is unknown. Some examples are profile measurements of very small fields, measurements in
the build-up region for high-energy photon beams and measurements of IMRT fields.
Insulatingliquidshavebeenproposedforuseinionizationchambersbecausetheypossess
certain advantages compared with air-filled chambers. The high density of liquids compared
to air results in a higher ionization density, allowing very small-volume chambers to be
constructed which still have a sufficiently large signal, thus providing high spatial resolution.
Second, when measuring in a medium such as water, liquids produce negligible perturbation
effects as their density is very similar to water. Finally, for certain insulating liquids, the mean
restricted collision mass stopping power ratio water-to-liquid shows almost no variation with
electron energy over the range of energies used clinically. Therefore a liquid-filled detector
should have very little energy dependence.
The most extensive work on the development of liquid ionization chambers for clinical
dosimetry has been done by Wickman et al (e.g., Wickman (1974), Wickman and Nystrom
(1992)) This group has constructed and tested many chamber designs and has also studied
issues related to ion recombination in liquids (e.g., Johansson et al (1997)). There have
also been studies performed with commercial air ionization chambers filled with insulating
liquids (Francescon et al 1998). We carried out some previous work with one of the Wickman
chambers (Stewart et al 2001). We also tested an Exradin A14P chamber that had been
modified to reduce the electrode separation to 0.5 mm and filled with isooctane (Stewart and
Seuntjens 2003).
Inthispaperwediscussthedevelopmentandconstructionofanewtypeofliquidionization
chamber. We examine the properties of this chamber and investigate methods of correcting for
ion recombination effects. Finally we perform measurements with this new liquid ionization
chamber and compare it to other detectors.
2. Materials and methods
Our previous work with liquid ionization chambers inspired us to design an original liquid
ionization chamber. Several prototypes were constructed and tested with the most suitable
for measurements being the GLIC-03. GLIC stands for guarded liquid ionization chamber,
as the unique feature of this chamber is the inclusion of a guard electrode. Generally, in
liquid ionization chambers, a guard electrode has not been included since it is not required
to prevent in-scatter perturbation effects as it would be in air-filled chambers. The purpose
behind including it in this design, however, was to allow testing of the chamber both with and
without liquid in the sensitive volume. The GLIC-03 (figure 1) was constructed with graphite
electrodesandTefloninsulators. Thecollectingelectrodehasa1.5mmdiameterandtheguard
ring is 1.5 mm thick. The Teflon insulator between these electrodes has a thickness of 0.4 mm.
The outer cap and chamber body are made from Delrin in order to make the chamber liquid
tight. The end face of the cap consists of 1 mm Delrin and 0.5 mm graphite. A Buna-N o-ring
is used to seal the chamber volume. To prevent wear on the graphite outer electrode when the
cap is screwed on and off, a 1 mm thick stainless steel ring was glued below the graphite and

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Development of a guarded liquid ionization chamber3091
Figure 1. Schematic diagram showing the cylindrical cross section of the GLIC-03.
a brass foil provides improved electrical connection with the outer cap. The outer diameter
of the chamber is 21 mm. The length of the chamber body is 34 mm and holes were drilled
to insert 0.8 mm diameter stainless steel filling tubes. Tygon tubing was attached to the outer
ends of the stainless steel tubes and the ends are clamped closed after filling the chamber with
liquid.
The chamber was filled with 2,2,4-trimethylpentane, also called isooctane (Aldrich,
anhydrous, 99.8%), which was used without additional purification. Non-ultra purified liquids
have a limited ion mobility and therefore require the use of high electric field strengths in
order to reduce ion recombination. For isooctane with a purity of 99.5% without additional
purification, the measured ion mobility was 2.9 × 10−8m2s−1V−1for both positive and
negative ions (Johansson and Wickman 1997). We have used this value of ion mobility for our
calculations. Many other studies have been done with isooctane to determine properties such
as temperature dependence (Franco et al 2006) and ion recombination characteristics (e.g.,
Johansson et al (1997)) and the variation in stopping power ratio isooctane-to-air is very small
over the range of energies used clinically.
2.1. Measuring the air-filled chamber characteristics
Before filling the chamber with liquid, we examined its air-filled properties. For comparison,
measurements were also taken with an Exradin A14P chamber, which is similar to the GLIC-
03 in terms of design and sensitive volume. Measurements were done for 6 and 18 MV beams

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3092K J Stewart et al
Figure 2. Diagram showing the three Solid WaterTMphantoms used in this work. Phantom 1 was
used for reference dosimetry with the Exradin A12 and Exradin A14P. Phantom 2 was used for
reference dosimetry with the GLIC-03 and for PDD measurements with the GLIC-03. Phantom 3
was used for stability and ion recombination measurements with the GLIC-03.
from a Varian Clinac 21EX with a polarizing voltage of 300 V applied to the chambers. To
determine the absorbed dose to water calibration coefficient at each energy, NDw, a cross-
calibration procedure was followed. First of all, measurements were taken with an Exradin
A12 chamber (SN 310) which has a60Co absorbed dose to water calibration coefficient
established at NRC (National Research Council, Ottawa, Canada), the national standards lab.
This chamber was positioned with the centre of the chamber at 10 cm water-equivalent depth
in a 20 × 20 × 20 cm3Solid WaterTM(Gamex RMI) phantom (phantom 1 in figure 2).
The absorbed dose to water was determined using the procedure of Seuntjens et al (2005)
which describes a method to derive dose to water for a measurement at an equivalent point
in Solid WaterTM. The correction factor kphhas a value of 1.000 for 6 MV and 1.006 for
18 MV (Seuntjens et al 2005). Immediately following the measurements with the Exradin
A12, measurements were taken with one of the small volume chambers. For the Exradin
A14P, it was possible to place the chamber in the same Solid WaterTMphantom as was used
for the Exradin A12, using a Solid WaterTMsleeve to adjust the position to represent the same
point of measurement for each detector. Because of its larger diameter, another Solid WaterTM
phantom was needed for the GLIC-03 (phantom 2 in figure 2). This phantom consisted of
a 30 × 30 × 20 cm3Solid WaterTMblock with a hole of the same diameter as the GLIC-03
drilled in the centre. 30 × 30 cm2slabs of various thicknesses were added in front of the
chamber to adjust the point of measurement to 10 cm depth. For all measurements, the SSD
was 100 cm and the field size was 10 × 10 cm2at the surface of the phantom. Polarity and
recombination effects for the air-filled chambers were determined using the method described

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Development of a guarded liquid ionization chamber3093
inTG-51(Almondetal1999). Whenmeasuringwiththesmall-volumechambers, theleakage
current was measured and corrected for.
2.2. Determining the electrode separation
The sensitive volume of the GLIC-03 is a cylinder with a cross-sectional area defined by the
size of the collecting electrode and a height defined by the separation between the collecting
and outer electrodes. Since the cap of the chamber is screwed on until it is tight, this electrode
separation is not rigorously defined by the chamber construction. In order to determine the
electrode separation we performed a capacitance test. In this test the un-filled chamber was
connected to a Keithley 6517A electrometer and the polarizing voltage was incremented in
steps of 50 V from 0 to 400 V. The charge was measured after each increment. The slope of
the linear fit to the charge as a function of voltage represents the capacitance of the chamber.
Assuming a perfect parallel plate capacitor, the separation between the plates can be expressed
as a function of the capacitance, C, and the plate area, A, by
d =ε0A
C
,
(1)
where ε0is the permittivity of free space. Because the GLIC-03 has a large guard ring, we
have assumed that the sensitive volume behaves as an ideal parallel plate capacitor. The area
of the sensitive volume is not easily measured in our case, as the collecting electrode has a
small diameter and may not be exactly circular. As well, a small error in the determination of
the radius will be magnified by the fact that the radius is squared to determine the area. For
this reason we used the calibration coefficients obtained through cross-calibration to estimate
the sensitive volume. Since Vol= dA, where Volis the sensitive volume of the chamber:
?ε0Vol
C
The cavity dose calibration coefficient is related to the sensitive volume by
1
ρairVol,
where W/e is the energy required to produce an electron pair in dry air and ρairis the density
of air. ND,airis derived from the measured absorbed dose to water calibration coefficient after
correcting for the stopping power ratio water-to-air and neglecting perturbation effects.
d =
.
(2)
ND,air=W
e
(3)
2.3. Stability and reproducibility of the liquid-filled chambers
The GLIC-03 stability was tested in the 18 MV beam of a Clinac 21EX. The polarizing
voltage was set to 500 V and the lowest available dose rate setting (100 MU min−1) was used.
The chamber was inserted into Solid WaterTMphantom 3 (see figure 2), which was oriented
such that the sensitive volume of the chamber was directed vertically downwards and the
phantom was positioned at 100 cm SSD. This phantom also has a hole in which the Exradin
A12 chamber could be inserted to monitor changes in beam output. Both chambers were at
approximately the same depth of 15 cm in Solid WaterTM.
To determine the necessary pre-irradiation dose to achieve a stable reading, a series of
measurements were taken to monitor the response of the chamber. Immediately following a
new fill of the chamber with liquid, the chamber was given a series of six to ten irradiations
of 250 MU until a set of measurements showed acceptable stability. Between each series of
irradiations 2500 MU was delivered. For all other measurements, this pre-irradiation dose
was delivered initially so that a stable response was achieved. The stability of the chamber