Diagnostic Reference Levels Working Party Statement

It was further agreed that National DRLs would be reviewed at five-yearly intervals, and that individual medical physics departments and hospitals carrying out programmes of patient dosimetry in diagnostic radiology should be strongly encouraged to contribute data to NRPB for national collation. The list of examinations for which there are national DRLs will be extended when sufficient data on UK practice has accrued.

2. Local DRLs
IR(ME)R requires the Employer to establish DRLs for radiodiagnostic examinations. Employers should adopt a set of DRLs, having regard to national and European DRLs where available. In the first instance, the examination types and DRL values adopted can be those of the established national DRLs. Where a national or European DRL is not available for an examination type, there is no requirement to set a DRL locally for this examination. Any relevant local patient dosimetry data should be reviewed to identify examinations where established local practice will support the adoption of a DRL value lower than the equivalent national DRL. The local adoption of a DRL which is higher than the respective national value will need to be justified. For example, a case-mix which consistently requires examinations of greater duration and complexity than the norm may justify a higher patient dose. However, a local DRL higher than the equivalent national value cannot be justified solely on the grounds of the use of poor equipment and/or techniques. A hospital or Trust Radiation Protection Committee, Medical Exposures Committee or their equivalent would be a suitable forum for ratifying locally adopted DRLs.

Local adoption of DRLs makes employers responsible for the level at which the DRLs are set, in line with the concept of Clinical Governance. It is important to note that this does not mean that individual Trusts must derive their own DRLs from their own locally measured patient doses. Local measurements from one Trust may not produce statistically valid DRLs.

The Department of Health working party meeting in January 2000 agreed that local DRLs should be reviewed annually. Annual review of DRLs is intended to provide a formal mechanism for revision of locally adopted DRL values which may follow revised or new national DRLs, or additions to local patient dose data. Where examination protocols have been changed, the effect on the locally adopted DRLs should be considered, but it is not intended that this review requires annual patient dose surveys.

3. Reviews triggered by DRLs being 'consistently exceeded'
Employers are required to undertake a review if a DRL is ‘consistently exceeded’. Because of the known wide variability in doses between individual patients for the same type of examination, DRLs are defined as dose levels for typical examinations for groups of standard-sized patients (or standard phantoms). Therefore comparing the dose to an individual patient with a DRL has limited value. Rather, the distribution of doses on a representative group of close to standard-sized patients (or on a standard phantom) should be considered. For example, the mean value of this distribution can be compared with the DRL to determine whether a DRL is being ‘consistently exceeded’.

Regulation 32 of IRR 99 requires that a suitable quality assurance programme be provided for equipment used for medical exposures, which should include periodic measurements of representative doses to samples of average size patients. The basis of a suitable quality assurance programme is outlined in IPEM Report 77 [2]. This incorporates national recommendations for patient dosimetry [3] which state that measurements should be made at least every 3 years on each piece of equipment or whenever changes are made to equipment or procedures that are likely to significantly affect patient dose. The Working Party believes that these periodic patient dose assessments required by IRR 99 can usefully be used to determine whether DRLs are being consistently exceeded.

The average dose to a group of standard-sized patients measured by such surveys should be compared to the respective locally adopted DRL. Where the DRL is exceeded, the employer must instigate a review of local practice to establish reasons and implement corrective action, where appropriate. It is expected that this ongoing audit of compliance with DRLs can be achieved in most Trusts by the existing rolling programme of 3-yearly patient dose measurements. This patient dose assessment programme is distinct from the annual review of locally adopted DRLs discussed in section 2.

The Working Party intends to consider the application of DRLs to other types of x-ray examination, and to provide practical guidance on the required review processes.

References
Doses to Patients from Medical X-ray Examinations in the UK - 1995 Review. NRPB R289, 1996
Recommended standards for the routine performance testing of diagnostic x-ray imaging systems. IPEM Report 77, 1997
National protocol for patient dose measurements in diagnostic radiology. IPSM, CoR and NRPB 1992

From IPEM website