Radiotherapy and Oncology 48 (1998) 95–101
A dosimetric intercomparison of kilovoltage X-rays, megavoltage photons
and electrons in the Republic of Ireland
Andrew Nisbet a ,*, David I. Thwaites a, Martin E. Sheridan b
a
Department of Medical Physics and Medical Engineering, University of Edinburgh,
Western General Hospital, Crewe Road, Edinburgh, EH4 2XU, UK
b
Department of Radiotherapy Physics, Saint Vincent’s Private Hospital, Merrion, Dublin 4, Ireland
Received 19 August 1997; revised version received 24 March 1998; accepted 8 April 1998
Abstract
Background and purpose: A comprehensive dosimetry intercomparison has been carried out involving all the radiotherapy centres, all
external beam modalities and every radiotherapy treatment unit in the Republic of Ireland.
Materials and methods: Reference point measurements were made for all megavoltage photon beams. Doses were also investigated in
planned three-field distributions. One of these was in a homogeneous epoxy resin solid water phantom, whilst the second included a lung
equivalent insert. The intercomparison was also carried out for three electron energies in each centre. The position of the depth of maximum
dose for a standard field size was independently determined, as was the beam energy and a subsequent beam calibration was made. In
addition, a kilovoltage X-ray intercomparison was carried out on every kilovoltage quality.
Results: For 13 megavoltage photon beams a mean ratio of intercomparison measured dose to locally measured dose of 1.002 was
obtained (standard deviation 1.2%). For 12 electron beam measurements a mean ratio of intercomparison measured dose to locally
measured dose of 1.018 was obtained (standard deviation 0.8%). For four kilovoltage beams a mean ratio of intercomparison measured
dose to locally measured dose of 0.997 was obtained (standard deviation 1.9%).
Conclusions: The intercomparison 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 within the Republic of Ireland. In addition, it has established a
methodology for subsequent ongoing routine radiotherapy dosimetry audit and a baseline set of results to act as an initial reference
point.  1998 Elsevier Science Ireland Ltd. All rights reserved
Keywords: Dosimetry intercomparison; Kilovoltage X-rays; Megavoltage photons; Megavoltage electrons
1. Introduction
The demands on precision in dosimetry and treatment
delivery are determined by the steepness of the relevant
clinical dose–effect curves, both for tumour control and
for normal tissue complications. Consideration of the clinical data led to generally agreed recommendations on the
required accuracy in clinical dosimetry for radical curative
radiotherapy being given in International Commission on
Radiation Units and Measurements (ICRU) Report 24
[11], which pointed to a need for at least ±5% accuracy in
* Corresponding author. Department of Medical Physics, Sandringham
Building, Leicester Royal Infirmary NHS Trust, Leicester, LE1 5WW,
UK.
the delivery of absorbed dose to the target volume in the
patient. More recent reviews [5,20] have led to recommended tolerance levels on accuracy in dose delivery of
±3.5 and ±3%, respectively, being stated as one relative
standard deviation. Therefore, comprehensive quality assurance systems are necessary [2,4,27]. As part of this, the
fundamental basis of consistent dosimetry is the use of carefully designed and applied international or national dosimetry protocols and codes of practice (e.g. Refs. [1,3,8–
10,12,13,15,17]) ensuring traceability of dosimetry to
national and international standards. 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 essentially designed to
0167-8140/98/$19.00  1998 Elsevier Science Ireland Ltd. All rights reserved
PII S0167-8140 (98 )0 0041-3
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A. Nisbet et al. / Radiotherapy and Oncology 48 (1998) 95–101
establish the accuracy and precision of dosimetry at a given
level in the dosimetry chain and to assess consistency
between centres. By using a standard methodology, phantom(s), measurement technique and measuring system, differences between the way that different centres apply
dosimetry protocols and carry out their dosimetry can be
assessed. Using dosimetry intercomparisons as an audit is
widely recognized as being effective in revealing the presence of errors [27,29]. Dosimetry intercomparison techniques form the basis of the practical methodology employed
in the developing radiotherapy dosimetry audit networks
[6,7,23,25]. Whilst audit systems can employ mailed thermoluminescent detectors (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 [26,24], as have some of the rapidly
developing network of routine dosimetry audit programmes
[25].
There is an increasing number of national audit systems
being established, with networking to other national or
international efforts. No such nation-wide dosimetry intercomparisons or audits have been previously carried out in
the Republic of Ireland and there was a need for an initial
comprehensive national study to determine the currently
achieved dosimetric precision in radiotherapy across the
country. In addition, it was expected that such an exercise
would provide a methodology and a baseline set of data
which could be used as a standard for subsequent ongoing
routine audit to work from and refer to. Thus, this work
reports a systematic comprehensive dosimetry intercomparison covering all the radiotherapy centres, all external beam
modalities and every radiotherapy treatment unit in the
Republic of Ireland. The experimental work was carried
out over 8 consecutive days in August 1996.
2. Materials and methods
2.1. General methods
The methods employed in this dosimetry intercomparison
are largely based on those developed for the UK megavoltage photon [28] and electron beam intercomparisons [21]
and the Scottish(+) audit group [25]. The radiotherapy centres in the Republic of Ireland have traditionally followed
the UK dosimetry codes of practice and hence the 1990
Institute of Physical Sciences in Medicine (IPSM) code of
practice [15] has been employed for megavoltage photons in
this dosimetry intercomparison. For kilovoltage X-rays, the
1991 IPSM recommendations [16] have been followed,
whilst for electron beams the 1985 Hospital Physicist’s
Association (HPA) code of practice [8] with its 1992 addendum [17] have been used to determine the absorbed dose to
water for electron beams.
For each beam considered, the host department was asked
to measure the beam calibration immediately prior to the
intercomparison. Thus, the intercomparison measurement
was compared directly to the local department’s statement
of dose, without any significant effect of drift in the treatment machine’s monitor or control system.
2.2. Measurement systems
Megavoltage photon beam doses were measured with a
Nuclear Enterprises (NE) Technology NE2570 dosimeter
and an NE2571 cylindrical graphite-walled ionization
chamber. The chamber was calibrated against a secondary
standard dosimeter (Nuclear Enterprises model 2560/2561)
in a cobalt-60 g-ray beam and also in X-ray beams of nominal energies of 4, 6, 9 and 16 MV and with quality indices
of 0.63, 0.68, 0.72 and 0.765, respectively, according to the
IPSM 1990 code of practice, traceable to the UK National
Physical Laboratory (NPL) primary dosimetry standards.
The same electrometer and ionization chamber assembly
have also been employed for the kilovoltage X-ray dosimetry intercomparison. The chamber assembly was calibrated
over a range of superficial qualities against a secondary
standard system traceable to NPL primary standards and a
plot of air kerma calibration factor against beam quality
constructed to cover the range of kilovoltage qualities
encountered in the intercomparison.
For the electron beam measurements, the NE Technology
2570 dosimeter was employed with a Scanditronix Nordic
Association of Clinical Physics (NACP) type-02 plane parallel ionization chamber. An operating voltage of −250 V
was employed and ion recombination was evaluated both
during calibration in a cobalt-60 g-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 to using an operating voltage of less than 100 V [12]. The chamber was calibrated
against the secondary standard system in a cobalt-60 beam
according to the HPA 1985 code of practice [8], again traceable to NPL primary standards.
Ion recombination and polarity effects for each ionization
chamber were determined across the range of measurement
conditions encountered. Appropriate ion recombination factors were applied according to the operating dose per pulse
of the pertinent accelerators considered. This was estimated
from the gun pulse repetition frequency and the indicated
dose rate. Time considerations prevented the actual measurement of recombination in each centre. It should be
noted that an error of ±20% in the pulse repetition frequency
results in an error of no more than ±0.1% in the ion recombination correction factor. Polarity effects were determined
for electron beam measurements by selecting from a preprepared curve of correction factors for the mean electron
beam energy at the depth of measurement [22].
The stability of the measurement systems was tested
A. Nisbet et al. / Radiotherapy and Oncology 48 (1998) 95–101
before and after the intercomparison by repeating the chamber calibration and also by using Strontium-90 check
sources. The repeat calibration of the NE2571 chamber
was within 0.1% and that for the NACP chamber was within
0.2%. Agreement of the Strontium-90 check measurements
was within 0.1% for the NE2571 chamber and within 0.3%
for the NACP chamber.
A dedicated barometer ((Negretti and Zambra, aneroid
type M2236) and thermometer (mercury in glass, supplied
by NE Technology) were used throughout the study to
determine pressure and temperature corrections to the ionization chamber readings. Both were calibrated traceable to
UK national standards. The pressure and phantom temperature were measured regularly throughout each set of measurements. In each centre, a local value of pressure and
temperature was compared to identify and quantify any
systematic differences due to this cause.
2.3. Phantoms
The phantom employed for the megavoltage photon beam
measurements was made of WT1, an epoxy resin water
substitute material manufactured by Radiation Physics, St.
Bartholomew’s Hospital, London. The water equivalency of
this material has been confirmed and is the subject of
another paper. A schematic diagram of the phantom and
its dimensions is shown in Fig. 1. It has six 20 mm diameter
removable water equivalent rods, one of which is machined
to hold a Farmer-type ionization chamber. One hole is
centred at 50 mm depth from the largest flat surface of the
phantom and was employed for reference beam measurements. The other five positions made a target volume for a
planned three-field distribution. The phantom also has an 8
cm diameter hole which may hold either a WT1 insert or a
lung equivalent insert. Further details of the basic approach
to the design and use of the phantom can be found in the
paper by Thwaites et al. [28].
The phantom employed for the megavoltage electron
Fig. 1. Schematic diagram of the WT1 phantom employed for the megavoltage photon beam measurements. The dimension of the phantom perpendicular to the cross-section is 18 cm. The 8 cm diameter hole may hold
either a WT1 insert or a lung equivalent insert. Dimensions: a = 3 cm;
b = 5 cm; c = 6 cm; d = 10 cm; e = 15 cm; f = 20 cm; g = 30 cm.
97
beam measurements was made of WTe, an epoxy resin
water equivalent material specifically formulated for electrons and again produced commercially by Radiation Physics, St. Bartholomew’s Hospital, London. The water
equivalency of this material has been confirmed and is the
subject of another paper. The WTe solid water phantom
consisted of 250 × 250 mm sheets with thicknesses varying
between 1 and 50 mm. For all the measurements, 100 mm of
phantom material was positioned behind the ionization
chamber to provide backscatter. A maximum sheet thickness of 20 mm was used to set the depth of the chamber in
the phantom material and no problems with charge storage
were observed with this arrangement. One 10 mm sheet of
WTe was machined to hold the NACP chamber. Further
details of the basic approach to the design and use of the
phantom can be found in the paper by Nisbet and Thwaites
[21].
For kilovoltage X-rays, all measurements were carried
out in air.
2.4. Measurements
2.4.1. Megavoltage photon beam calibration
The megavoltage photon beam intercomparison was carried out on each treatment unit in each radiotherapy centre
visited and for each beam quality available. The dose at 50
mm depth in WT1 was measured for three field sizes of
50 × 50, 100 × 100 and 200 × 200 mm. The beam quality,
as represented by TPR200
100 , was also measured for each
photon quality using the independent set of WTe slabs.
The measured beam quality was employed to determine
the appropriate absorbed dose calibration factor for the
NE2571 ionization chamber. In addition, the light field
size was measured for each of the three field sizes and the
agreement between the mechanical front pointer, the optical
distance indicator and the set-up lasers was noted.
2.4.2. Megavoltage photon three-field planned distribution
To investigate the agreement between locally planned
dose distributions and independent measurements, the host
department was asked to plan a three-field treatment and
deliver a uniform dose of 2 Gy to an 80 × 80 × 80 mm
volume centred on the central cavity in the WT1 phantom.
Two plans were requested, one in a homogeneous phantom
and the other with the 80 mm lung insert in place. Isocentric
plans were produced by all four centres and each centre
employed a nominal 6 MV photon beam. In each case the
phantom was set up and the beams were treated following
the plan. Doses were measured at each of the five points in
the high dose volume for each field.
2.4.3. Electron beam measurements
The electron beam intercomparison was carried out on
one unit in each radiotherapy centre visited and for three
energies across the range. For each nominal energy, the
depth of maximum dose for a standard field size, the elec-
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A. Nisbet et al. / Radiotherapy and Oncology 48 (1998) 95–101
tron beam energy and the beam calibration were independently determined. The methodology as described by Nisbet
and Thwaites [21] was employed. The energy relationships
given in HPA 1985 [8] were used to determine the mean
incident electron beam energy and the subsequent mean
electron beam energy at the depths of measurement.
2.4.4. Kilovoltage photon beam calibration
In the kilovoltage intercomparison, a dose was measured
in air for a standard applicator size. A clamp stand was
employed to position the ionization chamber against the
face of the applicator and an inverse square law correction
was applied to correct for the offset of the chamber central
axis from the end of the applicator. The half-value layer
(HVL) as quoted by the host department was employed to
determine the air kerma calibration factor for the ionization
chamber. In order to estimate the effect of uncertainty in the
quoted HVL on the dose measurements, it can be assumed
that by employing a good-quality set-up [18], i.e. narrow
beam and scatter-free conditions, and using high purity
attenuating material, one would expect to be able to measure
the HVL to within ±0.01 mm aluminium (Al) HVL. A conservative estimate on the uncertainty in the HVL as quoted
by the local centre is therefore ±0.1 mm Al. This leads to an
uncertainty in the air kerma calibration factor of the order of
±0.1%. The locally quoted HVL has also been employed to
choose the backscatter and other applicable factors. For a 5
cm diameter applicator, the change in backscatter factor
between 1.5 and 2.0 mm Al HVL is 1.8% and likewise
there is a 2.7% change from 2.0 to 3.0 mm Al. Other factors
change by approximately 1% over this range. The total
uncertainty in dosimetry due to employing the locally
quoted HVL is therefore of the order of ±0.5%.
2.4.5. Procedural audit
A degree of procedural audit was incorporated at each
visit. A questionnaire was designed which dealt with the
quality control procedures in operation and 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 analyzed here but expected
quality control procedures, frequency of checks and tolerances were taken to be those in IPSM Report 54 [14]. The
questionnaire helped in identifying reasons for any differences observed in dosimetry between the intercomparison
and that measured locally. A full analysis of any identifiable
reasons for any differences in dosimetry were given with the
subsequent report.
intercomparison measured dose to the locally stated dose,
are shown in Fig. 2. The lighter areas show the results for
the reference field of 100 × 100 mm and the full histogram
shows the results for all three fields. The mean value of this
ratio for the 100 × 100 mm field size is 1.002 (standard
deviation 1.2%) with minimum and maximum values of
0.984 and 1.027, i.e. a spread in dose of 4.3% across the
four centres. The mean ratio for all three field sizes is 1.001
(standard deviation 1.2%) with minimum and maximum
values of 0.978 and 1.027. It should be noted that a mean
value close to unity suggests that there are no significant
systematic errors in the intercomparison dosimetry.
3.2. Planned dose distributions
Fig. 3 shows the average measured to calculated dose
ratio for the five points in the dose distribution with the
water equivalent insert. The results for the distribution
with the lung equivalent insert are shown in Fig. 4. The
lighter areas for both distributions are the results for the
central points. It should be noted that the locally measured
beam calibrations on the day have been taken into account
in the calculated dose distributions. The mean ratio of intercomparison measured dose to calculated dose for all five
points was 0.998 for the homogeneous phantom (standard
deviation 1.1%) and 0.997 with the lung insert (standard
deviation 1.3%). The mean ratio of intercomparison measured dose to calculated dose for the four central points was
1.005 for the homogeneous phantom and 1.009 for the phantom with the lung insert. This may indicate a small systematic difference between the central dose and the dose at the
peripheral points.
A test of the self-consistency between the two planned
dose distributions is to compare the central doses in the two
plans. The average of the ratios is 0.997 and indicates no
systematic difference. The normalized ratio (relative to the
intercomparison dose) of central point to reference point
dose is an indication of the self-consistency of the dose
distribution and reference point measurements for the
same beam. The mean value of this ratio is 1.003 (standard
deviation 1.2%) and indicates no systematic difference.
3. Results
3.1. Megavoltage photon reference point measurements
The intercomparison results, expressed as the ratio of the
Fig. 2. Results for the photon reference point measurements showing the
distribution of the ratio of intercomparison measured dose to locally measured dose.
A. Nisbet et al. / Radiotherapy and Oncology 48 (1998) 95–101
Fig. 3. Results for the dose distribution in a homogeneous phantom showing the variation in the average measured to calculated dose ratio.
3.3. Kilovoltage X-ray beam calibrations
Four superficial units were included in the intercomparison with quoted half-value layers of 1.35, 2.0 and 2.2 mm
Al and 0.4 mm Cu (equivalent to 8.0 mm Al). The mean
ratio of intercomparison measured beam calibration to
locally stated beam calibration was 0.997 (standard deviation 1.9%), with minimum and maximum ratios of 0.974
and 1.018. Again it should be noted that a value close to
unity suggests that there are no significant systematic errors
in the intercomparison dosimetry.
3.4. Electron beam calibrations
The intercomparison results, expressed as the ratio of the
intercomparison measured dose to the locally measured
dose, are illustrated in Fig. 5. The mean value of this ratio
is 1.018 (standard deviation 0.8%), with minimum and maximum values of 1.002 and 1.027. This systematic difference
may be partly attributed to the different calibration chains
employed for the intercomparison and the local centres. It
should be noted that the mean ratio of the air kerma calibration factor (Nf) for the local electron beam chambers (two
NACP chambers and two Markus chambers) determined
during the intercomparison and the air kerma calibration
factor quoted locally was 1.016 (standard deviation 0.6%).
The cross-calibration against the intercomparison Farmer
chamber was carried out in the WTe phantom material.
99
Fig. 5. Results for the electron beam measurements showing the distribution of the ratio of intercomparison measured dose to locally measured
dose.
Comparative calibration measurements carried out in perspex and WTe suggest 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 readings
for deviations due to the non-medium equivalence of the
chamber wall) for these chambers in these phantom materials [19]. Therefore, approximately 1% of the mean difference in electron beam calibrations may be attributed to the
different calibration chains. In one instance, the intercomparison determination of Nf was carried out in a 6 MV
photon beam, whereas the local chamber had actually
been calibrated in another centre’s cobalt-60 beam. In a
second centre, the chamber calibration was actually carried
out in a 6 MV beam and a correction factor was applied
locally to effectively correct to a cobalt-60 calibration. It is
possible that the correction values to correct from 6 MV
chamber calibration to cobalt-60 chamber calibration may
have been overestimated.
3.5. Electron beam energy
With six beams the agreement in energy between that
determined during the intercomparison and that quoted
locally was within ±0.1 MeV. With 10 beams the agreement
was within ±0.2 MeV and all but one were within ±0.3 MeV.
The maximum positive difference in nominal electron beam
energy was 0.27 MeV and the maximum negative difference
was 0.60 MeV. The difference of 0.60 MeV occurred for a
nominal 12 MeV electron beam and the subsequent difference in dose is estimated to be 0.5%. For the former case,
the difference in energy determination of 0.27 MeV
occurred for a nominal electron beam energy of 12 MeV
and the subsequent difference in dose is estimated to be
0.1%.
3.6. Depth of maximum dose
Fig. 4. Results for the dose distribution in the phantom with the lung insert
showing the variation in the average measured to calculated dose ratio.
With seven electron beams the determined depth of maximum dose was within ±1 mm and in 11 of the 12 beams the
100
A. Nisbet et al. / Radiotherapy and Oncology 48 (1998) 95–101
Table 1
Type A uncertainties
Source of uncertainty
Estimated from
Standard deviation
Dosimeter response
Temperature (°C)
Pressure (mbar)
Output for given monitor unit setting
Focus to surface distance
Repeated Strontium check source measurements
±0.3
±0.1
Variation in chamber reading
±1 mm in 1 m
±1 mm in 80 cm
±0.5 mm
See text
See text
0.2%
0.1%
0.01%
0.3% cobalt-60, 0.5% MV X-rays, 0.5% electrons
0.2% MV X-rays, 0.2% MV X-rays
0.25% cobalt-60
0.8% kV X-rays (for a typical FSD of 30 cm)
0.5% kV X-rays
0.1%
0.6% MV X-rays, 0.6% electrons, 0.5% cobalt-60,
1.4% kV X-rays
Inverse square law correction
Quoted HVL
Ion recombination
Total
difference was within ±2 mm. The maximum positive difference was 1.5 mm and the maximum negative difference
was 2.5 mm. The latter case occurred for a nominal 12 MeV
electron beam and the subsequent difference in dose is estimated to be 0.2%. The former case occurred for a nominal
12 MeV electron energy and the subsequent difference in
dose is estimated to be 0.4%. The mean difference was −0.6
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.
uncertainties. By considering the mean ratio of intercomparison measured dose to locally measured dose in the larger
UK intercomparison, an uncertainty of ±0.6% for electron
beams and ±0.3% for megavoltage photon beams may be
estimated. For kilovoltage X-rays, where the measurement
is in air, an uncertainty of ±0.2% is estimated.
The total uncertainties are therefore estimated at ±0.8%
for electron beams, ±0.7% for megavoltage X-rays, ±0.6%
for cobalt-60 g-ray beams and ±1.4% for kilovoltage Xrays.
3.7. Uncertainties
4. Discussion
All uncertainties are quoted as one standard deviation (1
SD). The type A uncertainties (random uncertainties) have
been estimated and are listed in Table 1.
The type B uncertainties (systematic uncertainties) which
are common to the intercomparison system and the local
departments assessed cannot obviously be identified in
such an intercomparison. They may be assessed by comparison to other systems [24,25]. However, specific types of
uncertainty on the intercomparison dosimetry which may
affect the results of an intercomparison may arise, for example, from the calibration of the ionization chambers and the
accuracy of the thermometer and barometer. In addition, the
use of the solid water phantom materials may increase the
Table 2 summarizes the results. In conclusion, the study
has demonstrated generally consistent basic radiotherapy
dosimetry for megavoltage photon and electron beam dosimetry and kilovoltage X-ray dosimetry at the level of beam
calibration with no beams being outside the intercomparison
tolerance level of 3%. It has provided quantitative information on the currently achieved precision of dosimetry for all
external beam modalities across the whole country. The
methodology described has been shown to identify causes
of differences in dosimetry to a fraction of a percent. Therefore, it would be possible to identify problems at the
selected tolerance limits and allow them to be investigated
and rectified. The intercomparison has given confidence in
Table 2
Summary of results
Number
Megavoltage reference point
measurements
Planned dose distribution
Homogeneous
With lung insert
Electron beam calibration
Electron energy
Dmax
Kilovoltage beam calibration
13
4
4
12
12
12
4
Mean ratio (measured/
quoted) or difference
1.002
0.998
0.997
1.018
−0.04 MeV
−0.6 mm
0.997
Standard
deviation (%)
1.2
1.1
1.3
0.8
–
–
1.9
Maximum negative
deviation
0.984
1.018
1.019
1.002
−0.6 MeV
−2.5 mm
0.974
Maximum positive
deviation
1.027
0.971
0.973
1.027
0.27 MeV
1.5 mm
1.018
A. Nisbet et al. / Radiotherapy and Oncology 48 (1998) 95–101
the basis of clinical delivery of radiation dose in radiotherapy treatment and in the consistency (precision) of dosimetry between different centres within the Republic of Ireland.
In addition, it has established a methodology for subsequent
ongoing routine radiotherapy dosimetry audit and it has
established a baseline set of results to act as an initial reference point. It is intended that the Republic of Ireland will
establish and continue with an independent audit system
which will occasionally cross-link to the UK audit network
via the Scottish(+) audit group.
Acknowledgements
The co-operation of the physicists in the radiotherapy
departments which participated in this dosimetry intercomparison is gratefully acknowledged.
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