Receptor dose and Patient Dose in Radiographic exposures - a 15 year review D J Peet*, N Tyler, M Pryor, P Hollaway, C Strudley and L Leavesley Regional Radiation Protection Service, Royal Surrey County Hospital, Guildford, UK Abstract A patient dose programme has been established locally for the last 15 years across 109 hospitals and 250 X-ray rooms in line with the National Protocol [4], in conjunction with a programme to look at routine performance of these rooms. Routine performance checks initially looked primarily at film density and AEC performance but with the introduction of Computerised Radiography (CR) across UK hospitals and a revision of recommended procedures in the UK [3], the emphasis has shifted to assessing receptor dose under AEC control. Results show a wide variation in film density in the early years indicating sub optimal performance and dose. The spread was reduced over later years. The introduction of CR has led to a variety of approaches by the CR companies, X-ray companies and local sites. Receptor doses vary widely as a result. Large variations within hospitals were also observed. The doses over the last 15 years are reviewed and compared against diagnostic reference levels and with the performance of the imaging chain. Results show that patient dose programmes and optimisation strategies were having an impact, but the introduction of CR requires renewed efforts to ensure images and doses are optimised. KEYWORDS: Radiography; Patient dose; QC; optimisation; CR Introduction In the UK routine performance measurements and patient dosimetry programmes are common place for diagnostic radiology facilities. Routine performance measurements are required to be carried out under UK regulations requiring QC programmes to be in place (Ionising Radiations Regulation 1999) [1] Patient dose assessments are required under the Ionising Radiation (Medical Exposure) Regulations 2000/2006 [2]. The methodology for these is well documented with Recommended Standards for the Routine Performance Testing of Diagnostic X-ray Imaging Systems [3] and a National Protocol for Patient Dose Measurements in Diagnostic Radiology [4]. The UK has been active in the setting of national and local diagnostic reference levels with published guidance as to the methodology and approach for this [5]. The DH have adopted new National reference levels [6] in line with the latest published figures and National reviews of dose [7]. The National Dose database is kept under constant review with third quartile doses for common examinations last published in 2007 [8]. A patient dose programme has been established locally for the last 15 years across 109 hospitals and 250 X-ray rooms in line with the National Protocol, in conjunction with a programme to look at routine performance of these rooms. Routine performance checks initially looked primarily at film density and AEC performance, but with the introduction of CR across UK hospitals and a revision of recommended procedures in the UK, the emphasis has shifted to assessing receptor dose under AEC control. Method A database was developed in Guildford with the aim of increasing the efficiency of routine surveys, by generating a paper survey form which included previous survey data; by streamlining data input and automating repetitive calculations; and by producing a report form with a summary of the results which could be sent to each hospital. 1
The aim was also to store the information about the hospitals surveyed, the X-ray equipment within those hospitals and the results of annual Physics radiation protection and quality assurance surveys carried out on that equipment. The database was to be designed to be flexible and to store the survey results without defining an exact measurement set for each piece of equipment. This would enable analysis of the results and allow reviews of measurement techniques, X-ray equipment performance and patient dose and also allow flexibility in measurement and retain a degree of professional judgement over the measurements required. A separate dataset was also developed to store patient dose data using the same tombstone data from the main database to enable analysis of dose against the performance measurements in a particular location and in a particular year. Simple queries were written to collate the results of AEC performance with film density or receptor dose once a hospital had switched to CR. The doses for common examinations were also grouped into 3 year periods and collated. The dose for the AP lumbar spine was expected to correlate with the standard setting of the AEC system. The dose for the PA chest examination was also reviewed as this is an examination normally carried out under manual control and therefore exposure parameters are more likely to be adjusted in relation to resultant image quality. Results The optical density of films taken under AEC control during routine performance checks using standard lumbar spine settings are shown in Figure 1. Figure 1: The normalised number of systems tested against the optical densities measured. 60 Normalised Number of Measurements (%) 50 40 30 20 10 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 Optical Density Range 2
The mean, standard deviation and number of optical density measurements are displayed in Table 1. Table 1: Variations in optical density measurements. Year 96 97 98 99 00 01 02 03 04 05 06 07 08 Mean Optical Density 1.24 1.29 1.29 1.31 1.34 1.31 1.29 1.31 1.37 1.29 1.32 1.33 1.27 Standard Deviation 0.40 0.34 0.30 0.31 0.25 0.24 0.29 0.26 0.27 0.23 0.24 0.45 0.17 Number of Measurements 77 92 117 205 171 168 167 451 291 139 88 46 46 Figure 2 shows the normalised number of measurements against receptor dose for the period of 2005 2008. Figure 2: The normalised number of systems tested against receptor dose measured. 25 Normalised Number of Measurements (%) 20 15 10 5 2005 2006 2007 2008 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 More Receptor Dose (µgy) The mean, standard deviation and number of receptor dose measurements are shown in Table 2. Standard deviation significantly increased in 2007 due to an increase in the number of installed units. Results from 2008 show that the spread of values has reduced following some optimisation. The numbers of measured systems are for the first six month period in 2008 and numbers are expected to be in the region of 1000 measurements for the 12 month period. 3
Table 2: Variations in receptor dose measurements. Year 05 06 07 08 Mean Receptor Dose (µgy) 3.5 3.6 3.8 3.4 Standard Deviation 1.4 1.7 5.1 1.1 Number of Measurements 490 460 663 503 Patient dose measurements over the same period are shown for AP lumbar spine and PA chest views in Figures 3 & 4 respectively. Mean values with standard error of the mean are plotted for each three year phase of measurements. This data has also been tabulated in Tables 3 & 4. Figure 3. Measured patient doses for lumber spine AP examinations between 1994 & 2008 Patient Doses for Lumbar Spine AP examinations over 15 years 12.0 Mean Entrance Surface Dose (mgy) 10.0 8.0 6.0 4.0 2.0 0.0 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 Year Mean Dose - Film Mean Dose - CR National Reference Dose 1980s National Reference Dose 1995 National Reference Dose 2000 (NDRL) National Reference Dose 2005 4
Figure 4. Measured patient doses for Chest PA examinations between 1994 & 2008 Patient Doses for Chest PA examinations over 15 years 0.350 Mean Entrance Surface Dose (mgy) 0.300 0.250 0.200 0.150 0.100 0.050 0.000 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 Year Mean Dose - Film National Reference Dose 1980s National Reference Dose 2005 Mean Dose - CR National Reference Dose 1995 & 2000 (NDRL) Table 3 Entrance surface dose measurements for AP lumbar spine examinations. Standard Mean dose (mgy) Year Deviation No. of records National Ref Doses (mgy) film CR film CR film CR 1980s 1995 2000 2005 94 6.9 3.9 17 97 5.1 2.1 10 00 4.7 1.8 12 03 3.4 3.4 1.6 7 10 7 6 5 06 4.0 4.2 2.8 1.6 14 9 08 3.9 3.6 1.7 2.1 3 14 5
Table 4 Entrance surface dose measurements for PA Chest examinations. Standard Mean dose (mgy) Year Deviation No. of records National Ref Doses (mgy) film CR film CR film CR 1980s 1995 2000 2005 94 0.148 0.072 45 97 0.121 0.056 23 00 0.133 0.057 30 03 0.068 0.015 15 0.3 0.2 0.2 0.15 06 0.087 0.136 0.033 0.085 20 12 08 0.065 0.126 0.008 0.059 3 21 Figure 5 Screen manufacturers used 1995 to 2008 3M Agfa Dupont Fuji Imation Kodak Figure 6 CR manufacturers systems in use Agfa Kodak Fuji Konica 6
Discussion Results of optical density measurements (figure 1) show that the QC programme was effective with a reduction in the standard deviation of measurements around the central target optical density of a single value in the range 1.1 1.5. Following a peak in measurements in 2003 a sharp decline in the number of systems tested using optical density is noted which corresponds with the fast introduction of CR in our area. Measurements of receptor dose were initiated in 2005 following the significant implementation of CR and therefore the loss of optical density as a marker of AEC performance. A receptor dose of 3.5 4.0µGy has been used as a target and these results are showing that a considerable number of systems are achieving this. The spread is relatively consistent over the 4 years of data. Optimisation is however only underway in this area from the beginning of 2008. It is expected that in the following 2009 round of surveys there will be a significant decrease in the variation due to recommendations made in the 2008 surveys. Patient dose over the period 1994 2008 shows a steady drop in doses which has been due to a number of optimisation factors including faster film screen combinations, new equipment and the standard use of AEC s The patient dose measurements for the AP lumbar spine view which is almost exclusively carried out under AEC control show that since the introduction of CR, mean doses remain close to those of film. It is hypothesised that this may increase by a small margin over the coming year as the systems are optimised for CR. Chest images predominantly use an out-of-bucky, medium kv, manual control exposure and are therefore not controlled by the AEC system. CR manufacturers data suggest that the equivalent speed for Kodak systems is closer to 300 or even 200 than the standard 400 speed film screen combination that had been in use in many hospitals. With the lower sensitivity of the CR systems image quality has been noted to adversely affected and therefore local optimisation has been carried out. This is clear in the dose data with the measured doses reported for CR systems being significantly greater than those still using film. Optimisation using film has been limited to an extent by the wide range of film types, screen types and their combinations. This has resulted in a wide variation in film screen speed and therefore corresponding dose and also a target density within a range of values. With this variation coupled with the processing variations due to such things as chemical types, temperatures and cycle times to name a few, the imaging chain variations were numerous. With the introduction of CR a number of these issues have been eliminated. CR readers and cassettes, for the same manufacturer, in general have the same response and therefore a number of variables have been removed. CR plates are coupled with a particular reader manufacturer removing the previous combinations possible and with the removal of chemistry, CR reader stability is also much less of an issue when compared to wet processing. Conclusions Optimisation has been taking place over the last 15 years with target values of optical density for AEC systems. At the same time doses for plain films have been reducing as the effect of a National dose measurement programme is felt. 7
The introduction of CR, if receptor doses are adjusted to the noise levels recommended by the manufacturer, is likely to result in an increase in dose over the next few years. To date AEC s have not been adjusted but are now being set at a receptor dose of 4mGy. In the UK National reference doses have not been adopted based on the latest dose review within the National dose database. In light of the changes in technology this appears to be a prudent approach as doses appear to be increasing in the short term following the introduction of CR. Further work is required to monitor this change. References [1] Health and Safety Commission Ionising Radiations Regulations 1999 (SI 1999 No 3232) (The Stationery Office, London). [2] Department of Health. The Ionising Radiation (Medical Exposure) Regulations 2000. Statutory Instruments 2000 No. 1059 (The Stationery Office, London). [3] Institute of Physics and Engineering in Medicine (2005) Recommended Standards for the Routine Performance Characteristics of Diagnostic X-ray systems used in Medicine Report No 91 (IPEM, York) [4] Dosimetry Working Party of the Institute of Physical Sciences in Medicine (1992) National protocol for patient dose measurements in diagnostic radiology. (NRPB, Chilton) [5] Institute of Physics and Engineering in Medicine (2004) Guidance on the establishment and use of diagnostic reference levels for medical x-ray examinations Report No 88 (IPEM, York) [6] Department of Health. Guidance on the establishment and use of Diagnostic Reference Levels (DRLs) as the term is applied in the Ionising Radiation (Medical Exposure) Regulations 2000 DH DRL Working Party (2007) http://www.dh.gov.uk/en/publicationsandstatistics/publications/publicationspolicyandguidance/ DH_074067 [7] Hart D, Hillier MC and Wall BF. Doses to Patients from Medical X-ray Examinations in the UK 2000 Review. (2002) NRPB-W14 (NRPB, Chilton). [8] Hart D, Hillier MC and Wall BF. Doses to Patients from Radiographic and Fluoroscopic X-ray imaging Procedures in the UK 2005 Review. (2007) HPA-RPD-029 (HPA, Chilton). 8