Application of international standards to diagnostic radiology dosimetry

Transcription

1 Application of international standards to diagnostic radiology dosimetry Poster No.: C-780 Congress: ECR 2009 Type: Scientific Exhibit Topic: Physics in Radiology Authors: I. D. McLean, A. Meghzifene, F. Pernicka; Vienna/AT Keywords: Dosimetry, phantoms, Calibration, DAP DOI: /ecr2009/C-780 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 14

2 Purpose Purpose: An international protocol on dosimetry in diagnostic radiology has recently been developed by the IAEA (TRS 457) for implementation in both Secondary Standard Dosimetry Laboratories (SSDL) and the clinical workplace. An analysis of the implementation of TRS 457 has been made under a coordinated research programme (CRP) with 11 participants (see Conclusion section). The work plan for the CRP is below (Table 1) and this communication will give some overview of activities 1, 4 and 5 as defined in the Methods section. Table 1. Activity list Activity 1 Setting-up calibration beam qualities at SSDLs Activity 2 Development of calibration procedures including the uncertainty budget at SSDLs Activity 3 Comparison of calibration of a selected instrument in selected beam qualities at SSDLs Activity 4 Evaluation of measurement procedures in hospitals, including: 1. Research the feasibility of implementing the procedures described in TRS 457 for making dosimetric measurements using phantoms and for patient data collection. 2. Report on the availability of dosimetric instrumentation and recommended phantoms, and the possibility of phantom fabrication if needed. 3. Create uncertainty budget for each type of dosimetric estimation, including the dose estimation from patient data. 4. Compare phantom measurements with patient dose data for each modality. Activity 5 Calibration of KAP meters at the SSDLs and at the clinical centres Activity 6 TLD dosimetry audit for SSDLs and clinical centres Page 2 of 14

3 Activity 7 The implementation of practical peak voltage (PPV) What does a secondary standard's laboratory do and look like? SSDLs have a role to calibrate radiation detectors for diagnostic radiology use that comply with International Electrotechnical Commission (IEC) standards. They are typically equipped with Xray systems with a stationary anode X-ray tube utilising a continuous tube current (e.g ma). In most cases, a shutter is incorporated allowing the timing control for an exposure. IEC-61674, IEC, Geneva (1997). Setup for diagnostic X ray calibration at IAEA laboratory Typical calibration X ray tubes and shutter system Page 3 of 14

4 Methods and Materials Methods and Materials: The coordinated research project, over the last 3 years, had two main directions. First (A), to investigate the suitability of dosimetry standards and protocols used to calibrate dosimetric instruments through SSDLs, including detectors for mammography, CT and kerma area product (KAP) meters. The second (B) was to investigate the clinical application of dosimetry protocols. For this communication the work of establishing reference beam qualities in SSDLs is examined, with special attention to areas of interest such as mammography and KAP meter calibration. The IEC has defined reference beam conditions for use in diagnostic radiology and the beams discussed in TRS 457 are summarised below Table: Radiation qualities for calibrations of diagnostic dosimeters used in TRS 457 Page 4 of 14

5 Radiation quality Radiation origin Material of additional filter an Application RQR Radiation beam no phantom emerging from X ray assembly General radiography, fluoroscopy and dental applications (measurements free in air) RQA Radiation beam with Aluminium an added filter Measurements behind the patient (on the image intensifier) RQT Radiation beam with Copper an added filter CT applications (measurements free in air) RQR-M Radiation beam no phantom emerging from X ray assembly Mammography applications (measurements free in air) RQA-M Radiation beam with Aluminium an added filter Mammography studies IEC-61267, IEC, Geneva (2005). Table: Calibration beams: RQR (IEC 61267) Beam quality Tube voltage HVL1 Homogeneity coefficient, h kv mm Al RQR RQR RQR RQR 5* RQR RQR RQR RQR RQR X ray setup with filter wheel used to create beam qualities. Note monitor chamber needed when comparing the standard and detector under calibration Page 5 of 14

6 Table: Calibration beams: RQR(M) (IEC 61267) Radiation quality X ray tube voltage Nominal first half-value layer Mo target kv mm Al RQR-M RQR-M 2* RQR-M RQR-M SSDL laboratories used TRS 457 to establish the above beam qualities for calibration of general diagnostic dosimeters and mammographic dosimeters. The calibration of KAP meters is also described in TRS 457 and the use of RQR beams for this was investigated. Clinical Applications Page 6 of 14

7 TRS 457 is a complementary publication to ICRU 74 and uses defined terminology and symbols. Quantity ICRU IAEA Incident air kerma Ka.i Ki Entrance-surface air kerma Ka,e Ke Air kerma-area product PKA PKA Air kerma-length product PKL PKL CT air-kerma index free in-air CK (integration -# to +#) Ca,100 (integration -50 to +50 mm) CT air-kerma index in the CK,PMMA (CK,PMMA,100) standard phantom CPMMA,100 These quantities are measured using protocols for two basic situations; (i) with the use of phantoms and (ii) to estimate dose to patients from radiographic and patient data combined with a characterization of relevant tube output. This is relatively straightforward for general radiography and also fluoroscopy where recommended phantoms can be utilised. In this communication the comparison of results for phantom and patient doses are reported. In the case of CT dose estimation TRS 457 utilises the IEC system of CT dosimetry which utilises on two circular PMMA phantoms of defined diameter. The practicality of the data collection methods for patients is reported on. Phantoms used for abdominal and chest dosimetry, also CT below: Page 7 of 14

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9 Results Results: Establishment of beam qualities: The required set of IEC beam qualities were successfully implemented in the SSDL laboratories in the project. While the calibration protocols developed for most radiographic applications, including plane X-ray, proved suitable and transferable to SSDLs it was noticed that the RQR (M) beams did not cover the full range of clinical use in mammography as shown below for a molybdenum target. Page 9 of 14

10 (courtesy C Hourdakis) In this graph it is noted that the compression paddle (not accounted for in IEC beam qualities creates clinical beams that easily exceed calibration conditions. In addition it is recognised that target materials other than W and Mo are also used and not covered by calibration standards. While well designed ionization chambers show little change in the energy correction factor kq, some solid state detectors may have considerable differences and so the definition of appropriate beam qualities is important clinically. More complexity was encountered with the calibration of KAP meters, where new beam qualities may be required to account for conditions appropriate to the high dose areas of interventional radiology (LIN 2007). Note that for thin objects (patients) that the X ray unit described above will use 60 kv and filtrations of 0.9 mm Cu. This has been confirmed by (Vano et al. 2008) Such beam qualities are not covered by RQR beams. As mentioned above this may not be of great concern for detectors which have little energy dependence such as ionisation chambers with the Page 10 of 14

11 noteable exception of KAP meters which have very thin metal (transparent) electrodes and display considerable energy dependence as seen below. This data comes from STUK and is adapted from Toroi et al (2008) The participants from STUK (above) have further shown that when the clinical beams as described by Lin et al are used that the behaviour of the energy dependant calibration factor cannot be characterised by the simply use of HVL, further emphasising the need for separate new beam calibration conditions. As is clearly seen this has significant implications for accurate dose estimation in interventional radiology. It is also clear that the use of only one energy correction term for all beam qualities will considerably increase the uncertainty of measurement used in patient dose estimation. Page 11 of 14

12 Other work in resulting from TRS457 confirms the need to calibrate field KAP meters in situ with the use of a reference instrument, rather than the independent calibration of KAP meters in standard laboratories. Clinical protocols and measurements Generally the clinical application was successful; however, more work is still required in implementation for effective use in the countries that contributed. Considering patient dose and phantom dose for measurements made on the same X ray units the figures below show that generally patient dose exceeds phantom dose. (courtesy P Homolka) Page 12 of 14

13 The average ratio of these results is also summarised in a single figure. Ratio of patient to phantom doses, dotted line corresponds to mean value, solid line in the box to median. The shaded area contains the 1st to 3rd quartiles. As seen there is a large spread in the ratio of patient to phantom dose. This is hardly surprising given the geographical spread of the data collected, including sites in Asia and Europe, where the average size of patient would be expected to vary significantly. While it might be felt that the collection of patient data is more reliable that the use of phantom data it should be considered that phantom dose values, which can be made rapidly, are valuable for dose intercomparison exercises. The area of CT dosimetry was found to be satisfactory for most contemporary needs; however, continuing developments in equipment design pose ongoing challenges, as does the diversity of acquisition parameters used by different scanners. Page 13 of 14

14 Conclusion Conclusion: Establishment of beam qualities: TRS 457 was found to give sufficient guidance for the successful establishment of IEC beam qualities to allow the calibration of appropriate detectors for use in diagnostic radiology. It was further found however that limitation exist in the IEC beam qualities as they do not cover a number of significant possible radiological setups that are used for mammography and interventional examinations. Clinical protocols and measurements Generally units and protocols useful for patient and phantom dose estimation are able to be applied to the clinical environment. Further it is clear that both patient and phantom dose are important in diagnostic dosimetry. Patient dose, while time consuming and demanding in terms of data collection, should result in accurate estimations. Phantom dose estimations on the other hand can be made rapidly and are useful in intercomparisons. Recommendations from areas covered in this communication. 1. Since existing IEC beam qualities do not fully cover important areas of diagnostic radiology, such as significant conditions for mammography and interventional radiology, particularly for small patients, new appropriate beam conditions should be agreed upon for definition by the IEC and then established at primary standards laboratories. 2. The determination of patient dose as described in TRS457 has significant clinical impact when the results are compared to appropriate dose reference levels. Together with relevant image quality data, this will inform the process of optimization. 3. CT Scanner type specific data sheets should be used to assist in data collection 4. Further work is needed to extend the CT procedures to accommodate the new developments in CT technology. 5. Field KAP meters should be calibrated (or cross calibrated) against a reference KAP meter or air kerma meter in situ in the clinical environment. Page 14 of 14