Go Back

Dose audit


Medical physicists must be able to determine radiation doses for specific patients and procedures, and to use appropriate phantoms to estimate the dose to patient populations.  Patient dose survey data is then compared to diagnostic reference levels (DRLs) in order to determine if the dose levels are appropriate at a particular facility.  Comparison of patient survey data to DRLs is mandated by regulation in some areas, although regulatory considerations should not drive optimization with DRLs—it is considered a good clinical medical physics practice.

Important Principles

Patient dose audits provide the tools to evaluate patient exposure to all diagnostic x-ray imaging modalities and determine if the doses are appropriate.  Without comparison to national DRLs an imaging department does not know if their doses are reasonable.  Differences in patient doses in the order of 20 times or more have been seen between facilities.  Even within facilities dose differences in the order of 15 times from one room to another are not uncommon.  Furthermore, dose differences in the order of 5 to 10 times have been seen between radiographers at the same facility, using the same x-ray equipment.  This is due to the fact that radiographers train at many different facilities and tend to use the techniques which they learned during their training.  Consequently, dose audits of facilities, rooms, and even radiographers, are important to assuring clinical image quality at the appropriate patient doses.

It is important to remember that DRLs are, typically, based on the 75th percentile of a distribution of measured doses.  This means that 25% of the facilities exceed the DRLs, while 75% are operating at lower levels.  In order to encourage these facilities to optimize their imaging systems, a concept of achievable dose has been introduced by NRPB (now Health Protection Agency, see Supplementary References) and is also recommended by the NCRP.  The achievable dose is set at the median (50th percentile) of the survey dose distribution.  All facilities should be able to operate at the achievable dose level as 50% are already below that level.

The medical physicist must be able to design the dose audit, determine the dose indicator to be used (e.g., entrance surface dose, dose-area product, etc.), select either patient or phantom measurements (and justify the selection), and choose a patient sample population and size to assure statistical validity.  The medical physicist must, also, instruct staff involved in the actual measurements on the appropriate use of dosimeters and completion of survey data forms, assuming patient measurements are being used.

Finally, the medical physicist must summarize the data, present the data to the staff, and start the process of optimization as a team effort.

Introduction to References

Patient dose audits are covered in the IAEA Handbook of the Physics of Diagnostic Radiology and in the IAEA document entitled “Dosimetry in Diagnostic Radiology: An International Code of Practice” in Appendix VII.  DRLs are discussed in several Essential and Supplemental References, and more information can be found in the European literature.