| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||
Correspondence |
DRLs are essentially investigation levels that, if exceeded, trigger a detailed investigation into the causes of unusually high exposures. They are just the first step in the optimization process – a means for identifying those situations in most urgent need of investigation because the doses appear to be towards the top end of the distribution seen on a national scale. The choice of DAP as one of the quantities in which DRLs are expressed is based mostly on the practicability of making DAP measurements during radiographic and fluoroscopic examinations in the clinical environment and the widespread installation of DAP meters on X-ray imaging equipment throughout the health service.
There are three essential requirements for the dose quantities to be used in DRLs; they should be unambiguously defined so that everyone clearly understands what is to be measured, they should be capable of simple, direct measurement (or calculation from indirect measurements) with readily available dosemeters of sufficient precision and accuracy, and they should provide an indication of the typical dose received by patients examined in a particular facility from a particular type of X-ray examination. Clear guidance on how DAP meters can be used to meet all these requirements is provided in the National Protocol for Patient Dose Measurements in Diagnostic Radiology [2] and in Guidance on the Establishment and Use of DRLs for Medical X-ray Examinations [3] published by IPEM. Other practical dose quantities will be needed for expressing DRLs in different imaging modalities such as CT, mammography and dental radiography. These have been clearly defined in many publications, including IPEM report 88 [3]. It is, therefore, rather puzzling to see Dr Moores describing this wide variety of dose quantities as an intolerable situation, most likely due to "the lack of a clear understanding of the meaning and relevance of the term "optimization" in the field of diagnostic radiology".
Dr Moores is correct in pointing out that the full optimization process (i.e. maximizing the ratio of benefit to harm) in diagnostic radiology, requires assessment of both the diagnostic efficacy of the examination and the resulting radiation risk. However, this is the purpose of the detailed investigation that is triggered by exceeding the DRL; it was never intended that the dose measurements made to determine whether the DRL is being exceeded should, by themselves, provide all the information necessary for complete optimization.
One of the first investigations to carry out on exceeding a DRL might well be to see if the sample of patients on whom DAP measurements were made was biased towards excessively large patients, since the DRLs are set for average-sized patients. The fact that males tend to be generally larger than females and consequently will on average have higher DAP values, is no reason, per se, for not using DAP measurements to trigger such investigations. A further investigation might be necessary to determine whether the excessive DAP values were due to the use of large entrance surface doses or large field areas (or both).
Optimization in diagnostic radiology does not mean simply maximizing image quality and minimizing patient dose, rather it requires radiologists to determine the level of image quality that is necessary to make the clinical diagnosis and then for the dose to be minimized without compromising this image quality. Radiologists, of course, subjectively assess the adequacy of their images every time they report on them, and it can be argued that no one else is in a better position to do so. What radiologists cannot do intuitively is to assess the patient dose, which is why practical patient dose monitoring techniques and DRLs are needed to trigger more detailed optimization investigations, if images that are locally considered to be "diagnostically acceptable" are being obtained with excessively high patient doses.
Dr Moores' plea for the development of "a scientific framework for the quantification of diagnostic outcome and therefore clinical benefit" will, no doubt, have an important role in these detailed optimization studies. But it is difficult to see how such a framework would eliminate the "general ignorance within the medical profession concerning the levels of radiation employed in diagnostic radiology", as he claims. Rather, the establishment and implementation of DRLs using practical dose quantities such as DAP, has probably done more to raise awareness of patient doses in the radiology professions than any other national initiative. The halving of the DRLs that has been achieved for many common types of X-ray examination in the UK over the past 20 years [4] with no apparent loss of diagnostic efficacy, provides ample evidence for the effectiveness of this simple first step in the optimization process.
Yours etc.,
Radiation Protection Division, Health Protection Agency, Chilton, Didcot, Oxon OX11 0RQ, UK
Received for publication October 18, 2005. Accepted for publication October 25, 2005.
References
Related articles in BJR:
| ||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| BJR | DMFR | IMAGING | ALL BIR JOURNALS |
