clinical implementation of the ipem 2003 code of …...practical course in reference dosimetry, npl...

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


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)


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


Basic Methodology




Zref = 0.6 R50,D – 0.1cm



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


Basic Methodology




Zref = 0.6 R50,D – 0.1cm





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


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


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


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


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.


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


(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


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]


Effective Point of Measurement, Peff

1.18x 1mm

NACP FARMER

1.7mm

0.125ccROOS

1.8mm1.7x 0.6mm 0.5mm

DIODE

Effective Depths


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


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

EARTH NEGATIVE

-100V

TYPE B

FLYING LEAD


Polarity (electrometer floating)

NACP

EARTH POSITIVE

+100V

TYPE A


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(!)


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


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



0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.050.99

1

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

1.09

4 MeV, Markus4 MeV NACP10 MeV NACP15 MeV NACP

1/VOLTS

1/R

ead

ing

oo 200 100 253350 VOLTS

Actual: Parallel Plate chambers


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



Roos chamber ion recombination

0.0700

0.0800

0.0900

0.1000

0.000 0.010 0.020 0.030 0.040 0.050

1/voltage

1/r

ea

din

g

6MeV

9MeV

12MeV

16MeV

20MeV


Recombination/Polarity – NPL Certificate


Recombination Correction

Recombination depends only on the Dose per Pulse (Dp) and

chamber voltage

Recombination is independent of energy

But different energies might use a different Dose per Pulse!!

Recombination is linearly related to Dp

Fion = 1 + (c-1)(Vcal/Vuser) + m(Vcal/Vuser) Dp

Fion = c + m x Dp (at Vuser = 100Volt)


Linear Accelerator Calibration

Roos or NACP with inside of front window at Zref

Dw = R x ND,w x ft,p x fion x fpol x %DD

From %Depth Dose curve establish correction for

dose at Zref depth to Dmax depth

Clinically quote Dose at dmax and calibrate linac for

1cGy/mu at that point (NOT Zref)

Negligible up to >12MeV

Can be 4-5% at higher energy


Spreadsheet

Calculates composite ND,W factors including polarity,

recombination and Stopping power extrapolation

User Electron Beam Definition 2Type data into yellow cells only

Accelerator Varian 2100cdField 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 107

(Gy/C) 14.03 13.82 13.71 13.46 13.33 13.33Polarity 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 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


Spreadsheet: Extrapolation to Higher Beam Qualities

PLOT Nw,u calibration and extrapolation Type extrapolation value

Sw,air

Nw,u x 107

(Gy/C) 14.10 14.09 13.99 13.82 13.81 13.61 13.46 13.325

R50,D (cm) 1.23 1.97 2.75 3.48 4.23 5.72 6.60 8.17

13.0

13.2

13.4

13.6

13.8

14.0

14.2

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0R50,D

Nw

,u



Sw,air lin regrssn

Nw,u x 107

(Gy/C) 14.10 14.09 13.99 13.82 13.81 13.61 13.46 13.325 13.294

R50,D (cm) 1.23 1.97 2.75 3.48 4.23 5.72 6.60 8.17 -0.23%

13.0

13.2

13.4

13.6

13.8

14.0

14.2

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0R50,D

Nw

,u




Sw,air lin regrssn cubic

Nw,u x 107

(Gy/C) 14.10 14.09 13.99 13.82 13.81 13.61 13.46 13.325 13.294 13.262

R50,D (cm) 1.23 1.97 2.75 3.48 4.23 5.72 6.60 8.17 -0.23% -0.47%

13.0

13.2

13.4

13.6

13.8

14.0

14.2

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0R50,D

Nw

,u



2,3,4 Rule of Thumb


Last Word

Feedback on implementation and 2003/1996

differences

[email protected]

Recommendations on 2 year calibration may be re-

examined

clinical implementation of the ipem 2003 code of …...practical course in reference dosimetry, npl...

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