Diagnostic Reference Levels (DRLs) in medical imaging

» Why do we need DRLs in medical imaging?

The optimization of patient protection in diagnostic radiology, diagnostic nuclear medicine or image guided interventional procedures requires the application of examination-specific protocols tailored to patient age or size, region of imaging and clinical indication in order to ensure that patient doses are as low as reasonably achievable for the clinical purpose of the examination. 

Surveys of dose estimates from different imaging modalities highlight the substantial variations in dose between some healthcare facilities for same examination or procedure and similar patient group (adults or children of defined sizes). Such observations indicate the need for standardization of dose and reduction in variation in dose without compromising the clinical purpose of each examination or procedure. Examination-specific or procedure-specific DRLs for various patient groups can provide the stimulus for monitoring practice to promote improvements in patient protection.

» Which dose quantities are used for setting DRLs?

DRLs should be set in terms of the practical dose quantities used to monitor practice. These dose metrics should be easily measurable. The following are commonly used terms:

  • For radiography, air kerma-area product (PKA) and entrance surface air kerma (Ka,e) are recommended DRL quantities;
  • For fluoroscopy and interventional radiology procedures, air kerma-area product (PKA) is the recommended primary DRL quantity. Air kerma at patient entrance reference point (Ka,r), fluoroscopy time and number of images are recommended as useful additional DRL quantities (a multiple DRL);
  • For CT, volume computed tomography dose index (CTDIvol) and dose length product (DLP) are recommended quantities;
  • For mammography and breast tomosynthesis, the recommended DRL quantity is one or more of incident air kerma (Ka,i), entrance surface air kerma (Ka,e), or mean glandular dose (DG), with the choice of quantity depending on local practices; 
  • For dental intra-oral radiography, the recommended quantity is incident air kerma (Ka,i), and PKA for dental panoramic radiography; 
  • For nuclear medicine, DRLs are set in activity administered to patient, and/or in administered activity per kg of body mass. 

The above dosimetric quantities, their symbols and closely similar quantities are summarized in the following table:

Table: Quantities suitable for setting DRLs

QuantityRecommended
symbols
Recommended
 unit
Other
 common
symbols 
used in 
literature
Closely
 similar
 quantity
Entrance
surface air
kerma
Ka,emGyESAKEntrance-
surface
dose (ESD)*
Incident air
kerma
Ka,imGyIAK 

Incident air
 kerma at the
patient 
entrance
reference
point**

Ka,rGyCAK
 (Cumulative
 air kerma)
 
Air
kerma-area
product
PKAmGy.cm
(radiography
 and dental), 
Gy.cm
(fluoroscopy)
KAPDose-area 
product (DAP)*
Volume
computed
tomography
dose
index
CTDIvolmGy Volume CT
 air kerma
index (Cvol)*
Dose-length
product
DLPmGy.cm Air
kerma-length 
product (PKL)*
Mean
glandular
dose
DGmGyMGD, AGD 

* Because "air kerma" and "dose in air" are numerically equal in diagnostic radiology energy range.

** Also names "cumulative dose", "reference air kerma" and "reference point air kerma" have been used in the literature. These quantities are not patient doses that can allow estimation of risk to individuals, but are dose indicators characterizing radiation exposure for the purposes of comparison of practice. There is no merit in setting DRLs in terms of other dose quantities, such as effective dose, that are derived from the well-defined monitoring quantities by coefficients that could vary depending on the particular dose model adopted.

» How are DRLs used by a healthcare facility?

For each diagnostic imaging system typical levels of dose in related quantities for each type of examination or procedure (and associated clinical indication) should be determined as the median values observed for representative samples of patients of a particular group (adults and children of defined sizes). 

Mean rather than median was earlier recommended, but the recent recommendations favor median values. These median doses should be compared with the relevant DRLs. Clinical protocols for performing a particular examination or procedure should be reviewed if the comparison shows that the facility’s typical dose exceeds the DRL, or that the facility’s typical dose is substantially below the DRL and it is evident that the exposures are not producing images of diagnostic usefulness or are not yielding the expected medical benefit to the patient. The resulting actions aimed at improving optimization of protection and safety will usually, but not necessarily, result in lower facility typical doses for the examination or procedure.

» What can be done for individual patients?

Individual procedures performed with particular imaging equipment should be optimized according to the specific clinical task and body dimensions of the patient. In that individualization optimization process, the DRL can be used as a starting point and as a benchmark to compare (but not to dictate) the individual applied dose to the operational-based dose values (DRLs). Automatic dose tracking tools may help in the optimization process. 

» Which examinations should have DRLs?

DRLs are intended to promote improvements in patient protection by allowing comparison of current practice. National and local DRLs should (ideally) be set for each examination or procedure, for each clinical indication and each patient group (adults and children of defined sizes). The examinations or procedures included should represent at least the most frequent examinations performed in the region for which dose assessment is practicable, with priority given to those that result in the highest patient radiation dose.

In order to allow meaningful comparison of truly similar examinations or procedures conducted for similar purpose and requiring similar technique, it is crucial to specify detailed descriptions of the examination or procedure, including a clinical indication (such as CT abdomen in relation to liver metastases), rather than simply broad categories of examination or procedure (such as CT abdomen). This usefully allows the comparison of ‘apples with apples’ rather than a mixed bag of fruit. For interventional practices the complexity of the procedures should be taken into account.

» How should we account for patient size?

The technique factors required for an examination or procedure and the resulting dose are dependent on patient size and each healthcare facility should establish specific protocols for each patient group as part of optimized practice. Protocols for paediatric examinations can, for example, be developed for patients grouped by ranges of weight or cross-sectional area, reflecting necessary changes in optimized technique. 

» Where should I start in the absence of well-established national and local support for DRLs (as, for example, a small facility in a less-resourced country)?

The priority for a medical facility might be to estimate typical patient dose quantities in relation to present practice for a few common examinations on adult patients, according to the following steps:

  • Record displayed values of radiation dose quantity for samples of 10 or more typical adult patients undergoing procedures for common clinical indications. For example, for CT, consider including examinations of head (e.g. in relation to acute stroke), chest (e.g. in relation to lung cancer) and abdomen (e.g. in relation to acute abdominal pain); 
  • Verify the accuracy, and if necessary, apply correction factors, of the displayed values of radiation dose quantities;
  • Calculate for each type of examination the median values of dose quantities (e.g. CTDIvol and DLP for CT); these are your typical dose levels (but not your local DRLs that are set for a group of imaging systems or a group of hospitals); 
  • Compare your typical dose levels (median values) with published DRLs for a similar practice in the absence of local or national DRLs, in order to provide a broad indication of your relative performance and urgency of need for improvement in your imaging technique;
  • Comparison of typical dose levels (median values) to DRLs is not sufficient, by itself, for optimisation of protection. Image quality or, more generally, the diagnostic information provided by the examination (including the effects of post-processing), must be evaluated as well; 
  • If your values are below published DRL, this does not necessarily indicate satisfactory performance. Imaging techniques should always be reviewed for potential reduction in their levels of dose without compromising the clinical purpose of the examination; 
  • If your values are above DRL, there is a more urgent need to investigate whether simple changes can be made to the imaging settings selected for an examination in order to reduce values of radiation dose quantities whilst still providing the required clinical information;
  • Levels of dose should be reassessed following revision of imaging technique in order to allow further comparisons (see steps above). 

» What are the potential pitfalls in comparing your typical dose values (medians) with published DRLs?

Published DRLs can prove useful in allowing comparison of median dose values in your facility, for a particular imaging system. There are several problems which can occur:

  • Published DRLs values from other countries (with potentially different imaging practices and technology) may not be relevant to your particular circumstances;
  • The types of examination or procedure specified for the published DRLs (as being with or without detailed clinical indications) may not be directly relevant to your particular practice; 
  • Published dose values may not have been obtained using the same methodology (e.g. total values or values per projection or per series) or in relation to the same standard condition like CT dosimetry phantom (diameter of 16 cm or 32 cm), or may be given in different dose quantity or unit; 
  • Published DRLs values may not be expressed in a different dose quantity or dose unit; 
  • The patient sample (number of patients and their body size) in the published survey may be different; 
  • Advances in technology, such as post-processing and iterative reconstruction in CT, will need to be taken into account when updating DRLs. 

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