A Dosimetric Intercomparison of Electron Beams in UK

7/28/2019 A Dosimetric Intercomparison of Electron Beams in UK

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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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UK radiotherapy dosimetry intercomparison 2395

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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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


No of beams 41 37 15

Mean 0.992 0.993 0.985SD (%) 1.5 1.5 2.0



Spread (%) 6.3 8.0 7.5

(ii) Markus chamber Energy


No of beams 13 12 11

Mean 1.012 1.002 1.003

SD (%) 1.7 1.7 1.5



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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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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determined calibration factor and locally quoted calibration factor for the NE2571 chamber.


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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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


16/17


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


17/17


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

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