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
Basic Methodology Calorimeter based Absorbed Dose to Water calibration in electron beams Pictures courtesy of NPL
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)
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
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 90 80 70 60 50 40 30 20 dmax Zref NPL determined Recombination f ion = m(dose per pulse) + c 10 0 0 10 20 30 40 50 60 70 80 90 R50 Depth (mm)
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
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
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
Sw,med Stopping Power Ratios (20MeV Electrons) 1.3 1.2 Water/Air 1.1 1 0.9 0 20 40 60 80 100 Depth (mm)
Sw,med Stopping Power Ratios (20MeV Electrons) 1.3 1.2 Water/Si Water/Air 1.1 1 0.9 0 20 40 60 80 100 Depth (mm)
Determination of R 50,D Protocol gives 3 options; Find R 50,D either from; R 50,D = 1.029 x R 50,I 0.063 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
% DOSE Depth Dose Measurement 100 90 80 70 Si diode NACP S w,air Corr. 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 DEPTH (mm)
(corrected) NACP vs Diode: Energy Dependence 100 90 80 70 6MeV NACP 9MeV NACP 12MeV NACP 16MeV NACP 20MeV NACP % Dose 60 50 40 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 Depth (mm) Bremsstrahlung tail
(corrected) NACP vs Diode: Consistent Error % Dose 100 90 80 70 60 50 40 6MeV NACP 9MeV NACP 12MeV NACP 16MeV NACP 20MeV NACP Diode 30 20 10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 Depth (mm)
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]
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
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
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
Polarity (conventional) NACP FLYING LEAD -100V EARTH NEGATIVE TYPE B
Polarity (electrometer floating) NACP +100V EARTH POSITIVE TYPE A
Reading Polarity: Measurement Accuracy 2400 2350 2300 2250 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
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)
Recombination Correction f ion 0.051 Actual: Farmer Well behaved 1/Reading 0.050 0.049 0.048 12mev 20mev Parallel lines means curves overlay when normalised This means recombination is independent of energy Unless dose per pulse, Dp changes 0.047 0.000 0.005 0.010 0.015 0.020 0.025 1/Voltage 200 100 75 50 Volts
1/Reading Recombination Correction f ion Actual: Parallel Plate chambers 1.09 1.08 1.07 1.06 4 MeV, Markus 4 MeV NACP 10 MeV NACP 15 MeV NACP 1.05 1.04 1.03 1.02 1.01 0.99 1 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 1/VOLTS oo 200 100 50 33 25 VOLTS
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
Recombination Correction f ion Roos chamber ion recombination 0.1000 1/reading 0.0900 0.0800 6MeV 9MeV 12MeV 16MeV 20MeV 0.0700 0.000 0.010 0.020 0.030 0.040 0.050 1/voltage
Recombination/Polarity NPL Certificate
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)
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
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 1.4 2.1 2.9 3.2 2.1 2.1 R50,I (cm) in water 2.4 3.6 4.9 6.5 8.0 8.0 R50,D (cm) in water 2.4 3.6 5.0 6.6 8.2 8.2 zref (cm) in water 1.3 2.1 2.9 3.9 4.8 4.8 Mean Surface Energy (MeV) 5.7 8.4 11.5 15.3 18.9 18.9 Stopping Power Ratio (Eqn D.2) 1.072 1.057 1.045 1.032 1.022 1.022 Reference Chamber Factor Summary: Calibration Factor x 10 7 (Gy/C) 14.03 13.82 13.71 13.46 13.33 13.33 Polarity Corr. 0.9981 1.0000 0.9998 1.0000 1.0000 1.0000 Recombination Corr. 1.0101 1.0101 1.0101 1.0101 1.0101 1.0101 Electrometer Corr. (nc) 1.0010 1.0010 1.0010 1.0010 1.0010 1.0010 % Depth Dose at Zref 100.0 100.0 100.0 99.1 95.4 95.4 Composite Factor (to cgy) 14.16 13.97 13.86 13.73 14.12 14.12
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 ] / 1.0323 Use SPR table in protocol or spreadsheet
Nw,u Spreadsheet: Extrapolation to Higher Beam Qualities PLOT Nw,u calibration and extrapolation N w,u x 10 7 (Gy/C) 14.10 14.09 13.99 13.82 13.81 13.61 13.46 13.325 R 50,D (cm) 1.23 1.97 2.75 3.48 4.23 5.72 6.60 8.17 Type extrapolation value S w,air 14.2 14.0 13.8 13.6 13.4 13.2 13.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 R50,D
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) 14.10 14.09 13.99 13.82 13.81 13.61 13.46 13.325 13.294 R 50,D (cm) 1.23 1.97 2.75 3.48 4.23 5.72 6.60 8.17-0.23% 14.2 14.0 13.8 13.6 13.4 13.2 13.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 R50,D
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) 14.10 14.09 13.99 13.82 13.81 13.61 13.46 13.325 13.294 13.262 R 50,D (cm) 1.23 1.97 2.75 3.48 4.23 5.72 6.60 8.17-0.23% -0.47% 14.2 14.0 13.8 13.6 13.4 13.2 13.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 R50,D
2,3,4 Rule of Thumb
Last Word Feedback on implementation and 2003/1996 differences tom.jordan@royalsurrey.nhs.uk Recommendations on 2 year calibration may be reexamined