Electron Beam Dosimetry at NPL · • Electron dosimetry now on the same footing as MV photons. NPL Management Ltd - Public Outline of Code • Section 1: Introduction • Section

NPL Management Ltd - Public

Electron Beam Dosimetry at NPL

Julia Snaith

Practical Course in Reference Dosimetry

19 – 22 January 2016


Overview

• IPEM Code of Practice

– Theory

– Dose conversion

– Beam quality

– Phantoms

• Electron beam calorimeter

• Calibration of secondary standards

Covered in greater detail during

electron practical session


The IPEM 2003 Code

of Practice

• 1989: NPL introduced

calorimeter-based protocols

for MV X-ray beams

• 1990: IPSM published

photon CoP based on this

• 1996: Air kerma based

electron protocol

• 2003: Calorimetry-based

protocol, referenced to

absorbed dose to water


The IPEM 2003 Code

of Practice

• Based on the NPL absorbed

dose calibration service,

which gives direct factors for

electron chambers in terms

of absorbed dose to water

• A simple procedure to follow

in the clinic and a

significantly reduced

uncertainty

• Electron dosimetry now on

the same footing as MV

photons


Outline of Code

• Section 1: Introduction

• Section 2: Code of Practice

• Appendices:

– A – Corrections

– B – Chambers

– C – Formalism

– D – Data

– E – Uncertainties

Everything needed to measure

absorbed dose to water in an

electron beam is described in this

document


IPEM 2003 Theory (1)

(1) define reference depth in water for beams at NPL

(2) use range scaling to get depth in graphite

(3) calibrate chamber against the calorimeter at NPL, in graphite, at

zref,g

ND

MD ref g

g

ref g

, ,

,

zref,g = zref,w R50,g / R50,w

zref = 0.6 R50,D – 0.1 cm



(4) theoretical conversion from graphite to water

N Np

p

s

sD ref w D ref g

ref w

ref g

w air

g air

, , , ,

,

,

/

/

(5) compare user and reference chambers at NPL, at zref,w in water

N NM

MD user w D ref w

ref w

user w

, , , ,

,

,

The formalism is described in detail in Appendix C.






zref,g

ND

MD ref g

g

ref g

, ,

,


zref = 0.6 R50,D – 0.1 cm


(1) Reference depth zref = 0.6 R50,D – 0.1 cm

R100

depth in water, z/cmR85 R50 Rp

• Electron energy as a specifier

can be confusing – what does

it refer to?

• Mean energy emerging

from linac?

• Mean energy at phantom

surface?

• Choose R50,D as the specifier:

• Defines reference depth

• Calibration factors on the

NPL certificate given as a

function of R50,D


(1) Reference depth zref = 0.6 R50,D – 0.1 cmDepth-dose curves for NPL Clinical Linac

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Depth,mm

Re

lati

ve d

os

e

4 MeV

6 MeV

8 MeV

10 MeV

12 MeV

15 MeV

18 MeV

20 MeV

22 MeV


(1) Reference depth zref = 0.6 R50,D – 0.1 cm

NPL

Royal London SL18

Edinburgh WG LA20

Mount Vernon SL20

Rogers SL75/20, 2100C, Therac 20

NKI SL75/20

NRC

R1

00

in w

ate

r (c

m)

R50 in water (cm)

Linear fit below R50 = 3.5cm: R100 = 0.60 R50 - 0.10

0 1 2 3 4 5 6 7 8 9

0

1

2

3

4

5

6 NPL

Royal London SL18

Edinburgh WG LA20

Mount Vernon SL20

Rogers SL75/20, 2100C, Therac 20

NKI SL75/20

NRC

R1

00

in w

ate

r (c

m)

R50 in water (cm)

Linear fit below R50 = 3.5cm: R100 = 0.60 R50 - 0.10

0 1 2 3 4 5 6 7 8 9

0

1

2

3

4

5

6

• Burns et al 1996






zref,g

ND

MD ref g

g

ref g

, ,

,


zref = 0.6 R50,D – 0.1 cm


(2) Range scaling

• If the reference depth in graphite, zref,g, is chosen such that

• then the electron spectrum at the reference depth will be similar for

each phantom.







zref,g

ND

MD ref g

g

ref g

, ,

,


zref = 0.6 R50,D – 0.1 cm


(3) The NPL electron beam graphite calorimeter

Electron beam

Core

1mm air gaps

Backscatter plates

Buildup plates Kapton insulator


The calorimeter core

• Disc 50mm diameter, 2mm thick

• Contains 2 bead thermistors

• Polystyrene sprung support

strips

• Octagonal plates near to the

core to minimise the effect of the

unirradiated ‘cold’ corners


Calibration of NPL transfer standard

chambers• Deliver 200MU alternately to calorimeter and chambers

• Use the calorimeter to determine Dose (Gy) / MU

• Use the chamber readings to measure Charge (C) / MU

• Combine these to determine chamber calibration factor:

Gy / C


Corrections to calorimeter dose

Dg = cg.∆Traw.funif.fgap.fdist.fdepth.fscat.fmat.fheat.fback


Corrections to the ion chamber reading

Mref,g = Mraw.fTP.funif.fdist.fdepth.fscat.fion.fpol.felec.fnon-lin.fback






zref,g


zref = 0.6 R50,D – 0.1 cm

ND

MD ref g

g

ref g

, ,

,




N Np

p

s

sD ref w D ref g

ref w

ref g

w air

g air

, , , ,

,

,

/

/


N NM

MD user w D ref w

ref w

user w

, , , ,

,

,



(4) Conversion from graphite to water

• The perturbation factors for the reference chamber (pref,w and

pref,g ), correct for the presence of the chamber in water and

graphite respectively

• sw/air and sg/air are the stopping power ratios of water and

graphite, respectively, to that of air for the spectrum at this

position.

N Np

p

s

sD ref w D ref g

ref w

ref g

w air

g air

, , , ,

,

,

/

/




N Np

p

s

sD ref w D ref g

ref w

ref g

w air

g air

, , , ,

,

,

/

/


N NM

MD user w D ref w

ref w

user w

, , , ,

,

,



(5) Compare user and reference chambers

in water

• Calibration at 6 beam qualities

• Polarity and recombination measured at

each beam quality

N NM

MD user w D ref w

ref w

user w

, , , ,

,

,

M(zref) = Mraw.fTP.fion.fpol.felec


(


(


A few notes – non-water phantoms

• Calibration in a water phantom recommended

• Solid phantoms allowed, especially for low energy beams

(see Code of Practice section 2.2.8)

• Order of preference:

• 1. Water

• 2. Epoxy resin materials

• 3. PMMA, polystyrene

N.B. Uncertainties increase


Non-water phantoms

• Solid phantoms offer significant

advantages:

• Robust

• Dry!

• Mechanically stable

• Easy to set up

• but we need absorbed dose to

water


Non-water phantoms – Code of Practice 2.2.8

• Solid phantoms offer significant

advantages:

• Robust

• Dry!

• Mechanically stable

• Easy to set up

• but we need absorbed dose to

water


A note on the recombination correction

• It is important that a chamber is operated in the linear region

1/V (V-1)

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035

1/I (

no

rmalised

)

0.998

1.000

1.002

1.004

1.006

1.008

1.010

1.012

1.014

1.016


Overview

• IPEM Code of Practice

– Theory

– Dose conversion

– Beam quality specifier

• Electron beam calorimeter

• Calibration of secondary

standards

• Use in the clinic – covered in

next presentation

• Code of Practice – very

thorough

– You will need to read the

entire CoP to implement it

Covered in greater detail during

electron practical session


Electron practical session

• Gaining familiarity with the

IPEM 2003 Code of Practice

through both calculations and

measurements.

• Calculations:

• fluence scaling factor

• range scaling factor

• stopping power ratio

• determination of ND,w

• Measurements:

• beam quality

• beam output, including

corrections for ion

recombination and

polarity

Documents

Electron Beam Dosimetry at NPL · • Electron dosimetry now on the same footing as MV photons. NPL Management Ltd - Public Outline of Code • Section 1: Introduction • Section