Assessing the standard dose to standard patients for x-ray investigations

The assessment of radiation dose is an essential part in the optimisation process in diagnostic radiology. The concept of Diagnostic reference levels for x-ray investigations requires that radiation doses assessed can be compared between different x-ray departments. This study addresses the necessity for correcting average dose area product values due to patient size differences when assessing the standard dose for x-ray examinations. Standard doses for the pelvis examination in 19 departments were assessed using two methods. One method was to use measured dose area products only for patients complying with different criteria (60-80 kg body weight, 21.9-23.9 cm equivalent diameter and 20-25 kgm body mass index). The other was to use a mathematical fit (exponential curve) to the data and derive the dose area product value for a “normal” sized patient. Three descriptors for size were used: body weight 70 kg, equivalent diameter 22.9 cm and body mass index 24.2 kgm. These assessed standard doses were compared to the average dose area product values not taking the patient size into account. The corrected standard doses were different from the standard dose not taking the patient size into account. Setting size criteria significantly increases the time of data collection to approximately the double. This study indicates that the interpolation technique should be preferred. Only with a common and carefully defined and described method the assessed standard doses that can be compared with each other and with the diagnostic reference level. The need for standardization in radiation dose assessments to patients in diagnostic radiology For decades the need for optimisation of x-ray examinations has been emphasized by different organisations [1,2]. The optimisation is not an easy task, it involves both knowledge about the anatomy and pathology and the understanding of medical physics. The assessment of radiation dose is an essential part in the optimisation process. Methodologies for measurements of radiation dose have been developed over the years and different types of dosimeters have been advanced for this purpose [e.g. 3,4]. The optimisation studies have been performed mainly as research studies, where the staffs have been specially trained and the measurements and evaluation could be performed with special attention, relying on the special competences available. Very few departments in Sweden have performed such measurements routinely. The results from different studies have been difficult to compare with each other due to the fact that different methods have been chosen and the results have been presented differently. For example, radiation dose could be presented as energy imparted to the patient, as effective dose or as some other quantity. A comparison of the radiation dose to patients in different departments could not be performed. A standardized method for measurements and evaluations should relieve such activities. For different types of x-ray examinations a standard dose should be assessed, i.e. the dose should be derived according to a strictly defined method. Actually the concept of diagnostic reference values (DRL) for x-ray examinations requires that radiation doses assessed can be compared between different departments. Diagnostic reference levels The concept of diagnostic reference levels for x-ray investigations has been introduced into the Europe Union’s legislation [5] and promoted by the International Commission on Radiological Protection (ICRP) [1] as well as by the International Atomic Energy Agency (IAEA) [2]. It is a tool for facilitating the optimisation procedure. Most member states in the EU have introduced DRL in their legislation, making the assessment of standard doses to patients mandatory. Introductions of DRL methodology of how to determine the standard dose for different types of examinations should be established. A number of details have to be specified both concerning the measurements and how the standard dose is derived from the measurements. One crucial point is to define the type of examination for which the diagnostic reference level is established. This is difficult e.g. for the examination of the abdomen with computed tomography because the exact specification of the examination is depending on the indication for the individual