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

Transcription

1 Assessing the standard dose to standard patients for x-ray investigations A. Almén and W. Leitz Swedish Radiation Protection Authority, SE Stockholm, Sweden Abstract. 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, cm equivalent diameter and kgm -2 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 -2. 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 1

2 patient. This is also the case for a common conventional x-ray examination such as the chest examination where additional projections could be added due to patient specific considerations. In the instructions the type of examination has to be defined as unambiguously as possible. The radiation quantity is specified together with the diagnostic reference level and guidance on how to perform measurements and calculations is essential. Measurements for patients or with phantoms or pure calculations could be considered. The diagnostic reference level can be set for single projections or for complete examinations. Regional solutions on specifying the reference dose level Different types of reference dose levels require different levels of resources in the hospitals. The availability of these resources is influencing the choice of strategy. Other practical aspects have certainly to be taken into account. Therefore regional solutions are needed. To some extent Sweden and its neighbouring countries have adopted the same concept [6]. In Sweden, DRL for 12 different investigations are included in the regulations [7]. The quantity of DRL and hence of the standard dose for the six conventional investigations is the average dose area product (DAP) for a group of average sized patients for a complete examination. The present study aims at evaluating different methods to assess the standard dose from measurements of the dose area product, specifically to investigate the influence on the result of patient characteristics such as weight and to use inclusion criteria and interpolation between measured data when assessing the standard dose. The Swedish study of radiation dose to patients A database including the radiation dose for about 6500 patients undergoing conventional or computed tomography was used. Radiation doses have been measured in different x-ray departments all over Sweden, for the conventional examinations as dose area product for the complete examination. The x- ray departments measured the radiation dose for at least 20 patients investigated in the same x-ray room and for the same type of examination. Other relevant data were also collected and recorded in connection with the measurements, e.g. patient weight, height, sex and age. Description of the patient and setting up criteria for patients characteristics The radiation dose to the patient should normally depend on the size of the patient. When using filmscreen image receptors the adjustment of the radiation dose to the patients is a prerequisite for a useful x-ray image. Digital image receptors do not need this adjustment of the radiation dose, but from a radiation protection point of view the adjustment should be done also for these types of receptors. When assessing the standard dose the variation of radiation dose for different patient size has to be taken into account. The standard patient has to be described in order to specify for what patient size the standard dose is to be assessed. The size of the patients can be described in different ways. It could be by weight and length, or using the calculated equivalent diameter d eq [8] or as the body mass index (BMI). The patient characteristics for a group of patients could then be described using either of these quantities. For a group of patients the mean values of these quantities could be used to describe the size of the patient that is representative for this group. When measurements are performed, radiation doses can be measured only for a limited number of patients for practical reasons. Criteria for including patients with a certain size could also be set up in order to get a more homogenous group. The criteria can be applied before measurements are or performed after the measurements. The criteria used in this study are presented in Table I. 2

3 Table I. Inclusion criteria used in the study. Criterion 1 Criterion 2 Criterion 3 Weight [kg] d eq [cm] BMI [kgm -2 ] The patient characteristics were studied for the whole group of 6500 patient and for smaller groups of patients, in this case the patients having a pelvis examination. The inclusion criteria were also applied on the different patient groups and the patient characteristics were studied for the subgroups of patients. The average characteristics of patients The average patient having an x-ray examination is 59 years old, is weighing 73 kg and has a body length of 170 cm, Table II. The distribution between females and males is even. Table II. Description of all patients and of subgroups applying the three different criteria. Patient group Number (Female/Male) All 6534 (53% / 47%) Criterion (57% / 43%) Criterion (52 %/ 48 %) Criterion (53 %/ 47 %) Weight (kg) Height (cm) Age When applying inclusion criteria the number of eligible patients is reduced with 40 % to 60 %. Criterion 1 has the lowest impact on the number of patients. The equivalent diameter and body mass index does indicate the shape of the body. However, these concepts are more difficult to use in the department were a quick selection of which patients are to be included is needed. Patient characteristics and dose area products were studied for the pelvis examinations. The departments included are large university hospitals and smaller regional hospitals, both in the public and private sector. In Table III mean values of patient characteristics are summarized for 19 x-ray departments performing examinations of the pelvis. The criteria specified in Table I was used and the mean weight of the resulting patients was calculated together with the percentage of patients that are complying with the inclusion criteria. 3

4 Table III. Mean weight for patients undergoing pelvis examinations, both for all patients in the specific department and for the subgroups using the inclusion criteria. P is the percentage of the patients complying with the inclusion criteria. Depart. All Criterion 1 Criterion 2 Criterion 3 Weight Weight P Weight P Weight P (kg) (kg) (kg) (kg) A B C D E F G H I J K L M N O P Q R S The mean values of the weight of patients having a pelvic investigation vary from 66 to 78 kg (mean value for all, 72 kg). These differences could be due to regional variations or of pure statistical nature. With the different criteria the mean weight of the group of patients is changed significantly, in some cases it is increased and in others decreased. The percentage of patients complying with the criteria in each group varies from 33 % to 80 %. The average change is around 3.5 kg using criteria 1 and 2. Using criterion 3 the change is approximately 7 kg. For single departments the change could be as much as 13 kg. The significance of different patient configuration The dose area product value is depending on the weight of the patient. How it depends is influenced by among other things the radiation quality, both the tube voltage and the filtration, and the size of the irradiated volume. Therefore all groups of patients for which radiation dose have been measured cannot be described with one and the same function. An example of the dependency in one department is shown in Figure 1. 4

5 12 10 Dose area product, Gycm Weight, kg FIG. 1. Dose area product as a function of weight of the patient. Dose area product varies between 0.5 and 9.7 Gy cm 2 for this particular group of patients. A single value, the standard dose, has to be found from such measurements. Assessment of standard dose Two methods were used; mean values for the various patient samples complying with the different inclusion criteria and interpolation techniques using data from all patients in the respective group. For the interpolation an exponential relationship between dose area product and weight, equivalent diameter and body mass index was assumed, respectively. The standard dose was then calculated for the weight of 70 kg, an equivalent diameter of 22.9 cm and a body mass index of 24.2 kgm -2. The mean dose area product was calculated applying the inclusion criteria (Table I). In Table IV, the different standard doses are presented using the different methods and size descriptors. 5

6 Table IV. Assessed standard doses, dose area product Gy cm 2, for the pelvis examination. 70 Mean value of dose area product Interpolation technique Depart. All kg cm kgm -2 kg cm kgm -2 A B C D E F G H I J K L M N O P Q R S Comparing the standard dose with reference dose If the diagnostic reference dose would be chosen as 2.6 Gy cm 2 (the third quartile of all the DAP values in Table IV), depending on what method is used between 3 and 7 of the 19 x-ray departments would the standard doses exceeding the diagnostic reference level. Three of the departments would exceed the reference level independently of the method used for assessing the standard dose. Summary of the investigation The patient characteristics have to be addressed when the standard dose is assessed. The weight of the patients varies significantly between groups of about 20 patients. The dose area product is dependent on the weight of the patient. Therefore methods that are taking into account the differences in patient characteristics have to be applied when assessing the standard dose from such groups of patients. Inclusion criteria can be used before measuring the radiation dose in the clinic. The criteria based on equivalent diameter and body mass index are not easy to apply in the clinic. The inclusion of patients based on the weight is manageable, but of course excluding patients will increase the time needed for the measurements of the standard dose. There is a risk of favouring measurements of light patients, getting a biased sample. Applying different inclusion criteria after the measurements of the group of patients could also be done. The number of patients remaining in the group has to be considered, a significant number of measurements are not useful after the exclusion. But if an interpolation method is applied all measurements could be used. The standard dose will depend on what method is used for the assessment and it is important that the same principles are applied in the different departments. A mean value for all patients without considering the patient size is of little value. Applying inclusion criteria before measuring the radiation dose is not preferred; it is not very practical. Applying inclusion criteria after the measurements will 6

7 reduce the number of patients used for the estimation of the standard dose, and more patients have to be included in the measurements. This study indicates that an interpolation technique should be preferred where all data are included in the assessment. The instruction in the clinic is then very straight forward, to perform measurements for a specific number of patients coming for a certain type of x-ray examination in a specific x-ray room. The mean weight of the patients for the country could also be used in the interpolation, which will indicate the population dose. Specific findings in the study:! Assessing mean values for a group of patients without restrictions of the sizes as the standard dose is not satisfactory.! The use of inclusion criteria or using the interpolations technique results in significantly different standard doses compared with calculation of mean dose area product for the group.! The use of inclusion criteria will increase the time for the data collection to approximately the double.! With the interpolation technique all measured dose area products will be used.! 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. References 1. International Atomic Energy Agency, Radiological Protection for Medical Exposures on Ionizing Radiation. Safety Guide, Safety Standards Series, No. RS-G-1.5, IAEA, Vienna (2002). 2. International Commission on Radiological Protection, Radiological Protection and Safety in Medicine. Publication 73. Annals of the ICRP, 26,No. 2, Pergamon Press, Oxford and New York (1996). 3. Kramer H.M., Schnuer K. (Ed), Dosimetry in Diangostic Radiology. Rad. Prot. Dosim., 43, No 1-4 (1992). 4. Bengtsson G, Blomgren P_G, Bergman K, Åberg L, Patient exposure and radiation risks in Swedish diagnostic radiology. Acta Radiol. Oncol. 17: , (1978) 5. European Commission. Council Directive 97/43/Euratom of 30 June 1997 on health protection of individuals against the dangers of ionising radiation in relation to medical exposure. Official Journal of the European Communities, L 180:22-27, (1997) 6. Gron P, Olerud HM, Einarsson G, Leitz W, Servomaa A, Schoultz BW, Hjardemaal OA, Nordic Survey of Patient dose in Diagnostic Radiology. Eur. Radiol., 10: , (2000). 7. Swedish Radiation Protection Authority, The Swedish Radiation Protection Authority s Regulations and General Advice on Diagnostic Standard Doses and Diagnostic Reference Levels within Medical X-ray Diagnostics. Regulations of the Swedish Radiation Protection Authority SSI FS 2002:2, Stockholm (2002). 8. Lindskoug B A, The reference man in Diagnostic Radiology Dosimetry. Brit. J. Radiol., 65: , (1992). 7