Guidance

National Diagnostic Reference Levels (NDRLs): process to generate, adopt and maintain

Updated 20 November 2024

This page outlines the process for the generation, adoption, and maintenance of UK National Diagnostic Reference Levels (NDRLs). Specifically, it presents the steps by which reference dose index values from national dose surveys can become NDRLs.

In 2016, the Department of Health, now the Department of Health and Social Care (DHSC), assigned the formal adoption of NDRLs to the UK Health Security Agency (UKHSA) NDRL Working Party, and the publication of these values to UKHSA. Note, this was via UKHSA’s predecessor organisation, Public Health England.

The UKHSA NDRL Working Party

The UKHSA Working Party on National Patient Dose Surveys and NDRLs exists to support the process for adoption of new NDRLs, to ascertain needs for national values in new or existing areas, as well as new methods for acquiring data, and in addition to provide overall guidance in this area.

The working party (WP) was established in 2014. It consists of representatives from UKHSA, radiology professional bodies, experts in dose audits or dose data extraction, the DHSC and national regulators. Representatives of national dose audits that follow the process outlined here are also invited to attend relevant meetings.

The professional bodies currently represented on the working party are the British Institute of Radiology (BIR), the Institute of Physics and Engineering in Medicine (IPEM), the Society and College of Radiographers (SCoR), the Royal College of Radiologists (RCR), Society for Radiation Protection (SRP), and the British Institute of Cardiovascular Imaging (BSCI).

Purpose

UKHSA continues to undertake national dose audits for routine examinations on core modalities in diagnostic imaging. However, to enable a wider range of NDRLs to be established, a formal process has been developed to also incorporate results from other suitably run national dose surveys.

The additional surveys may be for specific modalities, examinations, or procedures; where the dose audits are run by experts in their field, ideally undertaken through a professional body, and following certain guidelines and principles.

The process has already been successfully implemented for:

• coronary computed tomography (CT) angiography
• CT in nuclear medicine: positron emission tomography-computed tomography (PET-CT) and single-photon emission computed tomography (SPECT-CT)
• Radiotherapy Planning CT
• screening mammography
• paediatric CT

This document sets out an overview of the process to enable the results to be adopted as NDRLs. The adopted values will then be published on the UK NDRL webpage.

Overview of process

First steps

A national dose survey is required to establish dose values which are representative of UK practice. The survey would be run either by UKHSA, or ideally by a professional body, and incorporate a multi-disciplinary team (whether as core team members, or in advisory roles).

The choice of modality or examination may be based on a lack of any existing NDRLs for that specific purpose, exam or modality, or a need to update existing values. For any modality the choice of exam is often guided by high doses or high frequency. Specialist exams that are complex are often not easy to incorporate into routine UKHSA surveys, and these can be ideal candidates for surveys run by radiology professional experts.

Involving the UKHSA NDRL WP from the outset is advisable if the results of the survey are to be put forward for possible adoption as NDRLs. It can also be advantageous for those running the survey to discover whether any existing or complementary survey is already underway.

It would be beneficial for UKHSA, or a member of the UKHSA WP, to be invited to observe or participate in the national survey as it unfolds, or alternatively provide regular updates to the UKHSA WP. A representative from the survey will also be invited to the UKHSA NDRL working party meetings.

Ideally, a terms of reference document will be produced to ensure a transparent approach, showing the target audience, focus of the study, the mechanisms of operation, timescales and so on.

Any successful survey needs suitable, realistic timelines – for example, start date and call for data, reminder staging dates, final call dates, possible extensions and write-up dates.

Consideration should also be given to how any proposed NDRLs will be updated as necessary in the future.

Methodology of survey

The survey should follow appropriate published guidance. Documents to consult include:

• IPEM 88 (under revision)
• ICRP Report 135
• European Diagnostic Reference Levels for Paediatric Imaging (PiDRL) project (RP 185)
• European Study on Clinical Diagnostic Reference Levels for X-ray Medical Imaging (EUCLID) project (RP 195)
• other similar international reports

Recent UKHSA surveys and their reports may also provide useful guidance and information on the methods used. These UKHSA guidelines have evolved from the National Protocol for the original UK dose surveys, developed and updated according to changes in dose data acquisition, and incorporating relevant international guidance (1 to 8).

Dose data is dependent on patient size, therefore specific aspects of methodology to be considered are patient data sample size and the size of patients within each sample. These are inter-related due to statistical requirements, a need to obtain meaningful data, and a pragmatic approach is required depending on data acquisition methods and availability of patient size information. The smaller the sample size, the more important it is to have an agreed or specified patient size or weight range (weight is often used as the most pragmatic indicator of patient size).

For submissions with much larger numbers of samples, a weight or size restriction is less important. ICRP 135 provides helpful guidance on this topic, some of which has been based on the earlier UK DRL protocols. The current UKHSA surveys generally align with ICRP 135 guidance.

Representation across the UK and types of hospitals

The survey ideally needs to provide representative data from all nations in the UK (England, Scotland, Wales and Northern Ireland). In a similar manner, a reasonable distribution across types of hospitals and radiology departments (large, medium, small; specialist, generalist; NHS and independent) is desirable.

In total, for a UK representative survey, submissions from a high percentage of installed systems would be ideal. In reality, recent surveys tend to include around 30% of systems.

With examinations or system types without current DRLs it would be hoped that the submission level would be higher. For example, the first CT survey in 1989 had an 80% submission rate, reflecting the interest of users for this relatively new modality at the time, for which little dose information was known.

Interim publications and status reports

At appropriate times, status updates of the survey should either be sent via email to UKHSA, and thereby to the NDRL WP, or by direct input to a UKHSA NDRL WP meeting.

Submission of presentations to professional conferences is helpful – to raise awareness and give progress.

Final publication

A peer-reviewed paper is normally required.

Alternatively, output could be a formal report (maybe published under the professional body, or co-badged with UKHSA and bodies on the WP).

Dose audit results into NDRLs

Following the survey, analysis and publication, the survey lead should submit the proposed NDRLs to UKHSA alongside the peer-reviewed, published paper or organisation report. (NDRL values are generally rounded values from a survey).

The proposed NDRLs, methods, peer-reviewed publication and so on will be circulated by UKHSA to the UKHSA NDRL WP (or to selected relevant members of the working party) for discussion and agreement to accept. A critical review from at least 2 members of the WP is required. The comments will then be circulated anonymously to the WP.

Following agreement, UKHSA will update the UK NDRL webpage to include the agreed values and the reference to the peer-reviewed publication, or organisation report in which the values are given.

Review of NDRLs

After a period of no more than 5 years from the date of publication on GOV.UK, an NDRL value will be reviewed at a UKHSA NDRL WP annual meeting. This review may take place earlier when considered necessary. The review decision will be documented in the meeting minutes.

If the NDRL is still suitable

The WP will set a new review date no longer than 5 years hence.

If the NDRL is no longer suitable

The WP will decide whether it should be retained, updated or withdrawn.

If the NDRL is to be updated

The group (or professional body sponsor) who undertook the previous dose survey will be asked if a new dose survey can be undertaken.

If that is not possible, UKHSA will review their own capacity to undertake a dose survey using the established methodology of the previous survey.

If that is not possible, UKHSA will ask whether any other body represented on the NDRL WP is able to carry out a survey.

If none can, the NDRL WP will review again whether to retain or withdraw the NDRL.

References

1. IPSM (1992). National protocol for patient dose measurements in diagnostic radiology. Dosimetry Working Party of the Institute of Physical Sciences in Medicine. NRPB, Chilton.

2. IPEM (2004). Guidance on the establishment and use of diagnostic reference levels for medical X-ray examinations. Institute of Physics and Engineering in Medicine Report 88. IPEM, York.

3. Shrimpton, P C, Jones, D G, Hillier, M C, Wall, B F, Le Heron, J C and Faulkner, K (1991). Survey of CT practice in the UK. Part 2: Dosimetric aspects. NRPB-R249, Chilton.

4. Doses to Computed Tomography (CT) Examinations in the UK – 2011 Review. CRCE Public Health England 2014.

5. Dose to Patients from Radiographic and Fluoroscopic Procedures in the UK – 2010 Review HPA-CRCE-034. Health Protection Agency June 2012.

6. ICRP, 2017. Diagnostic reference levels in medical imaging. ICRP Publication 135. Ann. ICRP 46(1).

7. European Guidelines on Diagnostic Reference Levels for Paediatric Imaging N° 185 (PiDRL project) ISSN 2315-2826.

8. European Study on Clinical Diagnostic Reference Levels for X-ray Medical Imaging N° RP195 (EUCLID project).

9. UKHSA Medical radiation website, Diagnostic radiology: national diagnostic reference levels

10. Latest UKHSA surveys. Please contact UKHSA Medical Dosimetry Group: medicalradiationdoses@ukhsa.gov.uk

UK legislation references

11. Ionising Radiation (Medical Exposure) Regulations 2017

12. Ionising Radiation (Medical Exposure) Regulations 2017 guidance

13. The Ionising Radiation (Medical Exposure) Regulations (Northern Ireland) 2018