Diagnostic reference levels (DRLs)

A diagnostic reference level (DRL) is an indicative dose that is not expected to be exceeded under normal imaging conditions for a given diagnostic task.

A DRL is not a regulatory limit, it is a benchmark that when exceeded triggers a review. Conducting a local dose audit and comparing the results to a DRL provides an imaging facility with a simple method of identifying situations where they are delivering an unusually high patient dose.

More information on DRLs, how they are used, and the regulatory requirements for Australian imaging facilities, is provided on the NDRLS in more detail page.

Deriving the nuclear medicine DRLs

The DRLs were derived from a survey of Australian nuclear medicine facilities conducted in 2021/2022 (the NDRLS survey).

For more information regarding this survey, see the nuclear medicine statistics page.

Collaboration in development of the nuclear medicine DRLs

ARPANSA developed the nuclear medicine DRLs together with a liaison panel composed of representatives from:

The DRLs have been endorsed by the boards of the AANMS and ACPSEM, and the federal council of the ANZSNM.

Understanding your responsibilities

Diagnostic Imaging Accreditation Scheme

The DIAS requirements state that you need to conduct an annual comparison with the DRLs. An advisory statement clarifying how this can be done for nuclear medicine has been published here.

The relevant portion is reproduced below:

DRLs provide benchmarks against which diagnostic imaging practices can compare their median radiation doses, also called a facility reference level (FRL), and administered activity for common imaging procedures.

Procedures with national DRLs

Diagnostic imaging practices using ionising radiation are required to:

  • Be aware of the latest relevant national DRLs.

  • Have a program to collect radiation dose data, calculate FRLs and compare these to the established national DRLs.

  • Undertake an annual audit to review the comparison results and where the FRL exceeds the national DRL, investigate whether the imaging protocol can be optimised.

  • Maintain audit records which show the FRL calculation and comparison, the review process and any investigation, and submit these at each accreditation assessment.

  • Provide in the audit records a justification when the median radiation dose (the FRL) is higher than the national DRL.

  • Update practice policies within six months of new or revised national DRLs being published by ARPANSA.

Nuclear medicine and PET procedures with a fixed administered activity

Where the administered activity for a nuclear medicine or PET procedure is fixed in an imaging protocol, requirements for the procedures with national DRLs (see above) are applied with the exception that the:

  • Program to collect radiation dose data is not required

  • Annual audit compares the administered activity in the protocol to the national DRL

  • Audit records do not need to reference the FRL calculation

  • National DRL for each procedure is documented in the imaging protocol

Nuclear medicine and PET procedures using weight corrected administered activity

Where a nuclear medicine or PET procedure uses a weight corrected administered activity, the requirements for the procedures with national DRLs (see above) are applied.

Computed tomography component of nuclear medicine hybrid imaging 

The national DRLs published for CT in conjunction with SPECT or PET apply to procedures conducted for the purposes of attenuation correction or localisation. The requirements for procedures with national DRLs (see above) are also applied to these procedures.

Code of Practice for Radiation Protection in the Medical Applications of Ionizing Radiation (RPS 14) and the Code for Radiation Protection in Medical Exposure (RPS C-5)

While at a national level RPS 14 has been superseded by RPS C-5, not all states and territories have officially adopted RPS C-5. How RPS 14 - Section 3.1.8 (and eventually RPS C-5 - Section 3.2.15) is interpreted is ultimately up to your state or territory regulator. However, it is ARPANSA’s opinion that following the DIAS guidance above would meet the requirements of either Code.

Comparing your facility’s doses to the DRLs

The appropriate method of conducting DRL comparisons depends on the modality and how a facility prescribes administered activity. Generally, you should determine your Facility Reference Level (FRL) for a particular procedure and compare that dose/activity to the DRL.

For the CT component of hybrid imaging, your FRL is the median dose delivered to a sample of patients. The sample should be representative of the normal patient cohort (with regards to gender, weight and age) and ideally should include 10 or more patients. A CT DRL comparison is scanner specific; a separate FRL should be calculated for each scanner that is regularly used to perform a given scan.

For general nuclear medicine DRL comparisons, a facility can define their FRLs using either its prescribed activity or the median dose administered to a sample of patients (analogous to the CT comparison above). In cases where a facility has a patient specific prescribed activity (for example based on weight), the latter approach should be used.

For PET DRL comparisons, facilities can choose to compare against either the fixed or variable DRL. If a facility uses a fixed prescribed activity then it may compare that activity directly against the fixed DRL. Similarly, if a facility’s prescribed activity can be expressed in terms of MBq/kg, then the prescribed activity can be compared against the variable DRL.

In cases where a variable dose is prescribed that cannot be expressed in MBq/kg, a facility should collect administered activities from a sample of patients. The median dose delivered to the sample may be compared against the fixed DRL or (preferably) the DRL for each patient in the sample can be calculated and the percentage of patients administered an activity below the weight specific DRL can be determined. In the latter case, if 50% or more of patients received an activity below the DRL then the facility can consider its activities to be below the DRL.

Facilities are free to use their own resources to conduct DRL comparison, however ARPANSA dose provide an excel template that may aid facilities:

The template is intended for dose audits of the CT component of multimodality imaging and for the administration of radioisotopes where the amount of radiopharmaceutical delivered is varied depending on body habitus (for example via weight or BMI).

Interpreting DRL comparison results 

If your Facility Reference Level (FRL) exceeds the DRL for a particular protocol, it is an indication that you are delivering a higher dose than 75% of Australian imaging facilities for that procedure. You should consider whether you can safely reduce the dose that you deliver without adversely affecting diagnostic capability.

For the CT portion of hybrid scans, if you exceed the DLP DRL, but not the CTDIvol DRL, it is an indication that your scan length is longer than that normally used by most Australian imaging providers. Conversely, if your CTDIvol is above the DRL, it is an indication that your scans are higher quality than customarily used.

If your FRLs are lower than the DRL, and you are confident you are acquiring diagnostic quality images, you should not increase your doses to match the DRL.

Definitions: attenuation correction and localisation CT scans

An attenuation correction CT scan is generally a low quality CT performed to provide an attenuation map for the purposes of SPECT or PET reconstruction.

A localisation CT scan is one that is performed to identify the anatomical location of features seen within the SPECT or PET acquisition.

A single CT scan is often used for both purposes and the image quality and dose are normally only a fraction of the quality and dose expected from a diagnostic CT scan of the corresponding region.

Comparing SPECT/PET CT doses to the MDCT DRLs

The PET/CT and NM/CT DRLs are for CT scans performed for attenuation correction and localisation purposes only. If you conduct diagnostic quality scans on your patients (for example instead of the patient getting the same scan in a radiology department) then you should compare your CT doses to the MDCT DRLs (if there is an appropriate protocol).

Brain NMCT and PETCT DRLs

The current DRLs for CT scans associated with PET and SPECT scans of the brain have not been updated from those issued in 2017 and therefore do not include a CTDIvol (as this was a new addition for the 2023 DRLs). Ambiguity in which CTDI phantom various scanners report against during brain scans means that further analysis of the data collected in the most recent NDRLS nuclear medicine survey is required before the NM/PET brain CT DRLs values are updated.

Access to information FOI disclosure log Information public scheme