Nuclear medicine DRLs in more detail

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 two data sources. The first data source was a survey of Australian nuclear medicine facilities conducted in 2014/15 (the NDRLS survey), the second was an extensive survey conducted on behalf of the ANZSNM in 2008 (Diagnostic Reference Activities for Nuclear Medicine procedures in Australia and New Zealand).

For more information regarding these surveys, 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 on the Diagnostic Imaging Accreditation Scheme website. The relevant portion is reproduced below:

Where a practice provides general nuclear medicine imaging services and PET services (where weight correction is not used) existing imaging protocols will need to be amended to include the new adult DRLs. If the doses specified in the imaging protocol are higher than the established DRL, a justification for the higher dose will need to be provided.

For the CT component of multi-modality imaging, a practice should ensure that, for commonly performed examinations, the Dose Length Product (DLP) delivered to a sample of patients is recorded, and the median DLP compared to the DRL. A suggested sample size is between 10 and 20 patients. Where a practice performs very low volumes of a particular scan there is no requirement to conduct a survey for that procedure. A similar audit should be performed in cases where a practice administers a dose that is dependent on the weight of the patient. Where a weight based DRL is used (e.g. FDG PET scans), the proportion of patients who received a dose below the DRL should be calculated. In cases where there is a single DRL (e.g. Myocardial Perfusion Imaging studies) the median dose delivered at the facility should be compared to the DRL.

There are no DRLs for paediatric nuclear medicine imaging scans. Consequently, there is no requirement to conduct annual DRL audits of paediatric protocols.

Code of Practice for Radiation Protection in the Medical Applications of Ionizing Radiation (RPS 14)

How RPS 14 - Section 3.1.8 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 RPS 14 requirements.

Comparing my facility’s doses to the DRLs

You can compare your facility’s nuclear medicine doses using two methods:

  1. Using ARPANSA’s spreadsheet template

ARPANSA is not currently operating an ongoing nuclear medicine survey. However, we do provide a template to help you if you need to conduct your own audits.

FileNuclear Medicine DRL comparison template

  1. Using your own tools

You should compare your facility reference level (FRL) - which is the median dose delivered to patients undergoing a given protocol at your facility - to the DRLs. The median dose is simply the value in the middle of the distribution of delivered doses, i.e. the same number of scans will deliver a dose above the median as deliver a dose below the median.

To calculate your FRL:

  • Record the administered activity or DLP and patient weight for a minimum of 10, but preferably 20 or more, scans performed using the same protocol on different patients. The sample should be representative of your normal patient cohort (with regards to gender, weight and age). In general, a larger number of patients will provide a better estimate of your FRL.
  • Excel and other spreadsheet based applications have built in functions to determine the median, but if need be you can calculate it manually:
    • Sort the scans in terms of increasing dose.
    • If there are an odd number of patients, the median dose is the value in the middle of the list.
    • If there is an even number of patients, then the median is simply the average of the middle two values.

In order to prove that you have complied with RPS 14, Section 3.1.8 a), you should document your results. This documentation should include your FRL and the corresponding DRL.

25th percentile and median doses

The 25th percentile and the median doses are only provided to add more context when you conduct your dose comparison - for regulatory purposes, only the DRL comparison is required.

Interpreting the comparison results – low doses

Occasionally the doses for your nuclear medicine procedures may be lower than the DRL. If 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 scan quality is normally only a fraction of the dose expected from a corresponding 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).