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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY. Part 19.03: Optimization of protection in Mammography Practical exercise. Overview.

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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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  1. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY Part 19.03: Optimization of protection in Mammography Practical exercise

  2. Overview • To be able to apply quality control protocols to mammography equipment • To measure the breast entrance surface dose and determine the average glandular dose • Interpretation of results 19.03 : Optimization of protection in Mammography

  3. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 19.03: Optimization of protection in Mammography Topic 1: Entrance surface dose (measurements on patients)

  4. Entrance surface dose (ESAK) • The Entrance Surface Air Kerma (ESAK) free-in-air, i.e., without backscatter, has become the most frequent used quantity for patient dosimetry in mammography • ESAK can be determined under reference conditions either with AEC or manual exposure 19.03 : Optimization of protection in Mammography

  5. Entrance surface dose (I) (measurements on patients) • This simple method of determining ESAK requires dosemeters from a central laboratory to provide first line information on the level of radiation dose being delivered to patients. • It is intended for facilities where local resources or expertise are not available. 19.03 : Optimization of protection in Mammography

  6. Entrance surface dose (II) (measurements on patients) • Select 10 patients with a compressed breast thickness in the range 4 to 6 cm • For each patient and for each projection, position one calibrated TLD on the upper inner quadrant of the breast and x-ray the patient normally • Remove the TLD and keep it away from radiation 19.03 : Optimization of protection in Mammography

  7. Entrance surface dose (III) (measurements on patients) • Complete the questionnaire for dosimetry on patient (provided by the central laboratory) • When all 10 TLDs have been used, return them together with the completed questionnaire to the issuing dosimetry laboratory • Compare the mean value of ESAK with the reference value of 10 mGy. 19.03 : Optimization of protection in Mammography

  8. Calibration of output (I) (Purpose) • This method of determining ESAK relies on the calibration of the radiation output of the X-ray machine together with a recording of the tube loading on a series of patients • It is suitable for X-ray machines which have an AEC system and a post-exposure display or for units with manual exposure control only 19.03 : Optimization of protection in Mammography

  9. Calibration of output (II) (Test equipment) • Select a dosimeter with a dynamic range from at least 0.5 to 100 mGy • Accuracy > ± 10% • Precision > ± 5 • Calibrate the dosimeter in terms of air kerma free-in-air at an HVL as close as possible to 0.4 mm of Al. • Use post-exposure technique readouts (mAs) 19.03 : Optimization of protection in Mammography

  10. Calibration of output (III) (test method - patient measurements) • Select 10 patients with a compressed breast thickness in the range of 4 to 6 cm • For the same view on each patient, record the mAs after exposure value or the value set on units with manual exposure control 19.03 : Optimization of protection in Mammography

  11. Calibration of output (IV) (test method - output measurements) • Place the dosemeter at the reference point (45 mm above the cassette table, 60 mm from the chest wall side and laterally centred). The compression plate should be in place • Set manual exposure mode (same kV, anode, filtration, etc.) as used clinically • Record the air kerma readings per mAs for the exposure that covers the range of values recorded on patients. 19.03 : Optimization of protection in Mammography

  12. Calculations (I) • Calculate the mean value of the mAs for the 10 patients • Determine the output, i.e., the air kerma/mAs • Calculate the mean value of ESAK corresponding to the mean value of tube loading • Calculate the mean ESD by multiplying the mean ESAK by the appropriate backscatter factor if the HVL is known 19.03 : Optimization of protection in Mammography

  13. Calculations (II) If the information on the HVL is lacking, apply a backscatter factor of 1.09 Backscatter factor as a function of HVL (Jansen et al. 1994) 19.03 : Optimization of protection in Mammography

  14. Assessment of results • Compare the mean value of ESAK with the reference value of 10 mGy • If the mean value of ESAK exceeds 10 mGy per view, it is necessary to investigate the reason and corrective actions must be taken 19.03 : Optimization of protection in Mammography

  15. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 19.03 : Optimization of protection in Mammography Topic 2: Entrance surface dose (measurements with phantom)

  16. Entrance surface dose (measurements with phantom) • This method is an alternative to the method of placing TLDs on patients where this is not deemed to be acceptable due to interference with patient examination • It is intended for facilities where local dosimetry resources and expertise are not available 19.03 : Optimization of protection in Mammography

  17. Tests equipment (provided by a central laboratory) • TLDs: calibrated in terms of air kerma free-in-air at a HVL as close as possible to 0.4 mm Al. • Standard phantom: PMMA (thickness 45  0.5 mm, 150 x 240 mm2) • Developed film strip with labelled optical densities • Questionnaire 19.03 : Optimization of protection in Mammography

  18. Test method (1) • Set up the x-ray equipment for a cranio-caudal view with the compression plate present and a cassette loaded in the bucky • Position the phantom on the breast table, make sure that it completely covers the AEC device • Place the TLD at the reference point 19.03 : Optimization of protection in Mammography

  19. Test method (2) • Expose the phantom to the same conditions as used clinically for a standard-sized breast • Process the film in the normal way • Verify that the film density (base + fog included) at the reference point is in the optical density range of 1.20 to 1.80 (densitometer measurement or visual comparison with the film strip provided by the laboratory) 19.03 : Optimization of protection in Mammography

  20. Test method (3) • Replace the TLD with a second one • If the film optical density is in the range 1.20 to 1.80 make a second phantom exposure aiming for a density of 1.50 • Finally process a fresh, unexposed film • Estimate the density of the background film by visually comparing with the density with a densitometer or with the test strip 19.03 : Optimization of protection in Mammography

  21. Test method (4) • Complete the questionnaire provided by the central laboratory • Return the TLDs together with the three films and the questionnaire to the issuing laboratory • Compare the value of ESAK provided by the laboratory to the limiting value of 11 mGy corresponding to a net optical density of 1.00 (for different values of optical density, see table) 19.03 : Optimization of protection in Mammography

  22. Limiting values for ESAK as a function of net optical density 19.03 : Optimization of protection in Mammography

  23. Calibration of output (Purpose) • This method of determining ESAK with a standard phantom gives information on compliance of the technical set-up of the mammographic equipment with guidelines on radiation dose. • It is suitable for X-ray machines which have an AEC system and a post-exposure display or for units with manual exposure control only 19.03 : Optimization of protection in Mammography

  24. Calibration of output(Test equipment) • A dosemeter with a dynamic range from at least 0.5 to 100 mGy • Accuracy > ± 10% • Precision > ± 5 • The dosimeter should be calibrated in terms of air kerma free-in-air at an HVL as close as possible to 0.4mm of Al. • A standard phantom: PMMA block of 45 mm; 150x240 mm2 • A densitometer (resolution of 0.01 density units) 19.03 : Optimization of protection in Mammography

  25. Test method (determination of mAs (1)) • Set up the x-ray equipment for a cranio-caudal view with the compression plate present and a loaded cassette in the Bucky • Position the phantom on the breast table, make sure that it completely covers the AEC device • Expose the phantom at clinically used technique 19.03 : Optimization of protection in Mammography

  26. Test method (determination of mAs (2)) • Record the mAs • Process the film • Measure the optical density and verify that it is in the range 1.20 to 1.80 • If necessary, adjust the AEC density setting to achieve a suitable optical density and repeat the procedure described above 19.03 : Optimization of protection in Mammography

  27. Test method (measurement of output) • Remove the phantom and place the dosemeter at the reference point. The compression plate should be in place • Set manual exposure mode and mAd determined previously • Record the dose measurement. If it is not possible to select the exact mAs, make two exposure that bracket this value 19.03 : Optimization of protection in Mammography

  28. Calculations • Apply the appropriate calibration factor to the dose measurement • Interpolate the results if it was necessary to make two bracketing exposures • Express the result as ESAK 19.03 : Optimization of protection in Mammography

  29. Assessment of results • Compare the mean value of ESAK with the limiting value of 11 mGy corresponding to a net optical density on the film of 1.00. For optical densities deviating from this value see the table • If the mean value of ESAK exceeds 10 mGy per view, it will be necessary to determine the cause and corrective must be taken 19.03 : Optimization of protection in Mammography

  30. Entrance surface dose Limiting value : £10 mGy for 40 mm PMMA, £12 mGy for 45 mm PMMA, £20 mGy for 50 mm PMMA. Frequency : Annually Equipment :Dosimeter, PMMA block 150x240 mm2, densitometer 19.03 : Optimization of protection in Mammography

  31. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 19.03: Optimization of protection in Mammography Topic 3: The average glandular dose

  32. Determination of average glandular dose (AGD) • The AGD cannot be measured directly. It is derived from measurements of the ESAK (with a standard phantom) and HVL. The ESAK is used with tabulated conversion factors (derived from Monte Carlo calculations) to determine the AGD. 19.03 : Optimization of protection in Mammography

  33. Average Glandular Dose Table 1: Conversion factors gPB for calculating the AGD for a 50 mm « standard breast DGS » from the ESAK (Ka) measured at the technique as for 45 mm standard PMMA phantom (Dance 1990) 19.03 : Optimization of protection in Mammography

  34. Limiting values for AGD as a function of net optical density Table 2 19.03 : Optimization of protection in Mammography

  35. Calculations • Derive the HVL by interpolation. This may be done by plotting the logarithm of the dose measurements against the relevant Al absorber thickness • Derive, if necessary by interpolation, the conversion factor gPB for the measured HVL from table 1. Multiply the measured ESAK, Ka, at the mAs determined for the correct exposure of a standard phantom and the relevant gPB to obtain the standard AGD DGS (breast) = gPB x ka 19.03 : Optimization of protection in Mammography

  36. Calculations (example) • An exposure of the standard phantom in the AEC mode, at 28 kV and Mo-Mo anode-filter combination requires 94 mAs • 0.080 mGy/mAs (average of values measured at 90 and 100 mAs) was determined from the calibration of the output at the reference point • The measured HVL was 0.32 mm Al, yielding a gPB value of 0.187, derived by interpolation from Table 1 19.03 : Optimization of protection in Mammography

  37. Calculations (example) • The standard AGD (DGS) is: DGS = 94 [mAs] • 0.080 [mGy/mAs] • 0.187 [mGy/mGy] DGS = 1.4 [mGy] 19.03 : Optimization of protection in Mammography

  38. Assessment of results • Compare the value of standard AGD with the limiting value of 2.3 mGy, derived from the ESD limiting value for a film optical density of 1.00 • Table 2 provides information for other film densities 19.03 : Optimization of protection in Mammography

  39. Where to Get More Information European protocol for the quality control of the physical and technical aspects of mammography screening. http://euref.org/index.php?option=com_phocadownload&view=category&id=1&Itemid=8 American College of Radiology Mammography Quality Control Manual, Reston VA, 1999. 19.03 : Optimization of protection in Mammography

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