Clinical Implementation of the IPEM 2003 Code of Practice for Electron Dosimetry

Transcription

1 Clinical Implementation of the IPEM 2003 Code of Practice for Electron Dosimetry TJ JORDAN Royal Surrey County Hospital IPEM Electron Dosimetry Working Party: + DI Thwaites, AR DuSautoy, MR McEwen, AE Nahum, A Nisbet & WG Pitchford Practical Course in Reference Dosimetry, NPL 2016

2 Basic Methodology Calorimeter based Absorbed Dose to Water calibration in electron beams Pictures courtesy of NPL

3 Basic Methodology Calorimeter based Absorbed Dose to Water calibration in electron beams Individualised parallel plate electron chamber calibration at 6 electron beam qualities based on R 50,D (depth of 50% dose) Original NPL accelerator, R 50,D : 1.97 to 6.60cm [6 to 19 MeV] LESS than the range of hospital clinical beams Elekta Clinical Linac, R 50,D : 1.6 up to 8.9*cm [4 to 22* MeV] *(not guaranteed)

4 Basic Methodology NPL Calibration certificate Calorimeter based Absorbed Dose to Water calibration in electron beams Individualised parallel plate electron chamber calibration at 6 electron beam qualities based on R 50,D Uses reference depth defined as Z ref = 0.6 R 50,D 0.1cm NPL determined Recombination f ion = m(dose per pulse) + c

5 Basic Methodology Calorimeter based Absorbed Dose to Water calibration in electron beams 100 %Depth Dose Individualised parallel plate electron chamber calibration at 7 electron beam qualities based on R 50,D Uses reference depth defined as Z ref = 0.6 R 50,D 0.1cm % Dose dmax Zref NPL determined Recombination f ion = m(dose per pulse) + c R50 Depth (mm)

6 Basic Methodology NPL Calibration certificate Calorimeter based Absorbed Dose to Water calibration in electron beams Individualised parallel plate electron chamber calibration at 7 electron beam qualities based on R 50,D Uses reference depth defined as Z ref = 0.6 R 50,D 0.1cm NPL determined Recombination f ion = m(dose per pulse) + c

7 Basic Methodology NPL Calibration certificate Calorimeter based Absorbed Dose to Water calibration in electron beams Individualised parallel plate electron chamber calibration at 6 electron beam qualities based on R 50,D Uses reference depth defined as Z ref = 0.6 R 50,D 0.1cm NPL determined Recombination f ion = m(dose per pulse) + c

8 Determination of R 50,D Measurement of Depth- Dose For photons, Dose w Air ionisation x W/e x [µ en / ] w,air [µ en / ] w,air the Mass Energy Absorption Coefficient for water to air is nearly constant with depth and relative Depth-Ionisation is assumed to Depth-Dose in water. For electrons Dose w Air ionisation x W/e x S w,air As the electron energy spectrum decreases rapidly with depth (from incident energy to 0), the Stopping Power ratio S w,air is NOT constant Depth-Ionisation curve needs renormalising at every point by S w,air ratio to be representative of a Depth-Dose curve

9 Sw,med Stopping Power Ratios (20MeV Electrons) Water/Air Depth (mm)

10 Sw,med Stopping Power Ratios (20MeV Electrons) Water/Si Water/Air Depth (mm)

11 Determination of R 50,D Protocol gives 3 options; Find R 50,D either from; R 50,D = x R 50,I cm eqn. 2.1 Use measured Depth-Ionisation curve (Need %DD curve clinically, see later) S w,air corrected Depth-Ionisation curve To produce Depth-Dose curve Directly from diode measured Depth-Ionisation curve as S w,silicon constant until deep Measure Depth-Dose directly

12 % DOSE Depth Dose Measurement Si diode NACP S w,air Corr DEPTH (mm)

13 (corrected) NACP vs Diode: Energy Dependence MeV NACP 9MeV NACP 12MeV NACP 16MeV NACP 20MeV NACP % Dose Depth (mm) Bremsstrahlung tail

14 (corrected) NACP vs Diode: Consistent Error % Dose MeV NACP 9MeV NACP 12MeV NACP 16MeV NACP 20MeV NACP Diode Depth (mm)

15 Effective Point of Measurement, P eff An air ionisation chamber introduces an air bubble into the water phantom The effective point of measurement is where the fluence is equivalent to the fluence in the undisturbed medium For a parallel plate chamber P eff is just inside the front window [For a cylindrical chamber it is around 0.6 x Internal Radius forward of the physical centre]

16 Effective Point of Measurement, P eff ROOS 1.18x 1mm NACP FARMER 0.125cc DIODE 1.7x 0.6mm 1.8mm 1.7mm 0.5mm Effective Depths

17 Polarity In an electron beam a polarity effect may arise as some of the primary beam collides with the collecting electrode F pol = ( M + + M - )/2M M is reading with normal polarity M + and M - readings with respective polarity In an electron beam polarity can change with depth (and energy) as electrons scatter obliquely

18 Polarity NPL polarity negative volts (to front window) Collecting electrode positive wrt front window Uses conventional connection of separate HV Connects to flying lead, or outer braid of cable

19 Polarity (conventional) NACP FLYING LEAD -100V EARTH NEGATIVE TYPE B

20 Polarity (electrometer floating) NACP +100V EARTH POSITIVE TYPE A

21 Reading Polarity: Measurement Accuracy NACP: 4MeV, R50-200V +200V USE A GOOD REFERENCE CHAMBER Switch off and disconnect Earth cable Reverse polarity Reconnect and switch on Take readings until stable Repeat Do +/- volts consecutively (not as part of recombination study) If in doubt use F pol = 1.0(!) -50V Time

22 1/Reading Ion Recombination Theory Plotting 1/Reading vs 1/Voltage is a straight line This can easily be extrapolated to volts, or 100% collection efficiency 0 ( volt) 1/Volts Linear dependence permits use of the 2 voltage method f ion - 1 = (M 1 /M 2-1)/(V 1 /V 2-1) In particular, if V 1 = 2x V 2 then f ion = M 1 / M 2 (M is reading)

23 Recombination Correction f ion Actual: Farmer Well behaved 1/Reading mev 20mev Parallel lines means curves overlay when normalised This means recombination is independent of energy Unless dose per pulse, Dp changes /Voltage Volts

24 1/Reading Recombination Correction f ion Actual: Parallel Plate chambers MeV, Markus 4 MeV NACP 10 MeV NACP 15 MeV NACP /VOLTS oo VOLTS

25 Recombination Correction Use of lower voltages (~100V) on parallel plate ensures linear region but may give large correction factor (2-3% on Varian) Use of higher voltages will give a smaller correction but loss of linearity means the error on the volt intercept is high It is recommended to stay close to NPL calibration voltage (100V) to reduce uncertainty MEASURE LOCALLY and use NPL as check only Due to uncertainty in Dose per Pulse

26 Recombination Correction f ion Roos chamber ion recombination /reading MeV 9MeV 12MeV 16MeV 20MeV /voltage

27 Recombination/Polarity NPL Certificate

28 Recombination Correction Recombination depends only on the Dose per Pulse (D p ) and chamber voltage Recombination is independent of energy But different energies might use a different Dose per Pulse!! Recombination is linearly related to D p F ion = 1 + (c-1)(v cal /V user ) + m(v cal /V user ) D p F ion = c + m x D p (at V user = 100Volt)

29 Linear Accelerator Calibration Roos or NACP with inside of front window at Z ref D w = R x N D,w x f t,p x f ion x f pol x %DD From %Depth Dose curve establish correction for dose at Z ref depth to D max depth Clinically quote Dose at d max and calibrate linac for 1cGy/mu at that point (NOT Z ref ) Negligible up to >12MeV Can be 4-5% at higher energy

30 Spreadsheet Calculates composite N D,W factors including polarity, recombination and Stopping power extrapolation User Electron Beam Definition 2 Type data into yellow cells only Accelerator Varian 2100cd Field Size (cmxcm) 10x10 at F.S.D (cm) 100 Energy 1 Energy 2 Energy 3 Energy 4 Energy 5 Energy 6 Nominal Energy (MeV) 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV 20 MeV dmax (cm) in water R50,I (cm) in water R50,D (cm) in water zref (cm) in water Mean Surface Energy (MeV) Stopping Power Ratio (Eqn D.2) Reference Chamber Factor Summary: Calibration Factor x 10 7 (Gy/C) Polarity Corr Recombination Corr Electrometer Corr. (nc) % Depth Dose at Zref Composite Factor (to cgy)

31 Extrapolation to Higher (& Lower) Beam Qualities The previous NPL calibration covers a limited range of electron beam qualities The higher end was equivalent to a ~16MeV clinical beam To extrapolate to a higher energy use the ratio of stopping powers at the two (different) reference positions N w,user = N w,r50=6.6 x [S w,air, R50=User, dref ] / [S w,air, R50=6.6 ] N w,user = N w,r50=6.6 x [S w,air, R50=User, dref ] / Use SPR table in protocol or spreadsheet

32 Nw,u Spreadsheet: Extrapolation to Higher Beam Qualities PLOT Nw,u calibration and extrapolation N w,u x 10 7 (Gy/C) R 50,D (cm) Type extrapolation value S w,air R50,D

33 Nw,u Spreadsheet: Extrapolation to Higher Beam Qualities PLOT Nw,u calibration and extrapolation Type extrapolation value S w,air lin regrssn N w,u x 10 7 (Gy/C) R 50,D (cm) % R50,D

34 Nw,u Spreadsheet: Extrapolation to Higher Beam Qualities PLOT Nw,u calibration and extrapolation Type extrapolation value S w,air lin regrssn cubic N w,u x 10 7 (Gy/C) R 50,D (cm) % -0.47% R50,D

35 2,3,4 Rule of Thumb

36 Last Word Feedback on implementation and 2003/1996 differences Recommendations on 2 year calibration may be reexamined