RADIATION PROTECTION IN RADIOTHERAPY

1. RADIATION PROTECTION IN RADIOTHERAPY IAEA Training Material on Radiation Protection in Radiotherapy Part 10: Optimization of protection in External Beam Radiotherapy PRACTICAL EXERCISE IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

2. Objectives of Part 10 • Be familiar with the ‘design considerations’ as stipulated by appendix II in the BSS • Be able to apply these design considerations in the context of radiotherapy equipment • Be aware of relevant international standards and other documents which provide specification for external beam radiotherapy equipment Part 10, Practical 3

3. Part 10 : External Beam Radiotherapy IAEA Training Material on Radiation Protection in Radiotherapy Practical 3: Calibration of a 60-Co unit using TRS 398 IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

4. Contents • Differences between TRS 277 and TRS 398 • Step by step procedure to be followed for calibration of a photon beam from a 60-Co unit following IAEA TRS 398 • Interpretation of results Part 10, Practical 3

5. What Minimum Equipment is Needed? • 60-Co unit with front pointer • Water phantom, spirit level • Calibrated ionization chamber and electrometer combination • IAEA TRS 398 protocol Part 10, Practical 3

6. IAEA TRS 398 • Assumes user has a calibration factor for exposure ND for the ion chamber/ electrometer combination in use • Determines absorbed dose to water Part 10, Practical 3

7. IAEA TRS 398 • Published in 2000 • Very general - can be used for photons (kV, MV), electrons, protons and heavy ions • Straight forward process Part 10, Practical 3

8. Advantages of absorbed dose calibration The exposure/ KERMA way • Easier for the user • Less factors required • Get NDw directly - only conversion for beam quality required Part 10, Practical 3

9. Assume you have a NE 2505/3 3A ion chamber and Farmer electrometer • Chamber volume 0.6cc • Internal radius 3.15mm • Internal length 24mm • Get absorbed dose to water factor - usually provided by the SSDL for a Cobalt reference beam: • ND,w = 9.95 10-3 Gy/div Part 10, Practical 3

10. The formalism • DwQ (zref) = MQ NDCo kQCo with DwQ (zref) - the dose in the users beam quality Q at reference location zref MQ - the corrected chamber reading NDCo - the absorbed dose to water factor for Cobalt as provided by the SSDL kQCo - a correction for beam quality difference between Cobalt and the user’s beam Part 10, Practical 3

11. Want to calibrate a Cobalt unit • kQCo =1 • FAD = 80cm • dmax = 0.5cm Part 10, Practical 3

12. Perform measurement in water phantom • Fill with water to correct depth • Let temperature equilibrate (>1 hour) • Level phantom • Insert chamber • Ensure linac settings and beam orientation correct PTW small water phantom Part 10, Practical 3

13. Reference conditions for 60-Co Part 10, Practical 3

14. Depth of measurement • Measurement depth = 5cm in water • Chamber position with geometric centre of the chamber at measurement depth • No correction for the effective point of measurement is applied - this is different from TRS 277! Part 10, Practical 3

15. Need correction for • Temperature (the higher the less molecules in chamber) • Pressure (the higher the more molecules in chamber) • kTp = P0/P (T + 273.2)/(T0 + 273.2) • with P and T the measured pressure (in kPa) and temperature (in oC) and P0 = 101.3kPa and T = 20oC as reference conditions Part 10, Practical 3

16. Need also correction for recombination of ions in the chamber • Effect depends on radiation quality, dose rate and high voltage applied to the chamber • Use two voltage method - normal voltage V1 and reduced voltage V2 (reduced voltage should be smaller than 0.5V1) with readings M1 and M2 , respectively ks = ((V1/V2)2 - 1)/ ((V1/V2)2 - (M1/M2)) Part 10, Practical 3

17. Corrections of electrometer reading MQ = Mraw kTP kelec kpol ks with • MQ and Mraw the corrected and the raw reading • kTP and ks the temperature, pressure and recombination correction • kelec a factor allowing for separate calibration of the electrometer - here 1 • kpol = (M+ + M- )/ 2M a polarity correction with M being the reading at normal polarity Part 10, Practical 3

18. Absorbed dose in 60-Co • Dw (zref) = MQ NDCo with Dw (zref) - the dose in the users beam quality Q at reference location zref MQ - the corrected chamber reading NDCo - the absorbed dose to water factor for Cobalt as provided by the SSDL Part 10, Practical 3

19. IAEA Worksheet Part 10, Practical 3

20. IAEA Worksheet Part 10, Practical 3

21. Part 10, Practical 3

22. IAEA Worksheet Part 10, Practical 3

23. Please fill in the sheet for ‘your’ Cobalt unit Conditions and readings on the next page...

24. Final information • Want to calibrate dose to dmax • Percentage depth dose for 10x10cm2, SSD 80cm at d5 = 78.8% • T = 28oC, p = 100.3kPa • Uncorrected readings for 1min exposure: 184.5, 184.2, 184.3 (for normal + polarity) and 185.0, 184.7, 184.6 (for - polarity) • Mean reading for 1/3 voltage 182.1 • Assume time is corrected for on/off effect (=timer error) Part 10, Practical 3

25. Questions? Let’s get started... Part 10, Practical 3

26. Result: 2.47 Gy per minute at depth of maximum dose Can you estimate the uncertainty of this? Part 10, Practical 3

27. Uncertainty analysis TRS 398 • Uncertainty from SSDL = 0.6% • User uncertainties: • stability of dosimeter 0.3 • establishment of reference conditions 0.5 • dosimeter reading relative to timer 0.1 • correction factors used 0.3 • Total 0.9% Part 10, Practical 3