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Phys. Med. Biol. 42 (1997) 23932409. Printed in the UK PII: S0031-9155(97)83024-2
A dosimetric intercomparison of electron beams in UK
radiotherapy centres
A Nisbet and D I Thwaites
Department of Medical Physics and Medical Engineering, University of Edinburgh,
Western General Hospital, Crewe Road, Edinburgh EH4 2XU, UK
Received 4 April 1997, in final form 15 August 1997
Abstract. A dosimetry intercomparison has been carried out for all 52 radiotherapy centres
in the UK which possess electron treatment facilities. The intercomparison was carried out
on one treatment unit in each centre and for three energies across the range of available
energies. The position of the depth of maximum dose for a standard field size was independently
determined and a subsequent beam calibration made. The factor to convert the reading on a
calibrated ionization chamber to absorbed dose in an electron beam is energy dependent, andhence to carry out an independent calibration measurement also requires the beam energy to
be determined. In addition a quantitative measure of the difference in the calibration chains
between the intercomparison equipment and the host departments field instrument was carried
out. In order to provide a follow-up to the initial IPSM national photon intercomparison, a
photon beam calibration was measured in one photon beam in each centre. For 156 electron
beam measurements, a mean ratio of intercomparison measured dose to locally measured dose of
0.994 was obtained with a standard deviation of 1.8%. For the 52 photon beam measurements,
a mean ratio of intercomparison measured dose to locally measured dose of 1.003 was obtained
with a standard deviation of 1.0%.
1. Introduction
The demands on precision in radiotherapy dosimetry and treatment delivery are determinedby the steepness of the relevant clinical doseeffect curves, both for tumour control and
for normal tissue complications. Consideration of the clinical data leads to generally
agreed recommendations on the required accuracy in clinical dosimetry for radical curative
radiotherapy being given in ICRU Report 24 (ICRU 1976), which pointed to a need for at
least5% accuracy in the delivery of absorbed dose to the target volume in the patient. More
recently, Brahme (1984) proposed a tolerance value on accuracy in dose delivery of 3% at
the one standard deviation level, based on considering the consequences of dose variability
on loss of tumour control probability. Similarly Mijnheer et al (1987) proposed 3.5%
(one standard deviation) by considering limiting uncertainties for unacceptable increases in
normal tissue complication risks. It is important to recognize that these figures are to be
applied to the overall delivered dose, i.e. the dose received by the patient at the end point
of all steps in the radiotherapy process. This includes patient data acquisition, treatment
planning and the delivery of the prescribed, planned and accepted treatment to the patientunder day to day conditions over the course of the treatment. The accuracy requirements
Present address: Department of Medical Physics, Sandringham Building, Leicester Royal Infirmary NHS Trust,
Leicester LE1 5WW, UK.
0031-9155/97/122393+17$19.50 c 1997 IOP Publishing Ltd 2393
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2394 A Nisbet and D I Thwaites
on each contributing stage must be correspondingly better in order to achieve these overall
requirements.
Such considerations make it necessary to establish comprehensive quality assurance
systems at the local level in each centre (Bleehan 1991, AAPM 1994, Thwaites et al
1995). In particular, the basis of consistent dosimetry is the use of carefully designed and
applied national dosimetry protocols and codes of practice (e.g. HPA 1985, IPSM 1990,
1992, IPEMB 1996a, b), ensuring traceability of dosimetry to national and internationalstandards. However, even with protocols in place errors may occur, for example due to
inexact implementation or misinterpretation of recommendations, equipment problems or
mistakes. Dosimetry intercomparisons are designed to establish the accuracy and precision
of dosimetry at a given level in the dosimetry chain and to assess consistency between
centres. They also act as an audit that can reveal the presence of errors. Coupled with
procedural audit of the local centres quality assurance system and records, they provide a
powerful overall audit of local clinical dosimetry. Dosimetry intercomparison techniques
form the basis of the practical methodology employed in the developing radiotherapy
dosimetry audit networks (Svensson et al 1994, Hanson et al 1994, Derreumaux et al
1995, Thwaites 1996). Whilst audit systems can employ mailed TLD or site visits using
ionization chambers, the latter approach provides the most accurate and flexible means to
carry out a dosimetry intercomparison.
Dosimetry intercomparison methods and results have been reviewed recently byThwaites and Williams (1994) and Thwaites (1994). Briefly they can be divided into
three categories. As the first level in the dosimetry chain, primary standard laboratories
regularly carry out intercomparisons of their provision of standard dosimetric quantities
and agreement has consistently been within a few tenths of a per cent (Niatel et al 1975,
Boutillon et al 1994). Secondly there have been a number of intercomparisons carried out to
assess the consistency of treatment beam calibration between radiotherapy centres, making
measurements under fixed conditions which either duplicate or approximate reference
calibration conditions (Johansson et al 1982, 1986, Wittkamper et al 1987, Thwaites et al
1992, Hoornaert et al 1993, Wittkamper and Mijnheer 1993, Hanson et al 1994). The
third group essentially assess uncertainties at other levels of the clinical dosimetry chain,
and range from additional measurements in non-reference conditions to more complex
intercomparisons in anatomical phantoms (Worsnop 1968, Johansson 1987, Johansson et al
1986, Wittkamper et al 1987, Thwaites et al 1992). Tables 1 and 2 summarize some resultsfor photon and electron intercomparisons at the beam calibration level. A number of general
statements can be made as a result of reviewing intercomparisons (Thwaites et al 1992,
Thwaites and Williams 1994): nearly all have shown some clinically significant variations;
the standard deviations on the observed distributions increase on going from cobalt-60 to
megavoltage x-rays to electrons, and on going from reference point measurements to more
complex situations involving more factors; and standard deviations and the incidence of
major discrepancies decrease in repeated intercomparisons.
The UK-wide dosimetry intercomparison of megavoltage photons (Thwaites et al 1992)
provided quantitative information on the consistency of clinical dosimetry for megavoltage
photons. It improved the quality of clinical dosimetry by highlighting some problems, which
in turn led to improvements in practice. The study was a one-off exercise with the aim
of measuring the achieved dosimetric precision in radiotherapy across the UK, yet at the
same time it provided a baseline set of data which can be used as a standard for subsequentcontinuing regional audits in the UK radiotherapy dosimetry audit network to work from
and refer to. Indeed that intercomparison stimulated the establishment of, and provided
the basic methods, for on-going regional audits (Thwaites 1992, 1996, Bonnett et al 1994,
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Table 1. Ratios of measured to stated doses of recent photon dosimetry intercomparisons in
reference conditions.
Reference Region No Mean SD Range
Johansson et al Scandinavia
(1982) Co-60 22 1.001 0.014 0.05
x-rays 50 1.017 0.023 0.10
Johansson et al Europe(1986) Co-60 59 1.001 0.019 0.10
x-rays 16 1.024 0.033 0.14
Wittkamper et al Netherlands
(1987) Co-60 11 0.994 0.006 0.02
x-rays 40 1.008 0.02 0.10
Hanson et al Mainly USA
(1984) Co-60 and x-rays 740 1.008 0.019 0.14
Thwaites et al UK
(1992) C0-60 61 1.002 0.014 0.08
x-rays 100 1.003 0.015 0.10
Table 2. Ratios of measured to stated doses of recent electron dosimetry intercomparisons in
reference conditions.
Reference Region No Mean SD Range
Johansson et al Scandinavia
(1982) > 10 MeV 59 0.989 0.027 0.15
< 10 MeV 68 0.996 0.034 0.18
Johansson et al Europe
(1986) 425 MeV 148 1.017 0.047 0.24
Wittkamper et al Netherlands
(1993) 620 MeV 54 1.002 0.025 0.11
Thwaites and Allahverdi 1995). The megavoltage photon intercomparison also pointed to
the need for a similar assessment of the consistency of electron beam dosimetry in the
UK, partly because around 90% of UK radiotherapy centres now have electron treatmentfacilities, a significant number having obtained an electron capacity relatively recently,
and partly because the potential for problems in electron dosimetry may be greater than
for megavoltage photons. This has been observed in other intercomparisons which have
included electron beams (Johansson et al 1982, 1986, Wittkamper and Mijnheer 1993),
and some of the reasons for this are: the multiplicity of energies; the variation of energy
with depth; steep dose gradients; the greater influence of variations in chamber design and
phantom materials; and the possibility of larger chamber correction factors, i.e. polarity and
recombination.
This paper reports a systematic electron dosimetry intercomparison in all UK
radiotherapy centres possessing electron facilities. Whilst this work is a scientific study
of the currently achieved consistency in megavoltage electron beam dosimetry in the UK, it
is intended that as with the photon intercomparison the methodology developed for clinical
electron dosimetry audit may be subsequently adopted by regional audits and that the results
will provide a baseline data set. A basic photon intercomparison was also carried out as a
follow-up to the earlier IPSM study (Thwaites et al 1992) in order to assess its impact and
any subsequent changes.
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2. Methods
An epoxy resin solid water substitute phantom material, WTe, produced commercially by
the Radiation Physics Department at St Bartholomews Hospital, London was employed
for all the measurements. WTe is a formulation developed for use in electron beams. The
depth ionization curves measured in the material were shown to be in agreement with those
measured in water within the limits of measurement uncertainty. Fluence ratios of waterto solid water were found to be linearly dependent on the mean electron beam energy at
the depth of measurement, with corrections being no more than 0.5% (Nisbet and Thwaites
1997b). The water equivalency of the material for megavoltage photon beam use has
also been reported (Allahverdi et al 1997). The WTe solid water phantom consisted of
250 mm 250 mm sheets with thickness varying between 1 mm and 50 mm. The sheets
were measured with a micrometer and found to be within 0.1 mm of the nominal thickness.
For all the measurements 100 mm of phantom material was positioned behind the ionization
chamber to provide backscatter. Maximum sheet thicknesses of 20 mm were used to set the
depth of the chamber in the phantom material and no problems with charge storage were
observed with this arrangement.
A Nuclear Enterprises 2570 dosemeter with an NACP type-02 design parallel plate
ionization chamber (marketed by Scanditronix) were employed for the electron beam
measurements. An operating voltage of 250 V was employed and ion recombinationwas evaluated both during calibration in a cobalt-60 gamma-ray beam and also in the
subsequent determination of absorbed dose in an electron beam. This ensured that any
changes in sensitivity at different voltages were automatically taken into account and is
an acceptable alternative approach (IPEMB 1996a) to using an operating voltage of less
than 100 V (Burns 1991, 1994). A 10 mm sheet of WTe was machined to hold the
NACP chamber with the front surface of its entrance window flush with the sheet surface.
The chamber was calibrated against a secondary standard dosemeter (Nuclear Enterprises
model 2560/2561) in a cobalt-60 beam according to the HPA (1985) code of practice. The
calibration of the NACP chamber was repeated quarterly throughout the course of the study.
The standard deviation of the quarterly calibrations was 0.25%. In addition, to further
ensure consistency, the chamber response was checked with a strontium-90 check source
(available commercially from Gothic Crellon) before and after each visit to a radiotherapy
centre. The standard deviation of the strontium-90 consistency checks was 0.2%, with amaximum deviation from the standard response of 0.4%. Ion recombination and polarity
effects for the ionization chamber were determined across the range of measurement
conditions encountered. Ion recombination corrections were then determined for each beam
in a particular centre based on the value of the operating dose per pulse of the linear
accelerator for that modality and quality. The operating dose per pulse was estimated
from the gun pulse repetition frequency and the doserate as indicated on the accelerator
console during beam-on operation of the accelerator. Where such parameters were not
available on the console, the gun pulse repetition frequency and doserate as quoted by the
host department were employed. Time considerations prevented the actual measurement
of recombination in each centre. It should be noted that an error of20% in the pulse
repetition frequency results in an error of no more than 0.1% in the ion recombination
correction factor. This may be employed as a very conservative estimate of the uncertainty
in the use of the recombination correction. The polarity corrections were determined from
the mean electron beam energy at the depth of measurement. The values for the chamber
and electrometer employed in the intercomparison have been reported in a technical note
(Nisbet and Thwaites 1997a).
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For the photon beam measurements the Nuclear Enterprises 2570 dosemeter was used
with an NE2571 graphite-walled cylindrical ionization chamber. For the photon beam
intercomparison the chamber was calibrated against the secondary standard dosemeter in
a cobalt-60 gamma-ray beam and also in x-ray beams of nominal energy 4 MV, 6 MV,
9 MV and 16 MV, with quality indices (TPR200100) of 0.63, 0.68, 0.72 and 0.765 respectively.
Calibration was carried out according to the IPSM (1990) code of practice and is in terms
of absorbed dose to water, traceable to UK (NPL) national dosimetry standards. Similarconsistency checks as described above were also employed for this chamber. The standard
deviation of the quarterly calibrations was 0.1%. The standard deviation of the strontium-
90 consistency checks was 0.2%, with a maximum deviation from the standard response
of 0.4%.
A dedicated barometer and thermometer were used throughout the study to make
pressure and temperature corrections to the ionization chamber readings. An aneroid
barometer (Negretti and Zambra type M2236) was used, whilst the thermometer was
supplied by Nuclear Enterprises. The temperature and pressure were compared with those
measured locally to help quantify any differences in dosimetry.
The HPA (1985) code of practice with the IPSM (1992) addendum were used to
determine absorbed dose to water for the electron beams, and the IPSM (1990) code of
practice was employed to determine absorbed dose to water for the photon beams. The
electron beam intercomparison was carried out on one unit in each radiotherapy centre visitedand for three energies across the range of available energies. For each nominal energy the
depth of maximum dose for a standard field size, the beam calibration and the electron beam
energy were independently determined. The Ce conversion factor necessary to convert the
ionization chamber reading into a measure of absorbed dose to water (as defined and used
in the HPA (1985) electron dosimetry code of practice) is energy dependent. Therefore to
carry out an independent measurement of the beam calibration of an electron beam means
that a beam energy determination is also required. Depth ionization curves were used to
estimate the electron beam energy. They were measured with a 200 mm 200 mm field
size and a constant focus to surface distance (FSD) of around 1 m (dependent on the make
of accelerator). Strictly speaking, a constant focus to chamber distance should be employed.
However, given that the phantom dimensions were only 250 mm250 mm, there may have
been insufficient sidescatter if a large stand-off was used to provide sufficient space beneath
the applicator end to add the required thickness of phantom material and this problem wouldhave increased for higher-energy beams. The depth ionization curve measured was therefore
converted to fixed focus to chamber distance by applying an inverse square law correction,
i.e. by multiplying each reading by (f+d)2/d2, where f is the distance of the front surface
of the phantom from the source, and d is the depth of the effective point of measurement
of the chamber beneath the surface. Strictly speaking f should be the distance from the
chamber to the virtual electron beam source; however, it was not practicable to determine
the virtual electron beam source during the intercomparison and so the nominal focus to
surface distance was used. The difference in the energy determined by using this in the
inverse square law correction is insignificant. The energy relationships given in HPA (1985)
were used to determine the mean incident electron beam energy and the mean electron beam
energy at the depths of measurement.
The depth of maximum dose for the standard field size was determined by taking
readings at a number of depths 1 mm apart, around the quoted depth of maximum dose(three readings were taken at each depth). The chamber reading multiplied by the appropriate
air kerma to dose conversion factor for the mean electron beam energy at that depth gives
the absorbed dose to water, and hence the depth of maximum dose could be determined.
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The dose per monitor unit at the depth of maximum dose for the standard field size was
determined from the readings taken. The temperature in the phantom cavity was used to
apply a temperature correction. The ion recombination correction was calculated from the
dose per pulse, and fluence and polarity corrections were calculated from the mean electron
beam energy at the measurement depth.
It should be noted that the host department was asked to measure the beam calibrations
of the appropriate electron and photon beams immediately prior to the intercomparison.The determination of the depth of maximum dose and hence the beam calibration was the
first set of electron beam measurements to be carried out. This ensured that there was
no difference in dosimetry between the intercomparison and the locally measured beam
calibration due to drift in output caused by prolonged use of the accelerator. Data obtained
from the questionnaire (see below) enabled estimates of the Ce values around the quoted
depth of maximum dose to be calculated, and these values were initially employed in the
above determination of the depth of maximum dose. Subsequent use of the derived Cevalues from the intercomparison measure of beam energy ensured an independent check of
the beam calibration and depth of maximum dose.
Finally the ionization chamber used by the host department for electron beam dosimetry
was cross-calibrated against the intercomparison NE2571 graphite-walled cylindrical
ionization chamber in the photon beam normally used for the calibration of the host
departments electron chamber. This provided two outcomes. Firstly it gave a quantitativemeasure of any differences in the calibration chains between the intercomparison and the
host department. In addition it enabled a measurement of the photon beam calibration to
be carried out. The quality index as quoted by the host department was used to determine
the calibration factor to convert the corrected instrument reading (corrected for temperature,
pressure and ion recombination) to absorbed dose to water. It should be noted that in
the host departments measurements of beam calibration their own previously determined
chamber calibration factor was employed. The result from the cross-calibration with the
intercomparison NE2571 was only used to quantify the deviation in chamber calibration
factor to the total discrepancy in dose between locally measured beam calibration and
intercomparison measured beam calibration. The local centre was only given the result of
this chamber cross-calibration with the subsequent report.
A degree of procedural audit was incorporated at each visit. To achieve this aim a
questionnaire was designed which dealt with the quality control procedures in operationand the techniques used to determine dosimetric and related parameters. This questionnaire
was sent out before the visit and was then used as a basis for discussion with local staff.
The results from this part of the audit are not analysed here, but it should be noted that
recommendations on appropriate quality control procedures and frequency of checks are
given in IPSM Report54 (IPSM 1988). The questionnaire also helped in identifying reasons
for any differences observed in dosimetry between the intercomparison and that measured
locally. A full analysis of the reasons for any differences in dosimetry were given with
the subsequent report. The report quantified differences in dosimetry due to the different
calibration chains employed; the use of different thermometers and barometers; differences
in dosimetry due to differences in energy determined; differences in dosimetry due to the
difference in the depth of maximum dose determined; and differences due to recombination
and polarity corrections. In addition any deviation from the correct use of the HPA (1985)
code of practice and IPSM (1992) addendum were highlighted.The intercomparison visits started in May 1995 and were completed in May 1996. The
intercomparison was carried out at 52 radiotherapy centres in the UK, of which 50 were
NHS Trust hospitals and two were private. The results were communicated back to the
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UK radiotherapy dosimetry intercomparison 2399
individual centres as soon as possible following a visit. Any discrepancies outwith a pre-set
tolerance value of5% (3% for photon beam calibrations) were followed up to confirm
that the causes of the difference in dosimetry as outlined in the reports were addressed. The
confidentiality of the results of each centre was maintained
3. Results
3.1. Electron beam calibration
Figure 1 shows the distribution of nominal electron beam energies included in the study.
The nominal electron beam energies range in value from 3 MeV up to 22 MeV.
Figure 1. Distribution of nominal electron beam energies in the intercomparison.
The results, expressed as the ratio of the intercomparison measured dose to the locallymeasured dose, are presented in figure 2. The mean value of this ratio is 0.994 (standard
deviation 1.8%) with minimum and maximum values of 0.949 and 1.046, i.e. a spread in
dose of 9.7% across the 52 centres. It should be noted that a value close to unity for the
mean ratio suggests that there are no significant systematic errors in the intercomparison
dosimetry. A difference in dosimetry greater than 5% was observed for only one of the
results. This was due to the local centre employing a cylindrical ionization chamber in
the low-energy region for which the use of cylindrical chambers is not recommended.
Subsequent follow-up measurements confirmed agreement to within 1%.
The results have been subdivided into three energy regions: (i) a mean electron
beam energy at the depth of measurement, Ed < 5 MeV, the energy region for which
cylindrical ionization chambers are not recommended; (ii) 5 MeV Ed < 10 MeV; and
(iii) Ed 10 MeV. A summary of the results in the three energy regions is given in table 3.
There are no significant differences in the precision of dosimetry between these three energy
regions.
The results have also been subdivided according to the type of ionization chamber
employed locally for electron beam dosimetry. The results are shown in table 4. It can
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Figure 2. Results for the electron beam calibration measurements showing the distribution of
the ratio of intercomparison measured dose to locally measured dose.
Table 3. Variation of beam calibration with electron beam energy.
Energy
Ed < 5 MeV 5 MeV Ed < 10 MeV 10 MeV Ed
No of Beams 65 60 31
Mean 0.996 0.995 0.995
SD (%) 1.8 1.6 2.0
Max. negative deviation 0.949 0.959 0.951
Max. positive deviation 1.044 1.046 1.026
Spread (%) 9.5 8.7 7.5
Table 4. Variation of beam calibration with ionization chamber used locally.
Ionization chamber
NACP Markus NE2571 Others
No of Beams 93 36 21 6
Mean 0.991 1.006 0.995 0.984
SD (%) 1.6 1.7 2.0 1.6
Max. negative deviation 0.951 0.969 0.949 0.965
Max. positive deviation 1.046 1.044 1.024 1.004
Spread (%) 9.5 7.5 7.5 3.9
be seen that there are differences in the mean ratio of intercomparison measured dose
to locally measured dose for each chamber type. It should be noted that the calibration
procedure for parallel plate ionization chambers described in the IPEMB code of practice for
electron dosimetry for radiotherapy (IPEMB 1996a) is expected to improve the consistency
in dosimetry between different types of ionization chamber.
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Table 5. Variation of beam calibration with ionization chamber used locally and with electron
beam energy.
(i) NACP chamber Energy
Ed < 5 MeV 5 MeV Ed < 10 MeV 10 MeV Ed
No of beams 41 37 15
Mean 0.992 0.993 0.985SD (%) 1.5 1.5 2.0
Max. negative deviation 0.957 0.966 0.951
Max. positive deviation 1.030 1.046 1.026
Spread (%) 6.3 8.0 7.5
(ii) Markus chamber Energy
Ed < 5 MeV 5 MeV Ed < 10 MeV 10 MeV Ed
No of beams 13 12 11
Mean 1.012 1.002 1.003
SD (%) 1.7 1.7 1.5
Max. negative deviation 0.990 0.965 0.979
Max. positive deviation 1.044 1.033 1.025
Spread (%) 5.4 6.8 4.6
The results according to the type of ionization chamber have been further subdivided into
the three energy regions. The results are presented in table 5. It can be seen that the differ-
ences in the use of different ionization chambers are reflected across the entire energy range.
3.2. Electron beam energy
The results, expressed as the difference in energy between that determined during the
intercomparison and that quoted locally, are presented in figure 3. With 56 beams the
agreement was within 0.1 MeV; with 94 the agreement was within 0.2 MeV; 116 were
within 0.3 MeV; and 142 were within 0.5 MeV. The maximum positive difference
in nominal electron beam energy was 1.04 MeV and the maximum negative differencewas 0.72 MeV. In the latter case a depthdose curve had been used by the local centre
to determine the mean incident electron beam energy. The constant linking the mean
incident energy and the depth of 50% ionization is taken to be 2.4 in the HPA (1985) code
of practice; however, the constant 2.33 MeV cm1 is a more representative value of the
available data for a derivation of the mean incident electron beam energy from depthdose
data (ICRU 1984, Brahme and Svensson 1976). If this constant had been employed by the
local centre then the difference in energy determination would be 0.33 MeV. The difference
of 0.72 MeV occurred for a nominal 14 MeV electron beam, and the subsequent difference
in dose is estimated to be 0.4%. For the former case the difference in energy determination
of 1.04 MeV occurred for a nominal electron beam energy of 20 MeV, and the subsequent
difference in dose is estimated to be 0.4%. This difference occurred due to the fact that the
depth ionization curves used locally to determine the mean incident electron beam energy
(measured with a constant source to surface distance) did not apply an inverse square law
correction to convert to depth ionization curves effectively measured with a constant source
to detector distance. It should be noted that the mean ratio of intercomparison measured
energy to locally quoted energy was 1.000.
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Figure 3. Results for the mean incident electron beam energy determination showing the
deviation between intercomparison measured energy and locally quoted energy.
3.3. Depth of maximum dose
The results, expressed as the difference between that determined during the intercomparison
and that quoted locally, are presented in figure 4. With 97 electron beams the determined
depth of maximum dose was within 1 mm; in 140 beams the difference was within
2 mm; and 150 were within 3 mm. The maximum positive difference was 10.3 mm and
the maximum negative difference 9.5 mm. The latter case occurred for a nominal 20 MeV
electron beam, where the depthdose has a broad peak, and the subsequent difference in
dose is estimated to be 0.6%. The former case occurred for a nominal 9 MeV electron
energy and the subsequent difference in dose is estimated to be 7.8%; fortuitously, other
areas of disagreement were observed which acted in an opposite direction and the total
difference in dose was under 5%. The mean difference was 0.3 mm, this highlights the
fact that a large percentage of centres take the effective measuring position of a parallel
plate chamber to be on the front face of the chamber rather than at the inside of the front
face.
3.4. Air kerma calibration factor
Fifty five ionization chambers (27 NACP design parallel plate ionization chambers, 20
NE2571 graphite-walled cylindrical ionization chambers and 8 Markus chambers) were
cross-calibrated against the intercomparison Farmer chamber. The intercomparison was
carried out in the WTe phantom material. The mean ratios of intercomparison determined
calibration factor to locally quoted calibration factor was 1.006 (standard deviation 1.2%) for
the NACP chambers, 0.999 (standard deviation 1.3%) for the NE2571 ionization chambers
and 1.008 (standard deviation 1.2%) for the Markus ionization chambers. Comparative
calibration measurements carried out in Perspex and WTe confirm that the calibration factor
determined in WTe is 1.006 greater than that determined in Perspex for the NACP and
Markus chambers. This is in agreement, within measurement uncertainty, with reported
values of pwall (the perturbation factor to correct chamber reading for deviations from
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UK radiotherapy dosimetry intercomparison 2403
Figure 4. Results for the depth of maximum dose determination showing the deviation between
intercomparison measured depth and locally quoted depth.
Figure 5. Results for the air kerma calibration showing the deviation between intercomparison
determined calibration factor and locally quoted calibration factor for the NACP chamber.
perfect BraggGray behaviour due to the non-medium equivalence of the chamber wall) for
these chambers in these phantom materials (Laitano et al 1993). The spread in the ratio
of the intercomparison determined calibration factor to locally quoted calibration factor are
presented in figures 5, 6 and 7 for the NACP chamber, the NE2571 chamber and the Markus
chamber respectively.
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Figure 6. Results for the air kerma calibration showing the deviation between intercomparison
determined calibration factor and locally quoted calibration factor for the NE2571 chamber.
Figure 7. Results for the air kerma calibration showing the deviation between intercomparison
determined calibration factor and locally quoted calibration factor for the Markus chamber.
3.5. Photon beam calibration
Figure 8 shows the distribution of nominal photon beam qualities included in the study.
These are the photon beam energies which different departments are employing to calibrate
their parallel plate ionization chambers.
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Figure 8. Distribution of nominal photon beam qualities in the intercomparison.
Figure 9. Results for the photon beam calibration measurements showing the distribution of the
ratio of intercomparison measured dose to locally measured dose.
The results, expressed as the ratio of the intercomparison measured dose to the locally
measured dose, are presented in figure 9. The mean value of this ratio is 1.003 (standard
deviation 1.0%) with minimum and maximum values of 1.026 and 0.983, i.e. a spread in
dose of 4.3% across the 52 centres.
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Table 6. Comparison of results from 1992 photon intercomparison with 1997 photon
intercomparison.
Number in Mean
study ratio SD (%)
Co-60 (1992) 61 1.002 1.4
(1997) 16 0.998 1.1
MV x-rays (1992) 100 1.003 1.5
(1997) 36 1.005 0.9
Number within 3% (1992) 97%
(1997) 100%
The results from this study are compared with those obtained in the initial photon
intercomparison of 1992 in table 6. It can be seen that in this repeated intercomparison
both the standard deviations on the observed distributions and also the incidence of major
deviations have decreased.
4. Discussion
4.1. Uncertainties
All uncertainties are quoted as one standard deviation. The type A uncertainties (random
uncertainties) have been estimated as follows. The uncertainty in dosemeter response has
been estimated from the strontium check source measurements to be 0.2%. In order to
ensure an accurate temperature measurement the phantom material was generally placed
in the treatment room approximately two to three hours before the measurements began;
however, this is still probably an insufficient time for the phantom to reach equilibrium
and so the temperature in the chamber insert was continuously monitored throughout the
course of the measurements. It may be argued, however, that this may not have been anaccurate value for the temperature in the air of the ionization chamber, and so a relatively
high uncertainty in the temperature correction of0.1% (equivalent to 0.3 C) has been
estimated. The uncertainty in the barometer reading is estimated at 0.01% (equivalent
to 0.1 mbar). The uncertainty in monitor unit fluctuations has been estimated from the
variation in chamber readings over repeated sets of measurements; an uncertainty of0.3%
has been taken for cobalt-60 units, and 0.5% for the megavoltage x-ray and electron
measurements. As regards uncertainties in the focus to surface distance an uncertainty of
0.2% (equivalent to 1 mm in 1 m) has been assumed for accelerators and 0.25%
(equivalent to 1 mm in 80 cm) for cobalt units. The uncertainty in the method of estimating
the recombination correction factor is taken as 0.1%
This leads to an overall type A uncertainty of 0.6% for the megavoltage x-ray and
electron measurements and an overall type A uncertainty of 0.5% for the cobalt units.
Type B uncertainties (systematic uncertainties) may arise from the calibration of the
ionization chambers and the accuracy of the thermometer and barometer. In addition the
use of the WTe phantom material may increase the uncertainties. It is estimated, from the
results in sections 3.1 and 3.5, that these factors contribute approximately 0.6% to the
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UK radiotherapy dosimetry intercomparison 2407
uncertainty with electron beams and 0.3% uncertainty to the photon beams. The total
uncertainties are therefore estimated at 0.8% for electron beams, 0.7% for megavoltage
x-rays and 0.6% for cobalt-60 gamma-ray beams.
4.2. Sources of differences in dosimetry
The intercomparison has been designed to identify the reasons for the differences in doses
determined by the intercomparison and those measured locally, and a number of common
causes have been identified. Firstly the use of ion recombination corrections (IPEMB 1996a)
is not considered in a large number of centres. This effect depends upon both the dose per
pulse and type of ionization chamber employed, and can be a significant effect. Likewise
polarity effects are not considered in a large number of centres. Secondly the use of epoxy
resin solid water phantom materials is becoming increasingly common, and depth ionization
curves and Ce values identical to water are generally assumed. It has been reported that an
assumption of unity for the fluence ratios of epoxy resin solid water phantoms may introduce
a systematic error of the order of 1% in electron beam dosimetry (Thwaites 1985, Nisbet
and Thwaites 1997b). Specific factors for these materials are included in the recent IPEMB
(1996a) code of practice. Thirdly, significant differences in chamber calibration factors
were observed (see section 3.4). One possible area of disagreement arises from the use of
megavoltage x-ray beams to calibrate parallel plate ionization chambers. The photon beamsincluded in the current photon intercomparison were generally those which the local centres
employed for the calibration of these chambers (see figure 5). The HPA (1985) code of
practice and its IPSM (1992) Addendum recommends that the calibration be carried out in
a cobalt-60 gamma-ray beam and minimal provision is given for calibration in other beams,
due to lack of data on the wall perturbation correction factors for parallel plate chambers
in these situations. The magnitude of this effect may be of the order of 1% to 2% (Nisbet
and Thwaites 1995) and there is evidence that the correction can vary between chambers
of the same nominal design due to small differences in construction (IPEMB 1996a). The
recent IPEMB (1996a) electron code of practice recommends that parallel plate chambers
are calibrated in higher-energy electron beams because of these problems.
5. Conclusions
A total of 156 electron beams were included in the intercomparison. The mean ratio of
intercomparison measured dose to locally measured dose was 0.994 with a standard deviation
of 1.8%. The maximum positive deviation was 4.6% and the maximum negative deviation
was 5.1%. One electron beam lay outside the intercomparison tolerance level of 5%. The
reason for this discrepancy has been determined and subsequent follow-up of the dosimetry
has confirmed agreement to within 1%.
In addition 52 photon beams were included in the intercomparison and no beams were
outside the intercomparison tolerance level of 3%. The mean ratio of intercomparison
measured dose to locally measured dose was 1.003 with a standard deviation of 1.0%. The
maximum positive deviation was 2.6% and the maximum negative deviation 1.7%.
In conclusion the study has demonstrated generally consistent radiotherapy dosimetry
for electron beam dosimetry at the level of beam calibration. The methodology described
can identify problems at the selected tolerance limits, allowing them to be investigated and
rectified. This has been demonstrated in the study. It has also been shown that the standard
deviations and the incidence of discrepancies have decreased for megavoltage photon beams
since the earlier national exercise. This is in part due to the use of the intercomparisons
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2408 A Nisbet and D I Thwaites
themselves and in part due to the recent implementation of quality systems and regular
quality audit via the dosimetry audit network. This study has given confidence in the
basis of clinical delivery of radiation dose in radiotherapy treatment and in the consistency
(precision) of dosimetry between different centres.
Acknowledgments
The study was funded by a grant from the UK Department of Health (NHS
Management Executive Audit Group). The members of the IPSM Radiotherapy Dosimetry
Intercomparison Working Party, responsible for the submission of the grant application, are
gratefully acknowledged (D I Thwaites (Chairman), E G Aird, T Jordan, S C Klevenhagen
and A McKenzie).
The cooperation of the physicists in the radiotherapy departments which participated in
this dosimetry intercomparison is also gratefully acknowledged.
References
Allahverdi M, Nisbet A and Thwaites D I 1997 An evaluation of epoxy resin phantom materials for photon
dosimetry Phys. Med. Biol. submittedAAPM 1994 Comprehensive QA for radiation oncology: report of AAPM Task Group 40 Med. Phys. 12 581618
Bleehan N (Chairman) 1991 Quality Assurance in Radiotherapy, Report of Standing Committee on Cancer
Bonnett D E, Mills J A, Aukett R J and Martin-Smith P 1994 The development of an interdepartmental quality
assurance programme for external beam therapy Br. J. Radiol. 67 27582
Boutillon M, Coursey B M, Hohlfield K, Owen B and Rogers D W O 1994 Comparison of primary water absorbed
dose standards Measurement Assurance in Dosimetry (Vienna: IAEA) pp 95112
Brahme A 1984 Dosimetric precision requirements in radiation therapy Acta Radiol. Oncol. 23 37991
Brahme A and Svensson H 1976 Specification of electron beam quality from the central axis depth absorbed dose
distribution Med. Phys. 3 95
Burns D T 1991 Measurement of the Ce factor for the NACP ionisation chamber in water relative to the farmer
HPA values NPL Report RSA(EXT) 27 (Teddington: National Physical Laboratory)
1994 Potential errors in recombination corrections for electron dosimetry IPSM Radiotherapy Newsheet no 10
(July)
Derreumaux S, Chavaudra J, Bridier A, Rossetti V and Dutreix A 1995 A European quality assurance network for
radiotherapy: dose measurement procedure Phys. Med. Biol. 40 1191208
Hanson W F, Stovall M and Kennedy P 1994 Review of dose intercomparison at a reference point Radiation Dose
in Radiotherapy from Prescription to Delivery (Vienna: IAEA) pp 12130
HPA 1983 Revised code of practice for the dosimetry of 2 to 35 MV x-ray, and of caesium-137 and cobalt-60
gamma ray beams Phys. Med. Biol. 28 1097104
1985 Code of practice for electron beam dosimetry in radiotherapy Phys. Med. Biol. 30 116994
Hoornaert M-Th, Van Dam J, Vynckier S and Bouiller A 1993 A dosimetric quality audit of photon beams by the
Belgian Hospital Physicists Association Radiother. Oncol. 28 3743
ICRU 1976 Determination of absorbed dose in a patient irradiated by beams of X- or gamma rays in radiotherapy
procedures ICRU Report 24 (Bethesda, MD: ICRU)
1984 Radiation dosimetry: electron beams with energies between 1 and 50 MeV ICRU Report 35 (Bethesda,
MD: ICRU)
IPEMB 1996a The IPEMB code of practice for electron dosimetry for radiotherapy beams of initial energy from
2 to 50 MeV based on an air kerma calibration Phys. Med. Biol. 41 2557 604
1996b The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating
potential (0.035 mm Al4 mm Cu HVL; 10300 kV generating potential) Phys. Med. Biol. 41 260525
IPSM 1988 Commissioning and quality assurance of linear accelerators IPSM Report 54 (York: IPSM)1990 Code of practice for high energy photon therapy dosimetry based on the NPL absorbed dose calibration
service Phys. Med. Biol. 35 135560
1992 Addendum to the code of practice for electron beam dosimetry in radiotherapy (1985): interim additional
recommendations Phys. Med. Biol. 37 147783
-
7/28/2019 A Dosimetric Intercomparison of Electron Beams in UK
17/17
UK radiotherapy dosimetry intercomparison 2409
Johansson K A 1987 Dosimetry audits of radiotherapy institutions in Europe Proc. 6th Annual Meeting of ESTRO
(Lisbon, 1987)
Johansson K A, Horiot J C, Van Dam J, Jepinoy D, Sentenac I and Sernbo G 1986 Quality assurance control in
the EORTC co-operative group of radiotherapy, 2. Dosimetric intercomparison Radiother. Oncol. 7 26979
Johansson K A, Mattsson L O and Svensson H 1982 Dosimetric intercomparison at the Scandinavian radiation
therapy centres Acta Radiol. Ther. Phys. Biol. 21 110
Laitano R F, Guerra A S, Pimpinella M, Nystrom H, Karlsson M and Svensson H 1993 Correction factors for
calibration of plane parallel ionisation chambers at a cobalt-60 gamma ray beam Phys. Med. Biol. 38 3954
Mijnheer B, Battermann J J and Wambersie A 1987 What degree of accuracy is required and can be achieved in
photon and neutron therapy? Radiother. Oncol. 8 23752
Niatel M T, Loftus T P and Oetzmann W 1975 Comparison of exposure standards for 60-Co gamma rays Metologia
11 17
Nisbet A and Thwaites D I 1995 In-phantom calibration of NACP design parallel plate ionisation chambers in
photon and electron beams Radiother. Oncol. 37 (suppl 1, paper 11) (Abstracts of the 3rd ESTRO Biennial
Meeting on Physics in Clinical Radiotherapy, Gardone Riviera, Italy)
1997a Polarity correction factors and ion recombination effects for ionization chambers employed in electron
beam dosimetry Phys. Med. Biol. submitted
1997b An evaluation of epoxy resin phantom materials for electron dosimetry Phys. Med. Biol. submitted
Svensson H, Zsdanszky K and Nette P 1994 Dissemination, transfer and intercomparison in radiotherapy dosimetry:
the IAEA concept Measurement Assurance in Dosimetry (Vienna: IAEA) pp 16575
Thwaites D I 1985 Measurements of ionisation in water, polystyrene and a solid water phantom material for
electron beams Phys. Med. Biol. 30 4153
1992 The role of quality audit in clinical dosimetry (Proc. Quality in Radiotherapy, York, July 1992) Scope
1 141994 Uncertainties at the end point of the basic dosimetry chain Measurement Assurance in Dosimetry (Vienna:
IAEA) pp 23955
1996 External audit in radiotherapy dosimetry Radiation Incidents ed K Faulkner and R M Harrison (London:
British Institute of Radiology) pp 218
Thwaites D I and Allahverdi M 1995 The development of interdepartmental audit methods Radiother. Oncol. 37
(suppl 1, abstracts 52) (Abstracts of the Third Biennial ESTRO Meeting on Physics in Clinical Radiotherapy,
Gardone Riviera, Italy, 1995)
Thwaites D I, Scalliet P, Leer J W and Overgaard J 1995 Quality assurance in radiotherapy (European Society
for Therapeutic Radiology and Oncology advisory report to the Commission of the European Union for the
Europe against Cancer Programme) Radiother. Oncol. 35 6173
Thwaites D I and Williams J R 1994 Radiotherapy dosimetry intercomparison Radiation Dose in Radiotherapy
from Prescription to Delivery (Vienna: IAEA) pp 13142
Thwaites D I, Williams J R, Aird E G, Klevenhagen S C and Williams P C 1992 A dosimetric intercomparison
of megavoltage photon beams in UK radiotherapy centres Phys. Med. Biol. 37 44561
Wittkamper F W and Mijnheer B J 1993 Dose intercomparison at the radiotherapy centres in The Netherlands. 3.Characteristics of electron beams Radiother. Oncol. 27 15663
Wittkamper F W, Mijnheer B J and van Kleffens H J 1987 Dose intercomparison at the radiotherapy centres in
the Netherlands. 1. Photon beams under reference conditions and for prostatic cancer treatment Radiother.
Oncol. 9 3344
Worsnop B R 1968 Phantom thermoluminescent dosimeter comparison for a co-operative radiotherapy trial
Radiology 91 54553