Performance assessment of patient dosimetry services and X-ray quality assurance instruments used in diagnostic radiology

Applied Radiation and Isotopes PERGAMON

Applied Radiation and Isotopes 50 (1999) 137±152

Performance assessment of patient dosimetry services and X-ray quality assurance instruments used in diagnostic radiology S. Green a, *, J.E. Palethorpe a, D. Peach a, D.A. Bradley b a

RRPPS, P.O. Box 803, Edgbaston, Birmingham B15 2TB, U.K. Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia

b

Abstract Experiences of the Regional Radiation Physics and Protection Service (RRPPS) in performance assessment of diagnostic X-ray QA instrumentation and on-patient dosemeters are recounted. Issues relating to the provision of realistic and reproducible reference conditions for calibrated X-irradiations are considered and summary statistics from test measurements of dose and kVp meters are provided. For both dose and kVp meters it is indicated that as many as 25% of instruments used in routine use in the U.K. may require some adjustment before they can truly be said to be performing as the manufacturer intended. Results from intercomparison exercises for patient dosimetry services are also discussed. It is apparent that, for those centres participating in the exercise, dose assessments are generally being obtained to within a bias and a relative standard deviation of less then 10%. # 1998 Elsevier Science Ltd. All rights reserved.

1. Introduction It is perhaps most appropriate to begin this article with a review of what is considered to be a ``patient dosimetry service'' or an ``X-ray quality assurance (QA) instrument''. In the U.K., there are a large number of organisations which provide services to hospitals for the direct measurement of the incident radiation dose to individuals undergoing diagnostic X-ray procedures. These services almost exclusively use thermoluminescent dosimetry (TLD) technology, and operate from hospital departments (usually in the radiation protection sections of departments of medical physics), from national bodies such as the National Radiological Protection Board (NRPB) in the U.K., or from commercial companies. For the purposes of this article these are referred to as ``patient dosimetry services''.

* To whom all correspondence should be addressed.

The dose to the patient may also be measured via an in-beam transmission chamber such as a dose-area product meter but quite often there is no dynamic measurement of the radiation output. The patient dose will be dependent upon the performance of the X-ray equipment, which will have been checked at installation as part of a ``critical examination'', and then subsequently as part of a regular quality assurance programme. This programme will involve extensive radiation output measurements according to a standard protocol, performed on a regular basis, being typically an annual check in the U.K. for standard radiology equipment. During this survey, the radiation output and the applied kVp on the X-ray tube will be measured under a representative set of exposure conditions. These measurements may form the basis of retrospective dose assessments for patients exposed using this equipment. The readings of any in-beam monitoring equipment will be checked and re-calibrated if required. Further daily or weekly check measurements may be performed to ensure the consistent operation

0969-8043/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 9 8 ) 0 0 0 3 0 - X

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of the equipment between the major surveys of radiation output. The performance of the QA devices used during these radiation output and kVp measurements are obviously critical to the future assessment and control of patient doses delivered by the equipment. These devices are what we shall refer to as ``X-ray QA instruments''. Methods and procedures to enable the accurate calibration of these instruments will be discussed in this article, along with a summary of the performance of a sample of instruments in routine use which have been sent to RRPPS for their annual calibration. The calibration of instruments and patient dosemeters used in diagnostic radiology is a major activity for the RRPPS. This article will attempt to distil the experience which has been gained by this team during the past few years of such endeavour. The long history of work in RRPPS in the radiation protection ®eld is intended to provide a ®rm basis for discussion of instrument calibration, for both dose and kVp meters, and in areas such as choice of (or indeed use of) reference phantoms for calibration of on-patient dosimetry devices such as diodes and TLDs. A great deal of very valuable work has been published in these same areas by workers from the Physikalish-Technische Bundesanstalt (PTB), Germany and NRPB, U.K., and it is therefore sensible to build on their experience when de®ning calibration conditions for diagnostic radiology. As an example of the advantages of an ongoing dialogue between di€erent organisations providing dosimetry services to hospitals, recent intercomparison exercises performed in the U.K. under the auspices of the Personal Radiation Monitoring Group (PRMG) will be described. This group has been active in the area of radiation protection and personal monitoring for around 15 years, and has consistently organised national intercomparison exercises over this time. The activities of the Group in patient dosimetry are less well established, and the results of two recent intercomparison exercises covering diagnostic X-ray dosimetry will be presented. In the ®nal sections of this article, discussion will focus on the calibration of instruments used in routine quality assurance of diagnostic X-ray beams, i.e. dosemeters (generally ionisation chambers) and non-invasive kVp meters. Our experience from calibration of many hundreds of these instruments each year will be summarised in an attempt to quantify the proportions of instruments in routine use (in the U.K.), which are operating as intended by their manufacturer. Our discussion will begin with considerations relating to the provision of realistic and reproducible reference conditions under which calibrated X-irradiations are performed.

2. Calibration reference conditions 2.1. What makes a good calibration reference set-up? Over recent years there has been an ongoing debate in the ®eld of protection level instrument calibrations, on the necessity of performing measurements in realistic ®eld conditions. In brief the arguments evolved from two opposing points of view. The ®rst can be summarised as a belief that the calibration is only useful if it is performed under conditions which are closely representative of those encountered in routine use of the instrument, and is unreliable if the breadth of possible ®eld conditions are not simulated in some way or other during the calibration. The second is that the calibration conditions need only roughly approximate the ®eld conditions but must be clearly de®ned and be easily reproducible between laboratories and from year to year. It is clear that the de®nitions of closely and roughly approximate are quite subjective and will vary from one instrument type or dosimetry system to another. It is true for example that it is less important to provide a calibration in a realistic X-ray beam spectrum for an ionisation chamber based dosemeter than for a diodebased dosemeter, as the response of ionisation chamber based instruments varies much less slowly with photon energy than does that of diode based instruments. Overall, there are a number of recent developments in the ®eld of protection level calibrations which suggest quite clearly that the second point of view is in the ascendancy. The International Standards Organisation (ISO) has recently published documents describing appropriate reference calibration conditions in a number of ®elds (ISO, 1993, 1996, 1997) which, amongst other things, specify standard radiation sources for the calibration of surface contamination monitors and reference phantoms to provide backscatter for dosemeters used in personal monitoring. These documents have moved away, in the case of surface contamination monitors, from the use of radionuclides which mimic the emissions of widely used radiation sources. Favourable calibration sources (viz. those with convenient emission energies, emission intensities and suciently long half life) can now be those which contain a mixture of emitted radiations, but are de®ned for use as ®ltered radiation sources which consist of fairly pure emissions. Typically these consist of gamma rays in the absence of contamination from low energy X-rays (e.g. a 57 Co source with a stainless steel ®lter), and photons without contamination from alpha radiation (e.g. a 241 Am source with a stainless steel ®lter) (ISO, 1997). However, there remains a need for detailed work on each instrument and dosimetry system to establish its likely performance over the range of conditions which

S. Green et al. / Applied Radiation and Isotopes 50 (1999) 137±152

might be encountered in routine use. This is clearly embodied in U.K. regulations as the requirement for a ``Type Test'' which is a comprehensive test which puts the instrument at or outside its normal limits of operation to examine its characteristics in greater depth. Such a test is essential, but should not be confused with the role of an annual or routine calibration. As a general principle it is now sensible to devise calibration conditions which whilst they must be of a similar nature to those encountered in routine use of the instrument, their primary purpose must be to facilitate reproducible metrology between laboratories and from year to year. 2.2. Use of reference phantoms There are two distinct uses for phantoms in radiation dosimetry. These are: . to simulate as closely as possible the radiation exposure of a patient to assess the dose from a particular radiological procedure, for example the use of a Rando phantom with internal TL dosemeters, and; . to provide reference calibration conditions which are appropriate for the calibration of a beam or dosemeter system, for example the use of a water phantom for radiotherapy beam calibrations. Clearly the discussion above on the appropriate nature of calibration reference conditions extends to the need for and de®nition of appropriate reference phantoms for use in calibration of patient dosimetry systems. The quantity which is generally of greatest importance in routine measurements of patient dose in diagnostic radiology is termed the entrance surface dose. It is usually expressed as a dose to air, since air will yield a dose ®gure which is intermediate between a number of other suggested reference tissues across the energy range which is typical in diagnostic radiology (NRPB, 1992), and because the reference medium for dosemeter calibration is air via Primary Standard Facilities using air-kerma Primary Standards. Since the position of measurement is de®ned to be the entrance to a patient, it seems initially to be essential to use some kind of phantom during dosemeter calibration to simulate the backscattering e€ects of a patient on the response of the dosemeter. This would certainly be consistent with the approach used in personal monitoring where a number of reference phantoms have been suggested over recent years (ICRU, 1992). In fact the very wide range of phantom designs suggested over the years has caused some confusion in the more practical areas of dosimetry (Dennis, 1995). After some years of confusion, the situation in the radiation protection and personal monitoring ®eld now has greater clarity since the recent recommendations of the ISO (ISO, 1996). A single phantom design is now suggested for calibration

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of dosemeters for whole-body dose assessments in photon and neutron applications, which is a Perspex enclosed water phantom. The calculation of backscatter factors and conversion coecients from this phantom, for a range of beam qualities, has now been performed. It would therefore be sensible in patient dosimetry to take a lead from this work, and if the need for a phantom is established, to use a water phantom of the same construction and dimensions as the one de®ned by the ISO. However, there are reasons why phantoms may not be necessary in patient dosimetry, where the need has been clearly established in personal monitoring. Firstly, the response of typical dosemeters used routinely in diagnostic radiology exhibits a much lower variation with energy than those used in personal monitoring. This means that any di€erences in photon spectrum between the incident and backscattered radiation will cause a smaller e€ect on dosemeter reading in patient dosimetry than in personal monitoring. There is also a di€erence in the de®nition of the quantities which are being simulated via the phantom. The quantity of interest in personal monitoring Hp(10) (ICRU, 1992), is de®ned at a depth of 10 mm into the phantom, and so cannot simply be addressed without the use of a phantom. The quantity of interest in patient dosimetry, entrance surface dose, is de®ned as dose to air, on the surface of the patient, and so may be addressed without the presence of the phantom. The second argument here is not a particularly strong one, since it implies that the phantom in some way establishes the relationship between the measurable quantity (air kerma) and the de®ned dose quantity (in this case entrance surface dose). In fact the phantom cannot make this connection, since the quantity required is de®ned for a real patient not for a phantom. In fact the phantom can only provide a physical simulation of the backscattered radiation from a real patient. What is essential is that the calibration conditions of the dosemeter can be related, via measurements or Monte Carlo simulations of photon backscatter to the quantities which are indicative of radiation detriment, that is Equivalent and E€ective Dose (ICRP, 1990). Since a number of authors have published work on such conversion factors (IPSM, 1988 provides a good review) there appears to be no overriding necessity for use of a phantom as part of the reference dosimetry conditions employed in diagnostic radiology. There are other areas of radiological imaging where the use of phantoms has greater importance. In mammography applications, greatest attention is concentrated on the mean glandular dose, which must be de®ned with reference to a phantom. The debate in the sections above is only applicable to areas where

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entrance surface dose is the parameter of dosimetric interest. This completes our introductory discussion on the more philosophical areas of the nature of the dosimetric and calibration techniques which are of importance in diagnostic radiology. There is a need for guidance at the international level before these issues can be fully put to rest. In the absence of such de®nitive guidance, and because it is our belief that such an approach is appropriate, all instruments and patient dosemeters supplied to RRPPS for calibration are irradiated free in air. The discussion will now switch to more practical matters related to the overall characterisation of the radiation beams used in RRPPS for these calibrated irradiations.

2.3. X-ray facilities The RRPPS is fortunate to have a dedicated building with two separate rooms which are suitable for the delivery of calibrated X-irradiations. For the majority of our work in diagnostic X-ray and mammography calibrations, the standard radiation qualities are generated from the following X-ray sources: (1) A Pantak HF-320 calibration X-ray set for generating IEC (International Electrotechnical Commission, IEC, 1994) and ISO (International Organisation for Standardisation, ISO, 1993) radiation qualities. This X-ray set has an X-ray tube comprising of a tungsten target and a 3 mm thick Be window; the facility produces a stabilised constant potential. The

beam ®lters and monitoring devices have been designed in accordance with guidelines of the ISO. (2) A Wolverson X-ray set (single phase, full wave recti®ed) with a Mo target, Be window and 0.03 mm Mo ®ltration, providing incident radiation spectra for mammographic calibrations, as speci®ed by the IPSM (Robertson et al., 1992). Our secondary standard laboratory is shown in Fig. 1, where the Pantak X-ray set is shown, and a typical diagnostic X-ray dosemeter is in place for calibration. The beam collimation and ®ltration system has been removed to reveal the in-beam dose monitor chamber on the front of the X-ray tube. Also shown on the left hand side is the array of ®lters for the beam qualities used routinely in the laboratory. There is a camera with in-room monitor which is coupled to a parallel display outside this room in the control area (not visible in the photograph). To the right of Fig. 1 are the high voltage generators for the X-ray set. Also present, but not visible in this ®gure is a laser alignment system and a ®xed metal track for accurate positioning of instruments.

3. Beam characterisation 3.1. Spectrometry It has become routine practice amongst laboratories providing X-ray calibration in the radiation protection ®eld to undertake photon spectrum measurements on the X-ray beam as a means of demonstrating compli-

Fig. 1. The RRPPS calibration laboratory.

S. Green et al. / Applied Radiation and Isotopes 50 (1999) 137±152

ance with standard parameters for these spectra. These beam qualities, for example the ISO Narrow series of radiations (ISO, 1979) are designed to have a single symmetrical peak around the mean energy, and hence may be speci®ed in terms of mean energy and characteristic width, as well as the more usual ®rst and second half value layer (HVL). Within the diagnostic Xray ®eld, agreement on suitable beam qualities for calibration is still not fully achieved. Dosimetry protocols have been issued which give broad indications of appropriate beam spectra (Robertson et al., 1992) but it was not until the issue of document IEC 1267 by the IEC, that beam spectra and appropriate methods of achieving them were established (IEC, 1994). IEC document 1267 was a major step forward in the ®eld of dosimetry for diagnostic radiology areas. It described realistic beam spectra for incident (entering the patient) and transmitted (exiting the patient) radiation qualities, and the ways in which the speci®ed ®rst HVLs could be achieved. These include both adjustment of the applied kVp, and the inherent ®ltration. However, because of the nature of diagnostic X-ray spectra, their shape are not conducive to speci®cation in terms of a mean energy and width as is possible for the ISO spectra mentioned above. Probably for this reason, it has not become routine practice for laboratories involved in dosimetry for diagnostic X-ray applications to undertake measurements of the actual photon spectrum used in the calibration. At the RRPPS we have taken the view that the only sure way of knowing the X-ray spectrum striking the detectors for which we provide calibrated irradiations, is to measure that spectrum. This is not a simple

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measurement to make since the incident beam intensities are such that direct measurement in-beam would exceed the count-rate capabilities of most nuclear spectroscopy detectors and electronics. However, such measurements have been made by a number of authors (Roberts, 1980; Matscheko and Ribberfors, 1987) using the Compton Scatter technique. At RRPPS we have used the scatter chamber shown in Fig. 2. This enforces a 908 scattering angle and uses a Perspex scatterer (low atomic number material being used for the scatterer to avoid the production of interfering Xrays). A hyperpure germanium energy dispersive detector is inserted into the scattering assembly as shown in Fig. 2. Once the germanium detector pulse-height spectrum has been collected, spectrum unfolding is required to account for the response function of the germanium detector, and the change in photon energy which results from the Compton scattering process (Matscheko and Ribberfors, 1987). Spectrum measurements have been made for both the ISO Narrow and a selection of the IEC incident radiation qualities from 50 to 120 kVp (IEC identi®ers RQR3, 4, 5, 7 and 9). The measured IEC beam qualities are shown in Fig. 3. The characteristic ¯uorescence lines from the tungsten target are clearly visible in the higher energy beam qualities, and the anticipated Bremstrahlung shape is reproduced. Such spectra are clearly non-symmetrical and not amenable to simple characterisation in terms of mean energy and width. There is little in the way of published recommendations for X-ray beams which are characteristic of those encountered in CT or mammography appli-

Fig. 2. The Compton Scatter chamber used for the photon spectrum measurements reported herein.

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Fig. 3. RRPPS measurements of a selection of IEC speci®ed incident radiation qualities.

cations. For mammography, beams have been suggested at 25, 30 and 35 kVp (Robertson et al., 1992) produced by Mo target X-ray sets with 30 mm Mo ®ltration. The IEC (1994) de®ne a mammography beam at 28 kVp, which while it is useful, it is a single point whereas the beams required for annual calibration of instruments should cover the kV range over which they may be used. In the absence of more de®nitive guidance we have performed a spectroscopic characterisation of the 25, 30 and 35 kV beams described by Robertson et al., and use them routinely for calibration purposes (Green et al., 1996) The spectra that have been measured are shown in Fig. 4. For CT, the more highly ®ltered beam qualities encountered fall somewhere between the HVLs for the IEC incident and transmitted radiation qualities. Further guidance is clearly necessary before de®nitive CT beam qualities are available.

3.2. Applied tube potential (kVp) There is a further role for photon spectrometry which we have found to be very useful, and that is in

the determination of the end-point of the photon spectrum to give an absolute measure of the applied kVp. This technique is not applicable in the ®eld since it requires careful geometrical alignment of pin-hole collimators, and the use of substantial added ®ltration. Both of these are used to reduce the incident beam intensity to a level which can be accommodated by a germanium spectrometer placed directly in-beam. Through such measures it possible to obtain the sort of information shown in Fig. 5, which with a simple straight line approximation (also shown on Fig. 5), can be used to obtain a measurement of the applied kVp. The accuracy of such a measurement is determined mainly by the uncertainties associated with determining the end-point of the spectrum (Green et al., 1996), but also by the available reference photon sources for calibration (ICRP, 1983). Such measurements have for many years (until the 1990s) been part of the Primary Standard for kVp maintained by the National Physical Laboratory (NPL). However, the kVp Primary Standard is now based solely on a measure of the applied voltage and is maintained relative to the NPL Primary DC Voltage Standard. It is possible in all measurement systems to have sources of systematic error which remain uniden-

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Fig. 4. RRPPS measurements of IPSM speci®ed incident radiation qualities for mammography.

ti®ed. However, the magnitude of such systematic errors can be assessed by making measurements with techniques which are suciently di€erent that they are not subject to the same sources of systematic error. Because of our desire to ensure that all factors which might have an impact on our determined kVps are

properly accounted for, measurements have been undertaken with a number of di€erent techniques as follows: (1) Direct measurements of the end-point of the photon spectrum as described above.

Fig. 5. Illustration of the kVp calibration technique, based on direct photon spectrometry.

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Table 1 kVp calibrations from a traceably calibrated non-invasive meter, direct spectrum end-point measurement and from a traceably calibrated invasive divider Nominal kVp

Calibrated Non-Invasive meter

Spectrum End-point

Calibrated Invasive meter

60 70 90 120

60.72 0.12 71.12 0.14 90.52 0.18 122.120.24

612 0.6 712 0.7 91.42 0.9 121.72 1.2

N/A N/A N/A N/A

26 30 35

26.32 0.05 29.62 0.05 34.72 0.05

26.02 0.8 29.32 0.8 34.42 0.8

26.420.07 29.720.08 34.720.09

(2) NPL calibration of a non-invasive kVp measurement device. (3) Direct kVp measurements using an instrument calibrated at a laboratory certi®ed by the U.K. Accreditation Service, UKAS (used for mammography kVps only). Comparison of the results from the ®rst two techniques for nominal kVps from 60 to 120 is shown in Table 1. The larger uncertainties on the RRPPS spectrometry results mean that the two sets of data are in agreement within the uncertainties (which are quoted at the 95% con®dence level). Also in Table 1 are results from the three techniques listed above when applied to kVps in the region used in mammography applications, from 25 to 35 kVp. Once again the agreement exhibited between the di€erent techniques is excellent, giving a high degree of con®dence that the true kVp is known accurately, and providing a limit to the magnitude of any systematic errors in the measurement process. 3.3. Dosimetry The general form of calibration traceability employed in dosimetry proceeds as follows. A high quality dosemeter is calibrated by a Primary Standard Laboratory (usually a National Standards Laboratory such as the National Physical Laboratory in the U.K.). This instrument, which through its calibration has become a Secondary Standard Dosemeter, is then used at the Secondary Standard Laboratory in a radiation beam which closely matches the speci®cation of the beam at the Primary Standard Laboratory, to transfer the calibration from the Secondary Standard Dosemeter to a Tertiary Standard Dosemeter. This Tertiary Standard Dosemeter is then used to de®ne the radiation exposure for instruments and dosimetry devices for which calibrated irradiations are to be provided. It is advantageous to be able to provide irradiations that are within the dynamic range encountered in working practice, within the con®nes of the discussion

above on appropriate conditions for calibration. With the Pantak HF-320, instantaneous air kerma rates are available that range from those typically encountered at the entrance to image intensi®er systems (around 10 mGy min ÿ 1) up to a maximum of 50 mGy s ÿ 1, being within an order of magnitude of the maximum encountered in clinical practice and approaching the performance limit of most test instrumentation used in diagnostic radiology (Wagner, 1992). The experience of others (Cerra, 1982; Wagner et al., 1988) together with manufacturers speci®cation of air kerma rate linearity (Radcal, 1991), suggest that the linearity of our Secondary Standard Dosemeter (a Radcal 1515 with 10X5-6 and 10X5-6M chambers should be within 25% up to an instantaneous air kerma rate of 70 mGy s ÿ 1. To ensure that the accuracy of our dosimetry is maintained over a very wide dynamic range, the calibration of our Secondary Standard Dosemeter includes a test of the linearity of response as a function of applied air-kerma rate. Measurements carried out by the PTB in Germany have demonstrated a linearity of better than 20.5% from 6 mGy min ÿ 1 up to 80 mGy s ÿ 1. The PTB quoted uncertainties on their standard calibration beam qualities are 21.2% at the 92% con®dence level (CL) for the RQR series of qualities, covering the photon energies of interest in general diagnostic radiology and 21.3% at the 92% CL for the MV beam qualities which have the same nominal kVps and HVLs as the IPSM mammography spectra listed in Table 2. Using this approach, and recent guidance on the assessment and propagation of uncertainties (UKAS, 1995) it is possible to transfer a calibration to a customer instrument with an uncertainty not exceeding 24% at the 95% con®dence level. In the preceding sections we have established the methods for obtaining complete characterisation of the RRPPS radiation ®elds in terms of the X-ray spectrum, the applied kVp and the applied air-kerma. It is therefore now possible to discuss the utility of such facilities in providing irradiations of patient dosemeters and QA instruments, and to perform intercomparison

S. Green et al. / Applied Radiation and Isotopes 50 (1999) 137±152 Table 2 Basic speci®cation for the IEC RQR Series and IPSM mammography beams implemented at RRPPS Code

Approximate Tube kV

Nominal ®rst HVL (mm of Al)

RQR 3 RQR 4 RQR 5 RQR 7 RQR 9 IPSM-I25 IPSM-I30 IPSM-I35

50 60 70 90 120 25 30 35

1.5 2.0 2.5 3.3 4.5 0.28 0.34 0.37

exercises between patient dosimetry services. These areas will form the remainder of this article. 3.4. Basis for performance assessment of patient dosimetry services In the U.K., in the ®eld of personal monitoring, Approved Dosimetry Services are legally de®ned and are required to meet certain standards on their performance (HSE, 1996a). Through such performance criteria it is possible to provide a meaningful assessment of the services which provide personal dosemeters through intercomparisons and the legally required ``Performance Tests'' (HSE, 1996b). The situation is less clear in patient dosimetry, but it is worth considering the guidance given in the joint IPSM/NRPB/CoR report (NRPB, 1992) which suggests that individual dose assessments should have uncertainties associated with them which meet the following speci®cations at the 95% con®dence level for entrance surface doses above 0.1 mGy: Non-random uncertainty Random uncertainty Overall uncertainty

210% 220% 225%

These are requirements on the uncertainty estimates which are associated with an individual dose assessment, and therefore relate directly from a summation of component uncertainties from all the elements which go together to make a radiation exposure, and measurement of light output from, a TLD. They are not intended to be performance bands to be applied to the results of test irradiations undertaken as some kind of blind-test of a Patient Dosimetry Service. However, it is our intention in this part of the article to report the results of two blind intercomparison exercises which have been performed under the auspices of the Personal Radiation Monitoring Group (PRMG). In order to assess the results of such an intercomparison

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exercise, it is useful to have some indicator against which the performance of each centre may be judged. It is probably not statistically correct to compare the speci®cations mentioned above, which relate to uncertainties on any individual measurement, to the mean values derived from intercomparisons such as those described here. However, for the purposes of discussion these speci®cations have been used as follows: Non-random uncertainty is compared with the Bias, that is the di€erence of the mean measured dose to the delivered dose, expressed as a percentage of the delivered dose. This comparison can be made for each participant, at each of the beam qualities used in the intercomparison exercise. Random uncertainty is compared with the two times relative standard deviation at each dose point (see Eq. (1)), i.e. random uncertainty is compared with P 2 1=2 n i …Di ÿ Dn† 2  100  P …1† nÿ1 i Di where in a batch of n dosemeters D i is the ratio of the reported to the true dose for dosemeter i, and Dn is the mean ratio. Once again this comparison can be made for each participant and each irradiating beam quality. It is useful to compare these requirements with the practical limits on the achievable accuracy in beam dosimetry using standard chains of traceability as described above. This process would tend to yield an overall uncertainty at the 95% con®dence level of around 4%. Under some testing criteria used in Personal Monitoring (DOE, 1986), it is possible to subtract the uncertainty on the delivered air kerma from the observed variation in dosemeter response, BEFORE applying the testing limit (10% or 20% above). This has not been done in the exercises described here.

4. Intercomparison exercises for TLD-based dosimetry services in the U.K. 4.1. The PRMG and the participants in the intercomparisons The work of the Personal Radiation Monitoring Group (PRMG) has developed since its inception in 1980, to make it a valuable forum for discussion of problems in personal monitoring from the detailed technical level such as ®lm processing conditions, to the more scienti®c areas of calibration reference phantoms and dosemeter response correction algorithms. The PRMG now has 14 full members, and includes a number of major services from outside the NHS. Membership is limited to those organisations which

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provide the service from the U.K. and Ireland. The group now contains 11 services which provide ®lm whole body dosemeters, and three o€ering TLD for the assessment of whole body dose. In addition, the group includes 14 services providing dose assessments for body extremities (e.g. hands). All of these use TLD. The group has been very successful in promoting collaboration between its members, encouraging open and wide-ranging discussion. As a result of these e€orts, the group has been able to perform the 6monthly intercomparison exercises which involve all members and which began in the early 1980s (before such dialogue was commonplace in the radiotherapy ®eld). We can now count on the results of around 30 intercomparison exercises for whole body dosimetry, which can give us powerful data in assessing the performance of current dosimetric technologies when faced with revised dose limits and new dosimetric challenges. This culture has been very valuable in the execution of the exercises described in this article. Not all of the PRMG members are active in patient dosimetry, so the numbers of participants was lower than would normally be the case for the other exercises undertaken by the PRMG. However, the results of two intercomparison exercises are presented here which were undertaken in 1995 and 1996. For the ®rst exercise, 6 services submitted dosemeters, and for the second the number of participants increased to 8. These have been referred to in this article by the number allocated to each centre for the purposes of other intercomparisons organised by the PRMG, and the results are therefore presented anonymously. The centres involved consisted of the medical physics departments in the following locations: Aberdeen, Belfast, Birmingham, Cambridge, Cardi€, Manchester, London (St Bartholomew's Hospital and University College Hospital). 4.2. Intercomparison arrangements Each service provided 27 dosemeters to RRPPS for irradiation to a beam quality and air kerma which were not known to the issuing centre. 25 dosemeters were irradiated and two used as transit control dosemeters. The dosemeters provided by most participants were either LiF chips (TLD-100) or lithium borate chips, and were therefore re-useable dosemeters which had previously been through some sort of calibration and selection procedure according to the local practice of each participating centre. There was one exception to this which was the dosemeter submitted by centre 11. This was the LiF ``Extremity Tape'' dosemeter supplied by NE Technology, which is not a re-useable dosemeter and can therefore not be subject to prior tests of reproducibility or to individual calibration. In

fact these dosemeters were submitted with the intention of providing data on their performance under diagnostic X-ray conditions, and were not part of an active service for patient dosimetry. The intercomparison involved irradiations at RRPPS, Birmingham. A single dose point for each of 5 di€erent kVp settings was used, with 5 dosemeters exposed per dose point (25 dosemeters per centre in total). Centres were not told which dosemeters had been irradiated at which kVp, so no energy response corrections could be made. The details of the irradiating beam spectra are shown in Table 2. Irradiations were performed in conditions which approximated to being free in air, but actually involved the use of Styrofoam2 backing material. The dosemeters were spread over an area of diameter 10 cm at the centre of an irradiating beam of 15 cm diameter. All dosemeters which received the same applied air kerma were irradiated simultaneously, except for those from centre 1 in the second (1996) exercise, which were irradiated one month later. 5. Results and discussion As mentioned above, the data presented for centre 11 results from the use of a disposable dosemeter and is not representative of an active patient dosimetry service. These results will not enter further into our discussions, except to note that it may be possible, with an appropriate calibration reference energy which would dramatically reduce the bias for centre 11 shown in Tables 3 and 4, to meet the criteria for bias with this dosemeter. However, the dosemeter produces results which are on the limits for random uncertainty speci®ed above, falling marginally outside these limits on a number of occasions. Hence overall our results suggest that its use for patient dosimetry is not to be recommended. Before consideration of the results of the remaining centres, it is interesting to note that out of approximately 320 dosemeters used in the two exercises, there was one complete failure, and a further 4 readings (for centre 10 in the 1995 exercise) where the assessed dose is signi®cantly lower than the anticipated ®gure. The results for centre 10 are clearly unusual and the cause was addressed before the 1996 exercise where such rogue results are not present. Hence there is a rate of complete failures of 1 in 320. For the remaining centres (that is excluding centre 11), neglecting the e€ect of the rogue readings mentioned above, it is clear that at almost all irradiation points, the random uncertainty criteria are met. The single exception to this is in the results of centre 10 in the 1996 exercise, where while the presence of individ-

S. Green et al. / Applied Radiation and Isotopes 50 (1999) 137±152

147

Table 3 Results for the 1995 Intercomparison Exercise of Patient Dosimetry Services organised by the PRMG Centre No. kVp/applied air kerma 50 kVp/12.16 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) 60 kVp/11.17 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%) 70 kVp/11.61 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%) 90 kVp/12.17 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%) 120 kVp/11.75 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%)

1

5

7

10

11

12

11.72 12.78 11.67 11.59 12.14 11.98 0.99 ÿ1 8

11.88 11.41 12.54 12.65 12.94 12.28 1.01 1 10

11.50 10.60 11.20 10.30 12.60 11.24 0.92 ÿ8 16

10.46 12.24 11.99 11.73 12.58 11.80 0.97 ÿ3 14

16.46 16.87 20.36 15.29 19.13 17.62 1.45 45 23

12.02 11.99 11.92 12.01 12.47 12.08 0.99 ÿ1 4

11.55 11.83 11.55 11.74 10.80 11.49 1.03 3 7

10.12 10.99 9.57 10.88 9.97 10.31 0.92 ÿ8 12

10.90 10.50 failure 10.50 11.20 10.78 0.96 ÿ4 6

10.88 10.62 11.56 5.78 12.49 10.27 0.92 ÿ8 51

16.41 18.42 15.65 15.98 13.84 16.06 1.44 44 20

10.74 11.07 11.61 10.73 11.07 11.04 0.99 ÿ1 6

11.08 11.29 11.35 11.09 12.00 11.56 1.00 0 8

12.00 11.35 11.75 10.25 10.29 11.13 0.96 ÿ4 15

10.50 12.00 10.50 11.70 11.30 11.20 0.96 ÿ4 12

12.40 12.50 6.80 12.58 11.22 11.10 0.96 ÿ4 44

17.93 18.25 15.82 17.04 15.85 16.98 1.46 46 13

11.48 12.16 12.39 12.51 10.84 11.88 1.02 2 12

12.25 12.92 11.91 12.71 10.41 12.04 0.99 ÿ1 16

11.45 10.60 10.19 11.61 11.07 10.98 0.90 ÿ10 11

10.5 11.5 10.9 12.1 10.6 11.12 0.91 ÿ9 12

12.41 11.99 11.56 9.86 13.18 11.80 0.97 ÿ3 21

18.71 16.75 15.01 18.63 17.05 17.23 1.42 42 18

11.97 11.59 11.26 12.78 11.97 11.91 0.98 ÿ2 10

10.44 10.52 10.89 10.86 11.60 10.86 0.92 ÿ8 8

11.79 9.96 11.35 11.27 10.59 10.99 0.94 ÿ6 13

11.6 10.8 10.2 11.3 10.8 10.94 0.93 ÿ7 10

8.92 11.65 13.00 11.22 11.30 11.22 0.95 ÿ5 26

17.19 14.17 15.35 16.80 17.35 16.17 1.38 38 17

10.96 12.34 11.85 11.00

ual results which departed signi®cantly from the expected value has been eliminated, there remains a random uncertainty which is greater than exhibited by

11.54 0.98 ÿ2 12

the other centres and which occasionally fails to meet the 20% target ®gure for random uncertainty. For bias or non-random uncertainty, the ®gures in Tables 3 and

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S. Green et al. / Applied Radiation and Isotopes 50 (1999) 137±152

Table 4 Results for the 1996 Intercomparison Exercise of Patient Dosimetry Services organised by the PRMG Centre No. kVp/applied air kerma 50 kVp/10.79 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%) 60 kVp/10.92 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%) 70 kVp/11.37 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%) 90 kVp/11.85 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%) 120 kVp/12.00 mGy

Mean assessed air kerma Mean ratio (assessed/true) Bias (%) 2 relative standard deviation (s n ÿ 1) (%)

1

2

4

5

7

10

11

12

11.63 11.00 9.76 9.52 10.12 10.41 0.96 ÿ4 17

11.00 11.60 10.50 11.00 10.40 10.90 1.01 1 9

10.49 10.38 10.05 9.83 10.49 10.25 0.95 ÿ5 6

10.54 10.54 10.48 11.08 10.87 10.70 0.99 ÿ1 5

10.80 11.10 10.60 11.10 11.50 11.02 1.02 2 6

12.3 11.3 10.6 11.6 10.2 11.20 1.04 4 15

16.22 18.74 16.82 15.93 15.65 16.67 1.55 55 15

11.06 11.61 10.55 11.11 10.87 11.04 1.02 2 7

10.73 11.01 10.56 10.79 10.35 10.69 0.98 ÿ2 5

11.70 11.00 10.70 12.60 9.81 11.16 1.02 2 19

10.72 10.66 10.10 10.24 10.24 10.39 0.95 ÿ5 5

9.94 9.97 10.72 9.59 10.63 10.17 0.93 ÿ7 10

11.50 11.30 11.30 11.00 10.30 11.08 1.01 1 9

11.6 9.3 11.3 10.3 13.0 11.10 1.02 2 25

16.18 15.72 19.00 19.48 19.46 17.97 1.65 65 21

11.18 11.92 10.51 10.61 10.90 11.02 1.01 1 10

12.11 10.22 10.66 10.33 10.60 10.78 0.95 ÿ5 14

13.10 11.20 12.50 12.20 11.60 12.12 1.07 7 12

11.22 10.90 11.01 10.57 10.44 10.83 0.95 .5 6

10.96 11.78 10.33 10.90 11.07 11.01 0.97 ÿ3 9

10.30 11.30 11.10 11.20 11.40 11.06 0.97 ÿ3 8

11.6 12.9 10.4 9.8 10.2 10.98 0.97 ÿ3 23

20.12 20.18 16.23 17.33 19.04 18.58 1.63 63 19

11.75 12.60 11.99 11.67 11.52 11.91 1.05 5 7

10.65 11.30 10.74 10.45 10.02 10.63 0.90 ÿ10 9

12.90 12.10 12.90 11.70 12.40 12.40 1.05 5 8

10.71 10.75 11.34 11.04 11.03 10.97 0.93 ÿ7 5

11.39 10.95 12.40 11.15 11.12 11.40 0.96 ÿ4 10

12.20 11.30 11.30 11.40 11.00 11.44 0.97 ÿ3 8

13.4 11.2 12.6 10.1 12.3 11.92 1.01 1 22

17.37 16.21 17.80 21.96 18.57 18.38 1.55 55 24

12.01 12.03 13.41 12.52 11.70 12.33 1.04 4 11

11.27 10.59 11.26 10.23 10.85 10.84 0.90 ÿ10 8

12.40 13.20 13.00 12.90 10.90 12.48 1.04 4 15

10.46 10.75 10.88 10.49 10.53 10.62 0.89 ÿ11 3

10.78 10.06 11.27 11.99 11.52 11.12 0.93 ÿ7 13

11.00 10.70 11.00 11.40 11.50 11.12 0.93 ÿ7 6

13.7 14.1 14.6 14.4 14.2 14.20 1.18 18 5

18.80 17.70 19.99 17.81 19.28 18.72 1.56 56 10

11.96 12.63 11.97 12.62 12.29 12.29 1.02 2 5

4 show that at almost all dose points the criteria are met with the exception of the 120 kVp beam quality for centre 4 in the 1996 exercise.

Such ®ndings are the main bene®t of intercomparison exercises such as these, which provide an independent audit of the working procedures in a laboratory,

S. Green et al. / Applied Radiation and Isotopes 50 (1999) 137±152

and provide evidence on which to base some modi®cations in those procedures. One interesting point is that the only centre using lithium borate TLD is centre 2. Because of the low variation in the response with energy of lithium borate TLD, it is often considered superior to LiF (as used by all other centres). There is perhaps some evidence in Table 4 that the bias for centre 2 does not become negative (or more negative) at the higher kVp beam qualities. Hence the dosemeters used by centre 2 exhibit a smaller change in response with energy than those used by other centres. We might anticipate that this trend will become clearer in future intercomparisons. Before leaving the results of these intercomparisons, it is perhaps worth examining further the recommendations on uncertainties outlined above (NRPB, 1992). These are that no single dose measurement shall have uncertainties which exceed the ®gures stated above. If we examine more closely the results for centre 5 at 60 kVp in the 1996 exercise, we can extract the data shown in Table 5 from Table 4. Here the ``bias'' ®gures for individual dose measurements have been calculated and it is clear that we were being a little generous by considering the mean bias over ®ve dose assessments in the discussion to this point. If we were to strictly re¯ect the aspirations expressed in the IPSM/NRPB/CoR document, we would ®nd that the number of ``failures'' increases considerably. The completes our discussion of the intercomparisons of patient dosimetry services undertaken by the PRMG. For the remainder of this article we shall discuss the calibration and general performance of instruments used for beam calibration in diagnostic X-ray departments. 5.1. Characterisation of X-ray quality assurance instruments arriving at RRPPS The annual calibration of a wide variety of radiation measurement equipment is now a major activity for the RRPPS. The underlying work on dosimetry and kV determination described earlier in this article per-

Table 5 Further analysis of the results for one participating centre (centre 5) at 60 kVp in the 1996 Intercomparison Exercise Assessed air kerma (mGy)

Ratio of air kermas (Assessed/True)

Bias (%)

9.94 9.97 10.72 9.59 10.63

0.01 0.01 0.01 0.01 0.01

ÿ9 ÿ9 ÿ2 ÿ12 ÿ3

149

forms a sound basis on which we are able to transfer accurate and reproducible calibrations to the instruments which are sent to us. Our customers are predominantly from the U.K., but also include countries in mainland Europe. In fact since the compilation of the data described in this paper, our customer base has extended to Africa and Malaysia. We are able to make an assessment of the instruments sent to us for calibration to give at least some indication of the general status of instruments in current use. The data displayed in Figs. 6 and 7 have been derived from all instruments submitted to RRPPS for calibration in the 12 months from 1st May 1994 to 30th April 1995. Instruments have been divided into two broad categories, dosemeters and kVp meters, and no attempt has been made to further sub-divide the instrument according to type or manufacturer. Instrument response is judged against the expected performance as stated by the manufacturer. This is typically 22% for kVp meters and 25% for dosemeters, but may di€er for older instruments or from one manufacturer to another. For the diagnostic radiology kV range (50± 120 kVp), uncertainties at the 95% con®dence level are: delivered air kerma rates applied kVp

24% 21%

All instruments are calibrated in the reference radiation beams de®ned in IEC document 1267, and for which the basic parameters are shown in Table 2. Each of the pie charts shown in Figs. 6 and 7 is based on over 100 instruments. They have been subdivided into categories according to their performance on arrival at RRPPS as indicated on the ®gures. It is clear that for both types of instruments, only around 55% of the instruments arrived in a condition whereby they operated within the speci®cation laid down by the manufacturer. A further 25% (again similar for both categories) were instruments which were operational, but required adjustment to meet their speci®cation. It is this latter category which are of most concern since they could possibly yield misleading or inaccurate results during routine use. The value of an annual calibration is clearly demonstrated by these charts. The further (smaller) categories relate to instruments which arrived at RRPPS in a non-functional state and therefore required some kind of repair. We are also able to evaluate mean ®gures which describe the performance of each type of instrument. These ®gures relate only to those instruments which arrived at RRPPS in a working condition and for which no adjustment was necessary. Those instruments

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S. Green et al. / Applied Radiation and Isotopes 50 (1999) 137±152

Fig. 6. The performance characteristics for dosemeters submitted to RRPPS for calibration.

which were adjusted by RRPPS will, by de®nition, meet their speci®ed tolerance criteria. For simplicity of presentation we shall deal only with data derived at 70 kVp and hence only consider instruments which required calibration at this energy. DosemetersMean Response at 70 kVp 1.005 standard deviation (s n ÿ 1) 2.4% kVp meters Mean correction factor at 70 kVp ÿ 0.20 kV standard deviation (s n ÿ 1) 0.42 kV The data above demonstrates that if this group of instruments are taken as a single population, the per-

formance of the group exhibits mean values which are very close to those expected, and standard deviations which are small. Similar analysis has also been performed for instruments used in mammography applications calibrated by the RRPPS in the months from April 1995 to March 1996. Calibrations are performed in the IPSM mammography beam qualities for which the basic parameters are shown in Table 2. These results are not presented graphically but on the basis of approximately 60 kV meters and 15 dosemeters, we can calculate performance ®gures as shown below: Dosemeters

Mean Response at 30 kVp

Fig. 7. The performance characteristics for kVp meters submitted to RRPPS for calibration.

1.00

S. Green et al. / Applied Radiation and Isotopes 50 (1999) 137±152

kVp meters

standard deviation (s n ÿ 1) Mean correction at 30 kVp standard deviation (s n ÿ 1)

2.9% ÿ 0.11 kV 0.48 kV

151

Acknowledgements The authors would like to express their thanks to Dr Ahmad Shukri for his useful input to the discussions in RRPPS on phantoms, and to the Commonwealth Foundation for funding his sabbatical in Birmingham.

References The results once again show small mean values and standard deviations, but as for the diagnostic X-ray instruments they are a selected population (i.e. they are those instruments which were found to meet their speci®cation on arrival at RRPPS). However the low standard deviations on the populations do give some con®dence that the calibration metrology is applied with a high degree of consistency in RRPPS.

6. Summary and conclusions This article has attempted to review the prevailing recommendations for the provision of dosimetric calibration in diagnostic radiology. It seems that there is much to be learned from the debates which for many years have occupied scientists who have or continue to work in personal monitoring. In particular there is the question of the nature of the calibration to be provided, with emphasis now being placed on accurate metrology rather than on simulation of ®eld conditions. The use of phantoms as a part of the reference conditions for calibration is not yet established, and such phantoms have not been used in the work described here. The results of intercomparison exercises for patient dosimetry services performed by the PRMG have been described. The usefulness of such exercises in ensuring both the accuracy and consistency of patient dosimetry is self-evident, but there is a clear need for performance criteria against which to judge the dose assessments of the participating centres. Overall it is found that the services which took part in the exercises which we have described, were for the most part achieving dose assessments with a bias and a relative standard deviation of less than 10%. The performance characteristics of dosemeters and non-invasive kVp meters submitted to RRPPS for annual calibration have been reviewed in an attempt to ascertain the likely performance of instruments in routine use. It is found that for both dose and kVp meters, as many as 25% of instruments in routine use in the U.K., require some adjustment before they can be truly said to be performing as the manufacturer intended.

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