Kilovoltage X-ray dosimetry in the clinic. Philip Mayles The Clatterbridge Cancer Centre

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1 Kilovoltage X-ray dosimetry in the clinic Philip Mayles The Clatterbridge Cancer Centre

2 Kilovoltage Codes of Practice Germany DIN 1988 and 1996 UK IPEMB (IPEM) 1996 and 2005 IAEA TRS 277 2nd edn 1997 Holland NCS 1997 USA AAPM TG IAEA TRS

3 Formula for Medium Energy Measure at 2 cm deep IPEM COP Klevenhagen et al Phys Med Biol In phantom

4 Formula for Low Energy Measure in air IPEM COP Klevenhagen et al Phys Med Biol In air

5 Formula for Low Energy Measure in air AAPM COP Ma et al Med Phys In air

6 Formula for Low Energy Measure in air IPEMB recommends use of the in air protocol if the prescription is to the max dose for medium energy IPEMB Addendum Aukett et al Phys Med Biol In air

7 IPEM code for Very Low Energy Measure at surface of phantom Note that k ch is based on measurement comparing to in air measurement IPEMB COP Klevenhagen et al Phys Med Biol IPEMB Addendum Aukett et al Phys Med Biol In phantom

8 Grenz ray chamber PTW23342 In phantom

9 Grenz ray chamber PTW23342 In phantom

10 How are Low and Medium Energy defined? IPEMB based on HVL Medium energy mm Cu (>160 kv) Low energy 1 8 mm Al ( kv) Very low energy mm Al (8 50 kv)

11 How is Energy defined? Penetrative power Depth dose First Half Value Layer Energy spectrum Generating potential Filtration

12 Measurement of HVL AAPM COP Ma et al Med Phys In air

13 HVL mm Al HVL mm Al Specification of Beam Quality Standards Lab TECDOC 1455 Clatterbridge Standards Lab TECDOC 1455 Clatterbridge kv kv Low Energy Medium Energy

14 Does Beam Quality Matter? N k HVL mm Al Adapted from TRS398

15 Factors in Calculation: M Electrometer reading corrected for temperature, pressure, ion recombination, polarity effect and electrometer accuracy AAPM COP Ma et al Med Phys In air In phantom

16 Factors in Calculation: M Electrometer reading corrected for temperature, pressure, ion recombination, polarity effect and electrometer accuracy IPEM COP Klevenhagen et al Phys Med Biol In air In phantom

17 Factors in Calculation: M Recombination correction V H and V L are the high and low voltages and M H and M L are the meter readings Important for short SSDs AAPM COP Ma et al Med Phys In air In phantom

18 Factors in Calculation: μ en /ρ air AAPM and IPEMB agree to better than 0.3% but the sources of data were not independent In air

19 Dose is tissue dependent μ en /ρ air AAPM COP Ma et al Med Phys In air

20 Factors in Calculation: μ en /ρ 2cm AAPM COP Ma et al Med Phys In phantom

21 Factors in Calculation: B w Since the backscatter factor is fundamentally a water-kerma ratio, reliable measurements are non-trivial. Therefore for the application of this protocol backscatter factors should not be measured in the clinic AAPM COP Ma et al Med Phys In air

22 Factors in Calculation: B w Since the backscatter factor is fundamentally a water-kerma ratio, reliable measurements are non-trivial. Therefore for the application of this protocol backscatter factors should not be measured in the clinic Largest difference AAPM:IPEMB is 0.4% AAPM COP Ma et al Med Phys In air

23 Factors in Calculation: P stem 1.0 if field size for calibration is same as measurement Likely to be important only for plane parallel chambers Need to know the value for the reference chamber In air

24 Factors in Calculation: k ch AAPM: IPEM: P Qcham P sheath k ch In phantom

25 Factors in Calculation: k ch AAPM: IPEM: P Qcham P sheath k ch TRS277: k u p u In phantom

26 Phantom Materials Compared Solid Water (RMI Gammex) and Plastic Water (Computerized Imaging Reference Systems) to H 2 O Percentage Depth Dose Solid water: up to 2.4% difference (75kVp beam 40mm deep) Plastic water: up to 23.2% difference (75kVp beam 2mm deep) Hill et al Phys Med Biol 50 N331-N In phantom

27 Phantom Materials Compared Solid Water (RMI Gammex) and Plastic Water (Computerized Imaging Reference Systems) to H 2 O Percentage Depth Dose Solid water: up to 2.4% difference (75kVp beam 40mm deep) Plastic water: up to 23.2% difference (75kVp beam 2mm deep) Absolute dose measurement: Hill et al Phys Med Biol 50 N331-N In phantom

28 Are In Air and In Water measurements equivalent? 300kV 3.67mm Cu Ma et al Med Phys In air In phantom

29 Are In Air and In Water measurements equivalent? kV 3.67mm Cu Ma et al Med Phys In air In phantom

30 Issues with Backscatter Method Thanks to Rhydian Caines and Richard Clements

31 Depth Dose measurement Measurement of doses close to the surface is difficult Ma et al recommend using an NACP chamber but found differences of 2.4% With a diode the differences were up to 10% In phantom

32 New Devices New uses are being proposed Zeiss Intrabeam Ariane Papillon 50

33 Methodology for New Devices Ebert et al have carried out a thorough evaluation of the dosimetry However, the uncertainties in dosimetry probably place the priority on consistency between centres An intercomparison jig has been proposed by Armoogum and Watson Ebert et al Med Phys Armoogum and Watson Z Med Phys

34 Uncertainty Estimates

35 Summary Protocols give reliable results at the point of measurement Accurate measurement of percentage depth doses is problematic A protocol should be chosen which measures the dose close to the prescription point

36 Kilovoltage Codes of Practice Germany DIN 1988 and 1996 UK IPEMB (IPEM) 1996 and 2005 IAEA TRS 277 2nd edn 1997 Holland NCS 1997 USA AAPM TG IAEA TRS

37 TRS 398 Calibrate chamber in absorbed dose to water is a chamber specific factor which corrects for differences between the reference beam and the actual beam In phantom

38 Is TRS 398 the answer? Only PTB offers an absorbed dose to water calibration Other standards labs may offer a calibration based on one of the protocols Jurado et al have used it with calibrations from PTB IAEA TECDOC 1455 shows agreement within 3% at low energies Jurado et al BJR In phantom