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British Journal of Radiology (2007) 80, 452-459
© 2007 British Institute of Radiology
doi: 10.1259/bjr/60918774
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Full paper

Diagnostic reference levels for thorax X-ray examinations of paediatric patients

T Kiljunen, MSc 1,2 H Järvinen, MSc 1 and S Savolainen, DOC 2,3

1 Radiation Practices Regulation, Radiation and Nuclear Safety Authority, PO Box 14, FI-00881, Helsinki, 2 Department of Physical Sciences, University of Helsinki, PO Box 64, FI-00014, Helsinki, 3 HUS Medical Imaging Center, University of Helsinki, PO Box 340, FI-00029, Helsinki, Finland

Correspondence: Mr Timo Kiljunen, Radiation Practices Regulation, Radiation and Nuclear Safety Authority, Laippatie 4, PO Box 14, Helsinki, FI-00881, Finland. E-mail: timo.kiljunen{at}stuk.fi; timo.kiljunen{at}helsinki.fi


   Abstract
Top
 Abstract
Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
 References
 
Based on the Medical Exposure Directive of the European Commission, 97/43/Euratom, The Radiation and Nuclear Safety Authority (STUK) in Finland has the responsibility for setting national diagnostic reference levels (DRLs) for the most common radiological examinations. Paediatric patients deserve special attention because of the higher radiation risk compared with adults. The purpose of this paper is to present a method that takes into account patient size when setting DRLs in paediatric patients. The overall data consisted of patient doses collected from six hospitals during the years 1994–2001, and new measurements in two hospitals in 2004. In total, there were 700 chest examinations. The method established by the National Radiological Protection Board (UK) for setting DRLs was not considered feasible in Finnish practice. Patient doses correlated exponentially with the projection thickness, which was measured directly for each patient. Since 1 January 2006, paediatric DRLs for conventional chest examinations have been specified in Finland as a DRL curve by using both dose quantities (entrance surface doses (ESD) and dose–area product (DAP)) as a function of patient projection thickness.


   Introduction
Top
 Abstract
 Introduction
Methods and materials
 Results
 Discussion
 Conclusions
 References
 
In Finland, about 4.1 million medical X-ray examinations are performed per year, of which close to 9% are for paediatric patients [1, 2]. X-ray examinations contribute nearly 15% to the total exposure of the Finnish population. X-ray examinations are thus the most significant source of radiation exposure after natural radon. As even small radiation doses increase the risk of stochastic radiation detriment, patient doses must be kept as low as reasonably achievable (ALARA principle) [3]. For children under the age of 10 years, the probability of fatal cancer induction is two to three times higher than for the whole population [4]. A number of publications cover the possibilities for optimizing patient doses in paediatric patients for different sizes (or ages) of children [5–10].

The International Commission on Radiological Protection (ICRP) has recommended the use of diagnostic reference levels (DRLs) as a first step in the optimization of diagnostic radiography [11]. By using DRLs, it is possible to find those hospitals where radiation doses are exceptionally high and where practices may need to be improved. The Medical Exposure Directive (MED), 97/43/Euratom, of the European Commission (EC) requires the member states to promote the establishment and use of DRLs for diagnostic examinations in radiology and nuclear medicine [12]. The decree 423/2000 of the Ministry of Health and Social Affairs takes account of the requirements of the MED in Finland [13]. According to this decree, the Radiation and Nuclear Safety Authority (STUK) in Finland has responsibility for setting national DRLs for the most common radiological examinations.

Patient size is an important factor in estimating the dose received from X-ray examinations. For adults, the influence of size is minimized by ensuring that the mean weight of the sample of patients is close to the reference weight of 70 kg for a standard patient. Variation in size and patient doses is most noticeable among children, and the use of a single reference size (as suggested by the EC and Kyriou et al [14, 15]) is impractical. Even dividing the children into different age groups, e.g. 0–1 years, 1–5 years, 5–10 years and 10–15 years, may not solve the problem because of the remaining high variation in size within each group [16].

The purpose of the present study was to collate the results of the existing work reported earlier for patient doses in paediatric X-ray examinations in Finland, and to present a method that takes into account patient size when setting the paediatric DRLs for thorax X-ray examinations.


   Methods and materials
Top
 Abstract
 Introduction
 Methods and materials
Results
 Discussion
 Conclusions
 References
 
Previous data and patient dose measurements
The data on patient doses used in the present study comprise published material collected previously by STUK and new measurements carried out in two hospitals.

The previous data had been collected in two research projects. The first was part of an EU-financed research programme on the radiation protection of the patient in paediatric radiology, where paediatric patient doses were collected in four university hospitals during the years 1994–98 [17]. In the second, patient doses were determined as part of an optimization project in two paediatric departments during the years 1999–2000 [18, 19]. In both projects, the dose–area products (DAP) were measured using DAP meters, and the entrance surface doses (ESD) were determined by calculation. The total number of patients in these projects undergoing chest examinations was 655. Data from other X-ray examinations were also collected, but the numbers were insufficient for statistical analysis.

The new measurements were done in the South Karelian Central Hospital (EKKS) and in a central health clinic (Armila Hospital), both located in the same health care district. Patient doses were only measured for chest examinations, because the number of other paediatric examinations was small. The total number of patient doses measured was 44 in EKKS and 33 in Armila. In EKKS, chest examinations were performed using a Philips Optimus Digital Diagnost X-ray machine (Philips, Eindhoven, The Netherlands) with a direct digital image intensifier, which was equipped with a display for computing DAP. The mobile X-ray machine for neonatal children (Mobilett Plus; Siemens Medical Solutions, Erlangen, Germany) was equipped with an immovable DAP meter (Kermax Plus; Scanditronix-Wellhöfer, Uppsala, Sweden). In Armila Hospital, two conventional X-ray film machines (Philips Super 50 CP: Vertical Diagnost 1 and CS-64) were used, and a movable DAP meter (VacuDAP 2000; VacuTec, Dresden, Germany) was alternated between them.

Patient dose measurements were carried out using the International Commission on Radiation Units and Measurements (ICRU) procedure [20]. Both DAP meters and the dose display were calibrated for all scan values used in the paediatric chest examinations by:


Formula 001

where kDAP is the calibration coefficient, DAPcal is the measured DAP value defined by multiplying the field cross-section area A and the measured absorbed dose in air Dair. Dair was measured with the Radcal 9015 electrometer and Radcal ionization chamber 10x5–6 (Radcal; Monrovia, CA), both calibrated at the secondary standard dosimetry laboratory in Finland. DAPmach is the value of the DAP meter or the dose display when irradiated with the same beam parameters.

ESD was calculated using the calibrated DAP values and the field areas defined from all images as:


Formula 002

where BSF is the backscattering factor, which ranged between 1.33 and 1.46 depending mainly on the field cross-section A, tube voltage U and the filtration f used [21–23]. To ensure that the ESD and DAP values were correctly determined, the ESD was also calculated independently of the DAP measurements by:


Formula 003

The calculation was based on the X-ray tube yield YU,f measured by STUK for every X-ray machine used in the study with the scan values used in practice. The output depends on the tube voltage U and the filtration f. Q in Equation (3) is the product of tube current I and exposure time t (mAs), FCD is the focus–chamber distance used in X-ray output measurements, and FSD is the focus–skin distance used in examinations.

The uncertainty of patient dose measurements
The relative uncertainties of the measured ESD and DAP values were assessed using the standard equation for independent variables [24, 25]. For the ESD, the relative uncertainty becomes:


Formula 004

where terms of the form {delta}(p)/p represent the relative uncertainties of the various variables p defined above. For the tube output measurements, the {sigma}xmacr;(Y) represents the standard error of the mean value, and the {sigma}xmacr;(Y)/Y is the corresponding relative uncertainty. For the DAP, one obtains similarly:


Formula 005

where {sigma}(kDAP) is the relative uncertainty of the calibration coefficient:


Formula 006

The {delta}(Dair)/Dair term in Equation (6) represents the ionization chamber used in calibration, {delta}(A)/A is for measurements of the area of the radiation beam, {delta}(r)/r is for the positioning of the ionization chamber and the X-ray film and {delta}(DAPmach)/DAPmach is for the intrinsic error of a DAP meter including the linearity and the resolution of the reading.

Diagnostic reference levels
The National Radiological Protection Board (NRPB) has published a method for taking the patient size into account when setting DRLs [16, 26]. The method is based on the normalization factors FESD and FDAP defined as:


Formula 007

and


Formula 008

Using these factors, the ESD values for a sample of patients, ESDd, can be normalized to represent the ESD values of five standard sized patients, ESDs. The µeff in Equations (7) and (8) is the effective attenuation coefficient, s is the thickness of the standard sized patient, and d is the thickness of the "real" patient in the sample.

The method was applied to the data used in the present study with the following approximations. The coefficient µeff determined at 100 kV was used for all analysis. In the original study by the NRPB [16], the normalization factors for chest examinations were determined without the grid. However, an anti-scatter grid is often used in paediatric chest X-ray examinations in Finland. The influence of the grid in chest examinations was estimated from the abdomen and pelvis data by the NRPB [26]. Finally, the patient thicknesses were estimated from the weight and height data [26] from EKKS, as the thickness measurements performed by the hospital were inaccurate.

The intensity of radiation decreases exponentially with object thickness. The ESD can be approximated by the simplified equation [27]:


Formula 009

where D0 is the required dose at the image receptor (automatic exposure control), µ is the homogeneous attenuation coefficient and d is the patient thickness (or diameter). The National Institute of Radiation Hygiene in Denmark (SIS) planned to use a graphical method, where paediatric reference levels are given as a function of patient diameter [28]. SIS planned to use an equivalent cylindrical diameter, calculated by means of body mass and height, to approximate patient thickness.


   Results
Top
 Abstract
 Introduction
 Methods and materials
 Results
Discussion
 Conclusions
 References
 
The results of the patient dose measurements for thorax X-ray examinations are shown in anteroposterior (AP) (or posteroanterior (PA)) and lateral (LAT) projections in GoFigures 1 and 2Go, respectively, as functions of patient thicknesses. Patient doses in the Armila hospital were clearly below those at EKKS in both projections and with both dose quantities (ESD and DAP). A reasonable correlation was found for the exponential fits of the doses as a function of patient thickness except in the case of the mobile X-ray machine, which was used only for neonates with small differences in thicknesses.


Figure 1
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  		Figure 1. Entrance surface doses(ESD; a) and dose–area products (DAP; b) presented as a function of patient thickness with exponential curve fits in the anteroposterior (AP) or posteroanterior (PA) projection. In Armila Hospital, the doses were measured in two examination rooms with Philips Vertical Diagnost 1 (open squares) and CS-64 (solid squares). In EKKS, the immovable Philips Digital Diagnost (open circles) and the movable Siemens Mobilett Plus (solid circles) examination devices were used. The number of patients examined in AP or PA projection was 77.

 

Figure 2
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  		Figure 2. The entrance surface doses(ESD; a) and dose–area products (DAP; b) presented as a function of patient thickness with exponential curve fits in the lateral (LAT) projection. The doses were measured in two examination rooms with Philips Vertical Diagnost 1 (open squares) and CS-64 (solid squares) in Armila Hospital, and in one examination room with a Philips Digital Diagnost (open circles) in EKKS. The number of patients examined in LAT projection was 56.

 
The relative uncertainties (1 standard deviation (SD)) of the variables in the uncertainty calculation of ESD and DAP are presented in Table 1Go. The total uncertainty with the 95% confidence interval (CI) was found to be 19% for ESD and DAP. For DAP, this is the average uncertainty for a range of uncertainties from 17% to 32%, corresponding to the resolutions of the DAP display of 0.1 and 1.0 mGy cm2, respectively.


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  		Table 1. The relative uncertainty factors used in the uncertainty estimations(1 SD)

 
The results of applying the NRPB method to calculate DRLs are shown in Table 2Go, where third quartile DAP and ESD values are calculated with and without the anti-scatter grid. The normalized DRLs of standard sized patients increase weakly as a function of patient size, as presented in Figure 3Go. The uncertainties for the normalization factors, which were mainly caused by the approximations, were 12% for the FESD and, depending on patient thickness, 20–40% for the FDAP.


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  		Table 2. The third quartile patient dose values of standard sized patients

 

Figure 3
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  		Figure 3. The third quartile values calculated for entrance surface doses(ESD; solid circles) and dose–area product (DAP; open circles) by the NRPB method in anteroposterior (AP) or posteroanterior (PA) projection (a) and in lateral (LAT) projections (b). Error bars represent the relative error of the normalization factors used in the calculation.

 
For convenience, in order to calculate the mean and the third quartile patient dose values without using the NRPB method, the patient dose data were divided into five thickness groups. The patient dose values were calculated by comparing the mean values of the thickness groups of each hospital. As presented in Table 3Go, the ESD values (third quartile) ranged from 40 to 83 µGy in AP or PA projection and from 57 to 315 µGy in LAT projection. The DAP values (third quartile) ranged from 6 to 48 mGy cm2 in AP or PA and from 9 to 142 mGy cm2 in LAT projection. The tabulated (third quartile) values are shown in Figure 4Go along with the exponential fits.


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  		Table 3. The standard deviations(SD), mean and third quartile statistical patient dose values calculated for different thickness groups in anteroposterior (AP) or posteroanterior (PA) projection and in lateral (LAT) projection

 

Figure 4
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  		Figure 4. Calculated third quartile values for entrance surface doses(ESD; solid circles) and dose–area product (DAP; open circles) in chest examinations as a function of patient thickness in anteroposterior (AP) or posteroanterior (PA) projection (a) and lateral (LAT) projections (b). A very good exponential correlation (R2  =  0.99) was found between the patient doses and patient thickness in LAT projection. In AP or PA projection, correlation was good (R2  =  0.86 for ESD, R2 = 0.90 for DAP).

 

   Discussion
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
Conclusions
 References
 
Accuracy of patient dose determination
The estimated uncertainty of patient dose calculations was 19% for ESD measurements and, on average, 19% for DAP measurements. This uncertainty should be taken into consideration when the results of patient dose measurements are compared with the DRLs: when the doses are only slightly above the DRL, this may be explained by the uncertainty of measurements, and no strict conclusions on other reasons can be made.

According to the International Electrotechnical (IEC) standard [25], the resolution of the reading of the DAP meter should be less than or equal to 10% within the whole effective range of indicated values, and the combined standard uncertainty of 25% should not be exceeded. This indicates that a resolution of 0.1 mGy cm2 is needed for DAP in paediatric chest examinations. The maximum uncertainties in our study were 17% and 32% when DAP meters with resolutions of 0.1 mGy cm2 and 1.0 mGy cm2 were used, respectively, for measuring the smallest patient doses. When a DAP meter is designed and calibrated for paediatric purposes, its use in patient dose measurements is reliable, and the use of a DAP meter is convenient when performing the large number of patient dose measurements needed in patient dose surveys. The DAP reading can also be converted to the effective dose. However, there are variations in individual field sizes, which should be properly accounted for in calculations of the effective dose [8], e.g. by using the Monte Carlo based PCXMC software (STUK, Helsinki, Finland [20, 29]).

Paediatric patient doses in Finland
Patient dose in a single chest examination is relatively low but, as about 30% of all paediatric X-ray examinations are chest examinations [2], the contribution to the total paediatric radiation exposure is significant. If a chest examination is performed for a neonate, for instance, the examination is most likely to be repeated – in Finland, the mean number of repeated X-ray examinations during the first year of life was 7.9 [30]. As both the SDs and the mean values of the patient doses in each thickness group are high compared with the study by Cook et al [31], it is clear that there is a need for optimization in the examination protocols. STUK has published guidelines for paediatric X-ray examinations in Finland to be used as a helpful tool for deciding the optimal examination technique [32].

Application of the NRPB method
The method for setting the paediatric DRLs published by Hart et al [16, 26] was applied to the data used in the present study with a few approximations. First of all, the highest tube voltage used in measurements of the µeff in the paper for the abdomen and pelvis [26] was 100 kV, which was close to the average (105 kV) of voltages used in chest examinations in the present patient data. Second, an anti-scatter grid is frequently used in chest X-ray examinations in Finland in the case of small children but, in the original study by the NRPB [16], the normalization factors were determined without the grid for chest examinations. Owing to the approximations of the mean tube voltage and uncertainty over the use of the thorax grid, the total uncertainty of the method was increased to 12% for ESD and to 20–40% for DAP depending on patient thickness.

As the use of the grid was seldom reported in the Finnish questionnaire, the total number of patients included decreased from about 700 chest examinations to nearer 200. The amount could be increased to 380 patients in AP or PA projection and to 340 in lateral (LAT) projection by assuming that the grid was routinely used for children of more than 1 year old in the two hospitals.

In addition, the total filtration in our data varied a lot as copper was often used as an extra filter but, in the original publication [26], only 3–4 mm of aluminium was used. For these reasons, the method was not considered to be feasible for Finnish practice.

Setting of the DRL
Projection thicknesses of the patients, which were measured at the central axis of the radiation beam, were found to correlate well with both patient dose quantities (ESD and DAP). The correlation has been noted in previous examinations as well [5–10]. It is evident that patient thicknesses are used when deciding suitable exposure parameters and, when using the automatic exposure control (AEC), the impact of patient thickness is accommodated automatically [7, 30]. However, thickness measurements should be made accurately to ensure that no extra uncertainty is caused by these measurements.

Compared with adults, the number of paediatric X-ray examinations can be relatively small; therefore, it would be difficult to collect enough patient data (at least 10 patients [33]) if the DRLs were given separately for each thickness group. A more practical method is to present the DRLs as a function of patient thickness. When paediatric DRLs are presented as a curve, hospitals can compare their patient doses directly against the graph, and the need for a large number of patients is significantly reduced. By using the graphical method, it is simple to find out whether the paediatric exposure parameters are well optimized, as any unusual behaviour in a hospital's patient dose data can easily be compared with the DRL curve.

In Finland, paediatric DRLs were issued from 1 January 2006 for conventional chest and sinus X-ray examinations and for micturating cystourethrography fluoroscopy examinations [34]. In chest examinations, the DRLs were given as DRL curves for AP or PA projections and for LAT projection by using both dose quantities as a function of patient thickness x (cm). The shape of the DRL curves is A = B·exp(µx).

According to the guidance published by the EC [35], the DRLs should be set taking into account individual regional circumstances such as the availability of equipment and training, and the DRLs should be based on patient doses measured not only in well-equipped hospitals. Following this principle, and because the use of removable grid techniques in paediatric examinations in Finnish health care units is not always possible, the DRLs given by STUK are intended commonly for grid and non-grid techniques. However, in those hospitals where the removable grid technique is not available, the DRLs will most likely be exceeded, as the majority of the dose data from small patients were collected with non-grid techniques.

The graphical chest DRLs can be used in hospital in two different ways: patient doses can be inserted manually into the printed DRL graph or the doses can be compared by calculation using a spreadsheet program (Figure 5Go). If there are more individual patient doses above the DRL curve than below it, the DRLs are concluded to have been exceeded. The manual method is suitable for those small X-ray units that may lack knowledge and effective working time to use calculations for DRL comparisons. In the calculation method, an exponential curve is fitted to the patient dose data, and the fit is compared with the DRL curve. A simple Excel program for comparison of the paediatric chest DRLs can be freely downloaded from the STUK web pages.


Figure 5
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  		Figure 5. An example of comparing patient doses in hospital with the diagnostic reference levels(DRL) curve (solid line). Manually inserted patient doses are presented as solid circles, and the curve fitted to the patient dose data is presented as a dotted line. As there are more individual patient doses above the DRL curve and the curve fit is above the DRL curve, the DRL is found to have been exceeded.

 
According to a recent enquiry to university and central hospitals, the preliminary experiences in the application of the DRL curve are promising. As expected, using the DRL curve has simplified the procedure and reduced the workload when comparing patient doses with DRLs. Of even more importance to the hospitals, the graphical illustration of patient doses has provided an overview of the whole scale of the patient doses, which has then helped to choose an optimal imaging technique depending on patient size. Most of the criticism has related to the measurement of patient thickness in uncooperative children. One way to solve this problem is to measure the thickness from the X-ray images if both AP and LAT projections are taken. Within the next 3 years, there will be more experience to judge the practicality of the DRL curve because, under Finnish legislation, the comparison of patient doses with the DRLs will be carried out at least every 3 years.


   Conclusions
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
References
 
According to the uncertainty estimations in this study, the achievable accuracy of ESD and DAP determinations is close to 20%, which should be considered when comparing patient doses at a hospital with the DRLs. Mean DAP and ESD ranged from 5 to 39 mGy cm2 and from 34 to 66 µGy in AP or PA, and from 8 to 109 mGy cm2 and from 52 to 226 µGy in LAT projection depending on patient thickness. The results suggest that there is a need for optimization of paediatric thorax imaging in Finland. The method established by the National Radiological Protection Board (UK) for setting paediatric DRLs was not considered feasible in Finnish practice because of its complexity and the extra uncertainty introduced by the approximate match between our conditions and the NRPB parameters. Because exponential curve fitting for DAP and ESD values correlated well with patient thicknesses, the graphical method seems to be ideal for setting the DRLs when a sufficient number of patient dose measurements are not easily available. STUK has now specified paediatric reference levels for thorax imaging in Finland by using exponential curves that are a function of patient thickness.


   Acknowledgments
 
The authors would like to thank the personnel of the X-ray departments of the EKKS and Armila Hospitals for participating in patient dose measurements, and the members of the Radiation Metrology Laboratory of STUK for practical help in patient dose measurements and in the uncertainty estimations.

Received for publication March 12, 2006. Revision received August 14, 2006. Accepted for publication August 30, 2006.


   References
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
 References
 

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