Testing of the Implementation of the Code of Practice on Dosimetry in X-ray Diagnostic Radiology Hungarian Contribution Ferenc Giczi a*, Sándor Pellet b, Ian Donald McLean c and Ahmed Meghzifene c a Széchenyi István University, Department of Physics and Chemistry, Egyetem tér 1., H- 9026, Gyır, Hungary. b National Centre for Healthcare Audit and Inspection, Váci u. 174., H-1138 Budapest, Hungary. c International Atomic Energy Agency, Division of Human Health, Department of Nuclear Sciences and Applications, Wagramer Strasse 5, A-1400 Vienna, Austria. Abstract. Medical ionization radiation sources give by far the largest contribution to the population dose from man-made sources and most of this contribution comes from diagnostic x-rays. Radiologist constantly face the dilemma of trying to minimize patients exposure whenever possible, while still using exposures that are high enough to produce images of good quality to be able to provide proper diagnosis. Quality assurance provides a framework for achieving this goal. As the part of the QA, there is a need to control patient doses arising from x- ray diagnostic activity. Due to the increased demand for dosimetry measurements in diagnostic and interventional radiology, it has become important to provide traceability of measurements, which can only work satisfactorily if correct calibrations and measurements are performed. In order to advice Secondary Standard Laboratories and end-users in hospitals on how to calibrate diagnostic dosimeters and determine the patient doses resulting from x- ray examinations, the International Atomic Energy Agency initiated the development of a Code of Practice (CoP) for dosimetry in x-ray diagnostic radiology. For testing of various procedures described in the CoP before their broad implementation in practice the IAEA initiated a Coordinated Research Project (CRP) entitled Testing of the Implementation of the Code of Practice on Dosimetry in X-Ray Diagnostic Radiology. Hungarian partner is involved in the following activities: evaluation of measurement procedures general radiography, fluoroscopy, mammography, computed tomography, dental radiology - in hospitals, calibration of KAP meters and TLD dosimetry audit. The Coordinated Research Project is in progress. KEYWORDS: patient dosimetry; diagnostic radiology; medical radiation; x-ray exposure. 1. Introduction Medical ionization radiation sources give by far the largest contribution to the population dose from man-made sources and most of this contribution comes from diagnostic x-rays. Radiologist constantly face the dilemma of trying to minimize patients exposure whenever possible, while still using exposures that are high enough to produce images of good quality to be able to provide proper diagnosis. Quality assurance provides a framework for achieving this goal. As part of the QA, there is a need to control patient doses arising from x-ray diagnostic activity. Due to the increased demand for dosimetry measurements in diagnostic and interventional radiology, it has become important to provide traceability of measurements, which can only work satisfactorily if correct calibrations and measurements are performed. In order to advice Secondary Standard Laboratories and end-users in hospitals on how to calibrate diagnostic dosimeters and determine the patient doses resulting from x-ray examinations, the International Atomic Energy Agency initiated the development of a Code of Practice (CoP) for dosimetry in x-ray diagnostic radiology [1]. For testing of various procedures described in the CoP before their broad implementation in practice the IAEA initiated a Coordinated Research Project (CRP) entitled Testing of the Implementation of the Code of Practice on Dosimetry in X-Ray Diagnostic Radiology. The CoP provides methods for clinical dosimetry in general radiography, fluoroscopy, mammography, CT and dental radiography. For each modality, the procedures for phantom measurements as well as the procedures for measurements on patients are describes. Hungarian partner is involved in the following activities: evaluation of measurement procedures * Presenting author, E-mail: giczif@sze.hu 1
general radiography, fluoroscopy, mammography, computed tomography, dental radiology - in hospitals, calibration of KAP meters and TLD dosimetry audit. The authors illustrate their measurements in chest PA radiography using phantoms as well as patient dosimetry and summarize their preliminary results and conclusions connecting to the other activities in the CoP. 2. Clinical measurements 2.1 General radiography 2.1.1 Measurements using phantoms In general radiography the incident air kerma was the dosimetric quantity measured for the chest PA and lumbar spine AP projections. The CDRH chest and CDRH abdomen/lumbar spine phantoms were the standard phantoms used utilizing the x-ray exposure parameters for an average-sized adult patient. In case of chest examination, the technologist was asked to set up the x-ray equipment for a normal adult patient, including the selection of exposure parameters, focus to skin distance, collimation, etc. The phantom was positioned so that it rests directly against the front plate of the vertical Bucky (Figure 1.). The distance between the focus and the surface of the vertical Bucky was measured and recorded, d FTP. The dosimeter was placed in the probe holder, sufficiently above the phantom surface to reduce backscatter and positioned outside the AEC detectors. Distance d m between the reference point of the dosimeter and the Bucky surface was measured and recorded. The ionization chamber of the detector were exposed three times under automatic exposure control or manual control as appropriate. Dosimeter readings and exposure parameters used were recorded. Finally, the ambient temperature and pressure were recorded. Figure 1: Set-up for measurements using a CDRH chest phantom d m t P t FTD The dose calculation was performed in two steps. First the air kerma, K(d), at the measurement point (at a distance, d m, from the Bucky surface) was calculated from the mean value of dosimeter readings using Eq. 2.1. K(d) MNK,Q0 = k k (2.1) Q TP In Eq. 2.1, k TP is the correction factor for temperature and pressure, N K,Q0 is the dosimeter calibration coefficient, k Q is the factor which corrects for differences in the response of the dosimeter at the calibration quality Q 0, and the quality Q of the clinical x-ray beam. In the second step, using the inverse square law (Eq. 2.2), the incident air kerma, K i, to the standard chest was calculated. K i 2 dftd d m K(d) = (2.2) d FTD t p In Eq. 2.2, t P is the thickness of the phantom. Testing of the implementation of CoP methodology were performed on 3 chest radiography x-ray equipment. The main characteristics of the equipment and the measured data can be found in Table 1. 2
Table 1: The main characteristics and the measured data for the chest phantom measurement. Identification HU-1 HU-2 HU-3 Generator TOP-X HF EMERIX EDR-750B Vertical Bucky BA-1 BA-1 BA-1 AEC Yes Yes No kv 100 85 90 mas 10 6.72 6.0 HVL (mm Al) 5.2 3.5 3.9 d FTD (mm) 1555 1450 1560 d m (mm) 525 500 515 Dose measurements were performed with a Radcal 9015 dosimeter, calibrated by the Hungarian National Calibration Laboratory. Calibration coefficient N K,Q0 =1.008 mgy/reading. Reference conditions at the calibration were the following: beam quality: BIPM 100, P 0 =101.325 kpa, T 0 =20 0 C. It is important to note, that in Hungary, the calibration laboratory specify the calibration coefficient of diagnostic dosimeters only for one beam quality, so no beam quality corrections could be performed. It was a problem that in some cases we could not find the mark of the focus position on the tube housing. In these cases the d FTD was measures from the centreline of the x-ray tube housing. The value of t P is 225 mm for the CDRH standard chest phantom. Dosimetric quantities and correction factors are summarized in Table 2. Table 2: Dosimetric quantities and correction factors for the chest phantom measurement. Identification HU-1 HU-2 HU-3 M (reading) 0.1290 0.3064 0.1279 k TP 1.005 1.004 0.991 k Q 1 1 1 K(d) (mgy) 0.1307 0.3101 0.1279 K i (mgy) 0.078 0.187 0.078 Uncertainties (mgy) ± 0.006 ± 0.013 ± 0.006 The relative expanded uncertainty (k=2) of the results was 7.2%, applying the scenario 2 in the CoP [1]. 2.1.2 Patient dosimetry As the first step of patient dosimetry, the x-ray tube output was measured for a representative set of tube voltages and tube loadings which adequately sample the patient exposure parameters used for chest PA projections. Geometry of measurements can be seen in Figure 2. The same geometry was used for the HVL measurements. Figure 2: Set-up for measurement of tube output and HVL 3
The detector was positioned to a distance, d, from the focus and sufficiently far from the patient support, avoiding the possible influence of backscatter radiation. In manual exposure control, one of the representative value of tube voltage, tube loading and field size were selected. The ionization chamber was exposed three times, and the readings were recorded. Following the methodology of the CoP, the HVL of the beam was also measured. The measurements were repeated on three tube voltages. Calculation of air kerma, K(d), at the measurement point (at a distance, d, from the x-ray focus) was the same procedure, described earlier, in paragraph 2.1.1. Finally, the K(d) values were normalized by the tube loading. Results of the measurements on the chest radiography equipment, identified by HU-2 can be found in Table 3. Table 3: Measurement of tube output and HVL on the chest radiography equipment (HU-2). Tube voltage (kv) 70 85 96 HVL (mm Al) 3.0 3.5 4.0 M (reading) 2.728 4.123 5.297 Y(d) (mgy/mas) 0.0893 0.1349 0.1734 Calibration coefficient of the dosimeter was N K,Q0 =1.008 mgy/reading. The correction factor for pressure and temperature was k TP =1.004 and k Q =1, because of the reason mentioned earlier. The measurement point was at a distance of 570 mm from the x-ray focus. Patient and projection data was collected by the technologist performed the chest x-ray examination for 10 patients. The collected data and the calculated incident air kerma as well as entrance surface air kerma values can be found in Table 4. These data are connected to the workplace identified by HU-2. Table 4: Exposure parameters and patient dose values for chest PA projections (HU-2). Patient ID. Tube voltage (kv) Tube loading mas t P (mm) Field size (mm x mm) K i (mgy) K e (mgy) S. J. (male) 96 19.5 330 430 x 430 0.88 ± 0.06 1.29 ± 0.10 T. E. (female) 96 20.2 320 430 x 430 0.89 ± 0.06 1.31 ± 0.10 R. K. (female) 90 7.68 240 430 x 430 0.260 ± 0.019 0.382 ± 0.029 Sz. B. (female) 85 5.00 210 430 x 430 0.143 ± 0.010 0.210 ± 0.016 M. J. (male) 90 10.56 250 430 x 430 0.363 ± 0.026 0.534 ± 0.041 R. R. (male) 96 6.25 240 430 x 430 0.240 ± 0.017 0.353 ± 0.028 T. T. (male) 96 25.6 340 430 x 430 1.17 ± 0.08 1.72 ± 0.13 D. F. (female) 85 9.60 260 430 x 430 0.297 ± 0.021 0.437 ± 0.034 Sz. Z. (female) 90 12.8 270 430 x 430 0.455 ± 0.033 0.67 ± 0.05 W. E. (female) 90 9.28 240 430 x 430 0.314 ± 0.027 0.462 ± 0.036 For each patient, the incident air kerma was calculated from the exposure parameters recorded, using Eq. 2.3. K i 2 d = Y(d)P (2.3) It dftd t p In Eq. 3., the x-ray tube output, Y(d), measured at a distance, d, from the tube focus on the different tube voltages was interpolated from the tube output data in Table 3. P It is the tube loading during the exposure of the patient. The tube focus-to-patient support distance, d FTD, was 1450 mm. The relative expanded uncertainty (k=2) was 7.2%, applying the scenario 2 in the CoP. However, it has to be 4
mentioned, that the patient thickness, t P, was measured by the technologist. Consequently the uncertainty of the t P values was not easy to estimate. The entrance surface air kerma was calculated from the incident air kerma values, using the appropriate backscatter factor for water. The backscatter factor was determined by interpolations, based on the tabulated data and the measured HVL and field size used during the exposures. 3. Other activities In Table 5 a summary of the Hungarian contribution to the CoP can be found. Based on our experiences, the following conclusions can be drawn. Generally, the methodologies of the CoP are clear and easy to follow. Datasheets make the data collections in the hospitals easy and fast. There are only some problems can be mentioned. These problems are not serious, but could be appeared in other countries too. Table 5: Hungarian contribution to the CoP. Activity Phantom measurements Patient dosimetry General radiography Chest PA projection 3 measurements on 3 x-ray 30 patient dose measurements on 3 x-ray Lumbar spine AP projection 3 measurements on 3 x-ray 30 patient dose measurements on 3 x-ray Fluoroscopy 3 measurements on 3 interventional cardiology x-ray 14 patient dose measurements on 2 interventional cardiology x-ray Computed tomography Dental radiology 20 free-in-air measurements on 2 CT scanners, 20 dose measurements in CT head phantom, 20 dose measurements in CT body phantom. Bitewing projection - 17 patient dose estimations on 2 CT scanners 6 patient dose estimations on 1 dental x-ray unit Panoramic projection - - Mammography 4 measurements on 4 mammography x-ray Calibration of KAP meters 2 KAP meters 41 patient dose measurements on 2 mammography x-ray There was a general problem, that the calibration laboratory does not specify the calibration coefficient of diagnostic dosimeters for more than one beam quality. Consequently we could not perform the beam quality correction of the dosimeter readings. The other general problem was, that on some of the x-ray tube housing there were not the mark of the focus position. In these cases, we measured the distances from the centre of the housing. In general radiography, the technologist should measure the patient thickness. Our experience is that it can be made only with relatively high uncertainties. For mammography measurements, it s worth mentioning that the calculation of glandular dose is very time consuming. The frequent interpolations of the tabulated data may be the sources of calculation errors. A computer program helping the users should be very useful and is highly suggested. 5
4. Conclusion In Hungary the overall patient dose measurements in diagnostic radiology was started in 1989 [2-10]. However, the applied methods were frequently changed, following the methods found in the literature. Consequently, temporal trends in patient doses could hardly be followed. In the framework of the coordinated research project, we could test of various procedures described in the CoP. It can be concluded, that the methods of the CoP are clear and can be followed. Datasheets make easy and fast consistent data collection in the hospitals. Uncertainty estimation promotes the evaluation of the patient doses. The CoP could be the base methodology of nationwide surveys of patient doses arising from diagnostic application of x-rays and determination of diagnostic reference levels. Dissemination of the methods of the CoP can promote the comparability of patient doses. REFERENCES [1] INTERNATIONAL ATOMIC ENERGY AGENCY, Dosimetry in Diagnostic Radiology: An International Code of Practice, Technical Reports Series No. 457, IAEA, Vienna (2007). [2] PERNICKA, F., GICZI. F., et al., Comparison of TLD Air Kerma Measurements in Mammography, International Symposium on Standards and Codes of Practice in Medical Radiation Dosimetry, Vienna, Austria, 25-28 November 2002., Book of Extended Synopses, IAEA-CN-96-47P, p. 94-95. [3] MACCIA. C., PADOVANI. R., VANO. E., REHANI. MM., et al., Image Quality and Patient Dose Optimization in Mammography in Eastern European Countries, World Congress on Medical Physics and Biomedical Engineering, Sydney, Australia, 24-29. August 2003. [4] Optimization of the radiological protection of patients: Image quality and dose in mammography (coordinated research in Europe), 2005., IAEA-TECDOC-1447 [5] PELLET, S., GICZI, F., et al., A Pilot Study of Radiation Exposures Arising From Interventional Radiology Procedures, 2006. Second European IRPA Congress on Radiation Protection, International Radiation Protection Association, Paris, France, Proceeding of Full Papers on CD-Rom. [6] PELLET, S., FAULKNER, K., VANO, E., PADOVANI, R., GICZI, F., et al., Hungarian Contribution to the SENTINEL Project, 2006. International Conference on Quality Assurance and New Techniques in Radiation Medicine, International Atomic Energy Agency, Vienna, Austria, 13-15 November 2006. Book of Extended Synopses IAEA-CN-146/261P, p. 542-543. [7] GICZI. F., PELLET. S., et al., Study on the patient dose of photofluorography in Hungary, The Central European Journal of Occupational and Environmental Medicine, 1996. 2(2): 181-190. [8] PELLET. S., GICZI. F., et al., Hungarian patient dose survey for photofluorography applied in mass chest screening program, Radiation Protection Dosimetry, (Vol. 80, Nos 1-3, pp. 115-116 (1998) [9] GICZI. F., PELLET. S., VANO. E., MACCIA. C., et al. Image quality and patient dose optimization in mammography in Hungary, International Conference on the Radiological Protection of Patients, Málaga, Spain, 26-30 March 2001. Contributed Papers, p. 169-173. [10] PELLET. S., GICZI. F., et al., Patient doses for computed tomography in Hungary, International Conference on the Radiological Protection of Patients, Málaga, Spain, 26-30 March 2001. Contributed Papers, p. 210-213. 6