The DAP Meter

1. The DAP Meter Teuvo Parviainen STUK Translation Eija Grönroos, Elina Allenius, Heidi Varonen, Metropolia University of Applied Sciences STUK: Teuvo Parviainen, VM, PP, TKo

2. What is DAP? Dose-area product, unit Gy·m2 • Product of primary beam area and absorbed dose in air • In x-ray examinations air kerma and area product is KAP ≈ DAP • DAP doesn’t include scattered radiation from patient, phantom or environment • The distance between focus and the measuring level doesn’t affect the DAP value STUK: Teuvo Parviainen, VM, PP, TKo

3. Recording the information on procedures • Information on procedures involving exposure to radiation shall be recorded, on the basis of which the radiation dose from the procedure to a person examined or treated may be determined if necessary • A summary of the number of examinations and of radiation doses shall be prepared according to instructions to be separately issued, on the basis of which national appraisals of the radiation exposure caused by the medical use of radiation and its development shall be prepared Act of Ministry of Social Affairs and Health in Finland (423/2000) STUK: Teuvo Parviainen, VM, PP, TKo

4. Measurements and comparison of results • Radiation doses caused by X-ray examinations shall be regularly measured or numerically assessed • The activity to be administered to the patient in nuclear medicine examinations shall be measured using an activity meter • Dose and activity data shall be recorded and shall be systematically compared to reference levels • If a reference level is found to have been repeatedly exceeded, then the reasons for this shall be investigated and necessary measures shall be taken to reduce exposure to radiation Act of Ministry of Social Affairs and Health in Finland (423/2000) STUK: Teuvo Parviainen, VM, PP, TKo

5. Defining exposure The exposure can either • be measured for a group of patients or • be assessed by calcutarory means for a group of patients or • be measured by using a phantom identical to patient or The meter used for the defining of exposure needs to be calibrated STUK: Teuvo Parviainen, VM, PP, TKo

6. Why to define patient exposure? • For comparison of reference values • For dose optimization • For organ dose and effective dose estimation • For comparison of best practices STUK: Teuvo Parviainen, VM, PP, TKo

7. STUK: Teuvo Parviainen, VM, PP, TKo

8. Different methods of defining exposure • Measuring ESD from the surface of the phantom • Measuring ESD from the skin of the patient • Calculting ESD on the basis of radiation product • Useing DAP-meter • Calculating ESD from the DAP value STUK: Teuvo Parviainen, VM, PP, TKo

9. Measures in x-ray diagnosticsEntrance Surface Dose(ESD) ESD STUK: Teuvo Parviainen, VM, PP, TKo

10. Dose Area Product(DAP) DAP ≈ D · A Area A Dose D STUK: Teuvo Parviainen, VM, PP, TKo

11. Dose Area Product (DAP) 0.5 m 1 m 2 m Dose: Area: DAP: 40 mGy 25 cm2 1000mGy cm2 10 mGy 100 cm2 1000 mGy cm2 2.5 mGy 400 cm2 1000mGy cm2 STUK: Teuvo Parviainen, VM, PP, TKo

12. Dose Area Product (DAP) • DAP is surface integral of absorbed dose in air. • DAP is almost independent of measuring distance. • DAP does not include scattered radiation from the patient or phantom. • Approximately DAP≈Da*A. DAP-meter STUK: Teuvo Parviainen, VM, PP, TKo

13. Why to calibrate DAP meters? • The DAP-meter usually shows the ’right’ DAP measure only in some spesific situations • In some cases this level of exactitude might be enough • To improve the exactitude of measures the DAP- meter must be calibrated • The calibration must be done for different radiation beams commonly used STUK: Teuvo Parviainen, VM, PP, TKo

14. DAP meters dependence of the radiation beam quality STUK: Teuvo Parviainen, VM, PP, TKo

15. DAP meters dependence of the radiation beam quality STUK: Teuvo Parviainen, VM, PP, TKo

16. Calibration of the DAP-meter • The DAP-meter should be calibrated with several radiation beam qualities • The specificity and variability of the calibration determines the accuracy of the dose measurements on patients STUK: Teuvo Parviainen, VM, PP, TKo

17. STUK: Teuvo Parviainen, VM, PP, TKo

18. Use of the Calibration Factors and DAP-value • There is no need to use the calibration factors in the measuring situations • The calibration factors are used when processing and analyzing the results • The use of the calibration factors improves the accuracy of the measurement STUK: Teuvo Parviainen, VM, PP, TKo

19. Grounds for the Calibration • The value displayed by the DAP meter often has to be corrected with the calibration factor in order to get the accurate dose • The calibration factor can be aquired by comparing the results of the meter in use at the site and the calibration meter • It is recommended to perform the calibration simultaneously with those two meters • The ionization chamber of the reference meter has to be far enough from the ionization chamber of the other meter and from other scatter causing structures STUK: Teuvo Parviainen, VM, PP, TKo

20. Calibration Methods • The reference meter must be calibrated by a reliable facility • The reference meter can be either • an air kerma meter (area method) or • a DAP-meter (tandem method) or • a thermoluminescent dosimeter (TLD) A removable DAP-meter can be sent for calibration to a reliable facility (such as a dosimetry laboratory) STUK: Teuvo Parviainen, VM, PP, TKo

21. Area Method Ion Camber of DAP meter Measurements to be taken • Air Kerma • In the center axis of primary radiation beam • Distance from the focus approximately 50 cm to 150 cm • Field size • about 15 cm * 15 cm in the ionization chamber of the reference meter • accurately measured from film or a digital image Easiest way to measure is to use the same distance from focus with both measurements Ion Camber of Air Kerma Meter STUK: Teuvo Parviainen, VM, PP, TKo

22. Area Method • Air kerma can usually be measured with most of the commonly used voltages and filterings • Area method results an approximate value of the true DAP value • Inaccuracies can be created by penumbra, measurement mistakes of the field size, a very small field size, scattered ratiation etc • Remember to measure the field size accurately • When using the film system the density of the x-ray must not be too high (OD 1 – 1,5) • Use the midpoint of the penumbra when measuring from a film STUK: Teuvo Parviainen, VM, PP, TKo

23. Tandem Method • The reference value is measured by using a reference DAP-meter (calibrated for this purpose) • Distance between the two meters should be about 30 cm • Field size is about 8cm*8cm at the level of the reference meter • The radiation beam has to fit completely in the midarea of the ionization chamber of the reference meter Ion Camper of DAP Meter Ion Camber of Reference Meter STUK: Teuvo Parviainen, VM, PP, TKo

24. Tandem Method • With this method there is no need to measure field size and distance separately • There is no need to measure the field size accurately • A reference DAP-meter is needed • The reference meter has to be calibrated beforehand by a reliable facility STUK: Teuvo Parviainen, VM, PP, TKo

25. The Maintenance of the DAP-Meter • It is useful to check the calibration at least once a year by using one commonly used radiation value set, and once in three years by using several different radiation values • If the meter is repaired or adjusted, it must also be recalibrated • The electrical function of the meter can be checked up with a test button STUK: Teuvo Parviainen, VM, PP, TKo

26. References • STUK-TR 4,2008 (tekninen raportti): Annoksen ja pinta-alan mittaaminen DAP-mittarin kalibrointi röntgensäteilykeilassa • Act of Ministry of Social Affairs and Health in Finland (423/2000) • The Directive 69/29/Euratom • See more in course references STUK: Teuvo Parviainen, VM, PP, TKo