Using diagnostic reference levels (DRLs) has outlived its usefulness, according to an article by radiation protection expert Dr. Madan Rehani, PhD, in the British Journal of Radiology (BJR). Instead, hospitals should adapt acceptable quality dose (AQD) for their own needs.
Everyone is concerned about radiation dose. Patients, in particular, want to know if what they get in their examination is close to a "standard" dose. The trouble, however, is no standard dose exists, and the closest radiologists, radiographer or medical physicists have come to one is a DRL, which is a 75th percentile of the mean doses for a sample of patients close to the standard size, typically 70 kg, according to Rehani, a scientist at Harvard Medical School and Massachusetts General Hospital in Boston and formerly a specialist at the International Atomic Energy Agency in Vienna (BJR, January 2015, Vol. 88:1045, pp. 20140344).
The purpose of a DRL is to detect outliers (i.e., higher than 25th percentile cases) from a sample.
"In the absence of a standard dose, there has been an erroneous tendency to assume that being below DRL means adequate optimization," he noted. "Removing rotten pieces of fruit is different from picking good pieces of fruit. When you go to market you do not pick up all that is left after removing rotten pieces. You want choose good quality one," Rehani said.
We should do the same with patient doses. When we look around data, first we should pick up those cases of patients where image quality is acceptable, then look at their doses, find average dose and that should be "standard dose," according to Rehani. In the absence of such a standard dose so far, DRL has erroneously been used as standard. DRLs do not provide guidance on what is achievable with optimum performance. DRLs were not developed as a tool for optimization within the 75th percentile.
"DRLs provided good tools in previous years when the spread of doses were by a large order of magnitude and the shape of dose distribution curve was right-skewed asymmetric," he added. "There is no problem with DRL, but stopping at DRL and using DRL in ways it was not supposed to be used creates problems."
Some of the problems include the following:
- Using DRLs as a de facto dose limit that should not to be exceeded.
- No DRLs are available for "nonstandard" cases -- for example, larger patients.
- DRLs haven't been updated as technology has been updated.
- DRLs were not meant to be used for an individual patient.
- DRLs rely on retrospective analysis, but optimization requires a prospective approach.
Rehani proposes a solution to these problems by the use of AQD:
- Each facility determines averaged dose values for individual exams that has clinically acceptable image quality classified in weight groups of 10 kg body weight for adults (e.g., 41-50, 51-60, etc.) and a 5-kg slot for kids.
- AQDs are set for local, regional, and national situations.
- The AQD serves as a standard dose and may be compared with the AQD of another room in the same hospital or between other hospitals.
- The AQD can be prospectively adjusted in patients. For example, in CT, if anticipated dose length produce (DLP) value is likely exceeding the AQD standard deviation, action can be taken.
- Patients can be identified in whom image quality was not diagnostic or higher than necessary and used as an example to correct it in the future.
"Image quality is primary in this approach and dose is secondary, and the approach provides provision for covering all weight groups," he said.
Rehani acknowledges that with so much history and momentum behind DRLs, it will take some time for any new concept to gain traction, but that shouldn't prevent creativity and new approaches from emerging.
"Regulatory actions in future should require facilities to establish their local AQDs and compare these with those of national values," he stated. "It should be for national authorities to establish national AQDs, which will be averaged values derived from local AQDs."
AQD has a number of advantages, namely it starts with a facility rather than national levels and so promotes facility-based actions. It's based on clinically acceptable image quality rather than dose, and it focuses on optimization rather than exclusion of outliers.
To supplement perceived weaknesses in subjective assessment of image quality, this year there are likely to be further papers published that will compare subjective and objective image quality to put image quality assessment on better footing, Rehani wrote in an email to AuntMinnieEurope.com.
"The image quality assessment has been on weaker ground all these years," he wrote. "IAEA [International Atomic Energy Agency] is likely to motivate member countries to work on AQD. That will bring experience with new concept in the coming years."