Practical Course in Reference Dosimetry, NPL 2016 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
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Clinical Implementation of the IPEM 2003 Code of …...Practical Course in Reference Dosimetry, NPL 2016(f2014) Determination of R 50,D Protocol gives 3 options; Find R 50,D either
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Practical Course in Reference Dosimetry, NPL 2016
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(f2014)
Basic Methodology Calorimeter based Absorbed Dose
to Water calibration in electron
beams
Pictures courtesy of NPL
Practical Course in Reference Dosimetry, NPL 2016(f2014)
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 R50,D (depth of 50% dose)
Original NPL accelerator, R50,D: 1.97 to 6.60cm [6 to 19 MeV]
• LESS than the range of hospital clinical beams
Elekta Clinical Linac , R50,D: 1.6 up to 8.9*cm [4 to 22* MeV]
• *(not guaranteed)
Practical Course in Reference Dosimetry, NPL 2016(f2014)
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 R50,D
Uses reference depth defined as
Zref = 0.6 R50,D – 0.1cm
NPL determined Recombination
fion = m(dose per pulse) + c
NPL Calibration certificate
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Basic Methodology
Calorimeter based Absorbed Dose to Water calibration in electron beams
Individualised parallel plate electron chamber calibration at 7 electron beam qualities based on R50,D
Uses reference depth defined as
Zref = 0.6 R50,D – 0.1cm
NPL determined Recombination
fion = m(dose per pulse) + c
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90
Depth (mm)
% D
os
e
dmax
R50
Zref
%Depth Dose
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Basic Methodology
Calorimeter based Absorbed Dose to Water calibration in electron beams
Individualised parallel plate electron chamber calibration at 7 electron beam qualities based on R50,D
Uses reference depth defined as
Zref = 0.6 R50,D – 0.1cm
NPL determined Recombination
fion = m(dose per pulse) + c
NPL Calibration certificate
Practical Course in Reference Dosimetry, NPL 2016(f2014)
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 R50,D
Uses reference depth defined as
Zref = 0.6 R50,D – 0.1cm
NPL determined Recombination
fion = m(dose per pulse) + c
NPL Calibration certificate
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Determination of R50,D
Measurement of Depth- Dose
For photons, Dosew Air ionisation x W/e x [µen/]w,air
[µen/]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 Dosew Air ionisation x W/e x Sw,air
As the electron energy spectrum decreases rapidly with depth
(from incident energy to 0), the Stopping Power ratio Sw,air is
NOT constant
Depth-Ionisation curve needs renormalising at every point by
Sw,air ratio to be representative of a Depth-Dose curve
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Stopping Power Ratios (20MeV Electrons)
0.9
1
1.1
1.2
1.3
0 20 40 60 80 100
Depth (mm)
Sw
,me
d
Water/Air
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Stopping Power Ratios (20MeV Electrons)
0.9
1
1.1
1.2
1.3
0 20 40 60 80 100
Depth (mm)
Sw
,me
d
Water/Si
Water/Air
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Determination of R50,D
Protocol gives 3 options;
Find R50,D either from;R50,D = 1.029 x R50,I – 0.063 cm eqn. 2.1
• Use measured Depth-Ionisation curve
• (Need %DD curve clinically, see later)
Sw,air corrected Depth-Ionisation curve
• To produce Depth-Dose curve
Directly from diode measured Depth-Ionisation curve as Sw,silicon constant until deep
• Measure Depth-Dose directly
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Depth Dose Measurement
DEPTH (mm)
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90
% D
OS
E
Si diodeNACPSw,air Corr.
Practical Course in Reference Dosimetry, NPL 2016(f2014)
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100 110 120
Depth (mm)
% D
os
e
6MeV NACP
9MeV NACP
12MeV NACP
16MeV NACP
20MeV NACP
(corrected) NACP vs Diode: Energy Dependence
Bremsstrahlung tail
Practical Course in Reference Dosimetry, NPL 2016(f2014)
(corrected) NACP vs Diode: Consistent Error
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100 110 120
Depth (mm)
% D
ose
6MeV NACP
9MeV NACP
12MeV NACP
16MeV NACP
20MeV NACP
Diode
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Effective Point of Measurement, Peff
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 Peff is just inside the
front window
[For a cylindrical chamber it is around 0.6 x Internal
Radius forward of the physical centre]
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Effective Point of Measurement, Peff
1.18x 1mm
NACP FARMER
1.7mm
0.125ccROOS
1.8mm1.7x 0.6mm 0.5mm
DIODE
Effective Depths
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Polarity
In an electron beam a polarity effect may arise as
some of the primary beam collides with the
collecting electrode
Fpol = (|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
Practical Course in Reference Dosimetry, NPL 2016(f2014)
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
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Polarity (conventional)
NACP
EARTH NEGATIVE
-100V
TYPE B
FLYING LEAD
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Polarity (electrometer floating)
NACP
EARTH POSITIVE
+100V
TYPE A
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Polarity: Measurement Accuracy
NACP: 4MeV, R50
2250
2300
2350
2400
Time
Re
ad
ing
-200V
+200V
-50V
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 Fpol = 1.0(!)
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Ion Recombination
Plotting 1/Reading vs
1/Voltage is a straight line
This can easily be extrapolated
to volts, or 100% collection
efficiency
Linear dependence permits
use of the 2 voltage method
fion - 1 = (M1/M2 -1)/(V1/V2 -1)
In particular, if V1 = 2x V2 then
fion = M1 / M2 (M is reading)( volt)
0
1/R
eadin
g
1/Volts
Theory
Practical Course in Reference Dosimetry, NPL 2016(f2014)
0.047
0.048
0.049
0.050
0.051
0.000 0.005 0.010 0.015 0.020 0.0251/Voltage
1/R
ea
din
g
12mev
20mev
Actual: Farmer
50100 200 75 Volts
Recombination Correction fion
Well behaved
Parallel lines means curves
overlay when normalised
This means recombination
is independent of energy
Unless dose per pulse, Dp
changes
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Practical Course in Reference Dosimetry, NPL 2016(f2014)
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 positionsNw,user = Nw,R50=6.6 x [Sw,air, R50=User, dref] / [Sw,air, R50=6.6]
Nw,user = Nw,R50=6.6 x [Sw,air, R50=User, dref] / 1.0323
Use SPR table in protocol or spreadsheet
Practical Course in Reference Dosimetry, NPL 2016(f2014)
Spreadsheet: Extrapolation to Higher Beam Qualities
PLOT Nw,u calibration and extrapolation Type extrapolation value