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Commentary |
National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ, UK
After the publication of a National Protocol for Patient Dose Measurements in Diagnostic Radiology [1] in 1992, NRPB established a National Patient Dose Database (NPDD) to collate measurements of radiation doses to patients from routine X-ray examinations in hospitals throughout the UK. The purpose of the NPDD was to monitor trends in patient doses and to update and extend the national reference doses recommended in the National Protocol. More recently it has provided essential data for an estimate of the UK population dose from medical X-ray examinations [2], and for establishing diagnostic reference levels (DRLs) as required by British legislation [3]. A first review of data collected up to the end of 1995 was published in 1996 as an NRPB report [4]. A second review covering the subsequent 5-year period from January 1996 to December 2000 was published in 2002 [5]. This article summarizes the findings of the second review, and provides guidance on the information needed for subsequent reviews.
Second review of the database (1996–2000)
For the second review, 54 individuals working in radiology or medical physics departments kindly supplied information on dose measurements made at 371 hospitals from every region of the UK. 50 of these hospitals were in the independent sector and the rest were in the NHS. A wide range of hospital sizes was covered; from out-patient clinics to hospitals with over 1000 beds. Dose measurements were supplied in three ways: entrance surface dose (ESD) per radiograph; dose–area product (DAP) per radiograph; and DAP per complete examination. A much greater number of dose measurements was provided for the second review (181 000) than for the first review (52 000), covering many more types of examination.
To evaluate typical doses for each type of radiograph or examination, the mean dose for a sample of standard-sized patients in each radiology room is required. In order to reduce the probability that the mean doses per room for adults were biased by patients being predominantly larger or smaller than average, the data were selected prior to analysis. Doses measured on a sample of adults in a specific room were included only if the mean patient weight was in the range 65–75 kg, or, if patient weights had not been provided, if there were measurements on at least 10 adults in that room. With at least 10 patients being measured, it would be unlikely that they would all be over- or under-weight. All doses measured on children were accepted if there was also sufficient information on their size, e.g. height and weight, to allow the doses to be adjusted to those for standard sized children according to the methods published by NRPB in 2000 [6].
Results from the current review indicate a continuing downward trend in doses to adults. Generally, there has been a reduction since the 1995 review of about 16% in the mean dose for common radiographs and examinations.
The main purpose for the NPDD has become the provision of national reference doses for common X-ray examinations, based on the rounded third quartile of room mean doses. The latest reference doses are approximately half the size of those established in the mid-1980s that are quoted in the National Protocol [1], and are about 20% lower than those derived from the first review. As well as revising existing reference doses, the second review has made full use of the increased quantity of data by recommending additional national reference doses. For adults, these are in terms of DAP/radiograph, DAP/examination, and fluoroscopy time/examination, and for children in terms of DAP/examination. Table 1
shows adult reference doses for radiographs in terms of both ESD and DAP. Table 2 shows the latest adult reference values for complete examinations in terms of both DAP and fluoroscopy time. DAP is the preferred quantity because it takes account of the dose from both radiography and fluoroscopy. However, for those procedures involving little or no radiography and where no DAP meter is fitted, the fluoroscopy time is a practical alternative. Table 3 shows paediatric reference doses which have been derived using the methods described in report NRPB-R318 [6]. These reference doses are set for standard sizes of children corresponding to the ages shown. By comparison with the corresponding adult reference doses in Table 2, it can be seen that the reference doses for a 15-year-old are about a factor of two or three below those for an adult.
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have not been formally adopted as DRLs, as defined in the Ionising Radiation (Medical Exposure) Regulations (IR(ME)R) for Great Britain [3]. A Department of Health working party on DRLs for IR(ME)R was set up in 2000 and has the authority to decide which of these recommended reference doses to adopt as national DRLs for use with IR(ME)R. The working party could also consider any other relevant sources of national patient dose data when approving DRLs. Detailed practical guidance on the establishment and use of DRLs is being prepared by a joint working party organized by the Institute of Physics and Engineering in Medicine (IPEM), with representatives from the British Institute of Radiology, the College of Radiographers, the Royal College of Radiologists, and NRPB. The IPEM working party has previously published some preliminary guidance [7–9], and will publish the full guidance as an IPEM report in 2003. More information and discussion of the methods and results of this second review of the NPDD are given in reference [5]. The report has been published in full on the NRPB website, and can be freely downloaded. We wish to thank those people who contributed data and thus made the review possible. NRPB intends to continue to review the NPDD every 5 years, to provide regular updates and additions to national reference doses. Some new guidance for potential contributors is given in the following section, to encourage a continuing supply of relevant data.
Guidance for contributors to the national patient dose database
The original guidance for the contribution of data to the UK NPDD was provided in the National Protocol for Patient Dose Measurements in Diagnostic Radiology [1]. This 35 page document is still worth reading for its extensive discussion of the choice of dose quantities to be measured, and the practical techniques of dose measurements. However, during the last 10 years there have been a number of developments in imaging equipment and procedures, which have necessitated some revision of the guidance. It is recommended that contributors of data refer to NRPB report W14 [5] to see what use is made of the data provided.
Types of radiograph and examination
In the past, any suitable UK data supplied to us has been entered into the NPDD. We shall continue to do this, but those types of radiograph and examination which are of special interest are discussed below in order of priority.
Radiographs and examinations that already have national reference doses
There will be a continuing need to update these (see Tables 1–3). Although currently a reference dose is given for biliary drainage/intervention (since the dose for each of these was found to be similar) it would be preferable to provide separate doses for biliary drainage and biliary dilation/stenting. Although a reference dose is given for femoral angiograms, this is an ambiguous term which could either refer to angiography in the thigh or to angiography in the abdomen using catheter access through the femoral artery. The use of a descriptor which gives the anatomical location of most of the angiography would be preferable. For venograms of the leg, it would be helpful to note whether one or both legs were examined.
Doses for common radiographs on children
Examples of these are radiographs of the chest, abdomen, pelvis or skull. ESD or DAP values (measured or calculated from the exposure factors) are acceptable. Information must also be included on the size of each child: either the thickness of the body part being X-rayed; or both the height and weight of the patient.
Examinations which make a major contribution to collective dose
Of the 15 largest contributors to collective dose listed in NRPB-W4 [2], 6 already have reference doses, and 5 are CT examinations. NRPB is planning a new national survey of CT practice and patient doses in 2002/03. A separate national patient dose database for CT examinations will be established to enable national reference doses for CT to be formulated. Readers are encouraged to submit data to NRPB when requested. The four remaining procedures which make a major contribution to collective dose are: hip, mammography, percutaneous transluminal coronary angioplasty (PTCA) and peripheral arteriography. It is not intended to include doses from mammography in the database, since the NHS Breast Screening Programme already collects and analyses these. For coronary angioplasties, the number of artery dilations performed and the number of stents fitted should be provided to indicate the complexity of the procedure, and allow the comparison of patient doses for procedures of a similar complexity.
Other common high-dose procedures
Procedures performed at a rate in excess of 10 000 per year in the UK, and with effective doses greater than 4 mSv [2], include: angioplasty; vascular stenting; renal arteriography; and endoscopic retrograde cholangio-pancreatography (ERCP). For non-coronary angioplasty and vascular stenting, it is essential that information be given on the precise anatomical location of these procedures, e.g. carotid angioplasty. ERCPs should be divided into purely diagnostic examinations or interventional procedures.
Rare but very high dose procedures
Procedures with effective doses greater than 10 mSv include (starting with the most frequent) embolisation, radiofrequency cardiac catheter ablation, kidney stenting, mesenteric angiography and transjugular intrahepatic portosystemic shunt (TIPS).
Table 4 lists, in order of priority, all those radiographs and examinations which are of especial interest for the NPDD, but which are not listed in Tables 1–3.
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Data provision
The details requested for the database (on the patient, examination technique and imaging equipment) were originally specified in the forms in Appendix D of the National Protocol. They have been regularly updated, to cover developments in imaging equipment. The latest forms, which list the data that are currently of interest, are included with this guidance (Appendix). Essential items are highlighted, but contributors are encouraged to provide as much of the other information as possible. It is intended to put the forms on the NRPB website, from where they may be freely copied for the purpose of data collection.
Data are best supplied to NRPB in electronic format, e.g. as an Excel spreadsheet. In such a format they can be e-mailed or sent on a floppy disc. Data on paper are also acceptable, but the risk of transcription errors when entering information into the database is greater, so electronic formats are preferred.
All radiology departments are required to perform regular audits of patient doses by the Ionising Radiations Regulations 1999 [10] and IR(ME)R [3]. After using that information in local quality assurance programmes to check compliance with local DRLs, its value will be greatly increased by passing it on to the NPDD. Only then will the NRPB's 5-yearly reviews and national reference doses truly reflect current UK practice and provide reliable data to guide the setting of local and national DRLs as required by IR(ME)R. Any data submitted will be treated confidentially, and it will not be possible to identify the performance of individual hospitals in any publications about the database. Please send your data to David Hart at NRPB (e-mail: david.hart{at}nrpb.org, telephone: 01235 822647). With your continuing support for the NPDD, national trends in radiology practice and patient doses will continue to be monitored, and the UK's enviable reputation at the forefront of radiological protection of patients will be maintained.
DATA REQUESTED FOR THE NATIONAL PATIENT DOSE DATABASE
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Received for publication November 21, 2002. Revision received March 27, 2003. Accepted for publication April 14, 2003.
References
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