MV Photon Dosimetry - National Physical Laboratory · MV Photon Dosimetry Small fields Maria Mania Aspradakis Cantonal Hospital of Lucerne, Switzerland [email protected] 1

MV Photon Dosimetry Small fields Maria Mania Aspradakis Cantonal Hospital of Lucerne, Switzerland [email protected]

1 Practical course on reference dosimetry, NPL, 22nd Jan 2014

Learning Objectives

• Definition of small MV photon field

• Small field characteristics

• Challenges in dosimetry and the determination of

dosimetric parameters

2 Practical course on reference dosimetry, NPL, 22nd Jan 2014

Small MV photon field conditions

• Beam related small-field conditions

Lateral charged particle disequilibrium

Occlusion of the focal spot (direct-beam source)

• Detector-related small-field conditions

Detector response due to its size and composition

3

Lateral electron disequilibrium

(lack of lateral CPE)

4

Lateral charged particle loss

Minimum field radius required for lateral electron

equilibrium (rLEE)

5 Li et al MedPhys 22, 1995, 1167-1170 688.2973.5 20

102 TPRcmgrLEE

LEE

dmax

In broad fields

Full view of

extended direct

beam source from

the point of

measurement

radiation source

Radiation detector

measures in region

of uniform dose

Dose variation across a

broad field (‘dose profile’)

collimator

6

Occlusion of

the beam

focal spot

with

decreasing

collimator

setting

source occlusion by the

collimators

Partial view of

extended direct beam

source from the point

of measurement

Radiation detector measures

in non-uniform dose region

Very narrow dose profile

7

Overlapping penumbras

8

Definition of field size?

Detector size and construction

detector perturbation effects

9

• Volume averaging

• Energy dependence (of

stopping power and

energy absorption

coefficient ratios)

• Perturbation effects

Sauer & Wilbert MP 34, 2007, 1983-1988

Detector perturbation

the effect of volume averaging

OAR(x,y) is the off axis distribution of field A in orthogonal directions x and y

Kawachi el al (2008), Med Phys 35 (10)

•A chamber of cavity

length of 24mm

underestimates dose by

1.5% in the 6cm field

•A chamber of cavity

length of 3mm

underestimates dose by

0.5% in the 2cm field

Detector perturbation due to its construction

11

Ionization chamber:

• wall,

central electrode,

air cavity

Diode:

• housing, shielding, contacting

silicon chip

Perturbation depends on

field size

C. McKerracher, D.I. Thwaites /

Radiotherapy and Oncology 79 (2006) 348–351

Detector perturbation

the influence of detector density at small field sizes

Scott et al PMB, 57 (2012) 4461-4476

in watervoxeldetector

in watervoxelwater

D

D

in waterdensitydetectorwithvoxel-water

in waterwater

D

D

15MV

Definition of small MV photon field

• For the selected energy and medium, the field size is not

large enough to ensure lateral CPE

• The entire beam source, as viewed from the point of

measurement, is partially shielded by the field-defining

collimators

• The detector not small enough and perturbs fluence

significantly

13

Small MV photon fields

characteristics

• changes in beam spectra with collimating method,

accelerating potential, field size and depth

• dose profiles: overlapping penumbra & apparent

widening of field

• drop in beam output

14

Photon fluence spectra in air variation with collimation method

15 Sanchez-Doblado et al (2003), PMB 48: 2081

Photon energy fluence spectra in water variation with accelerating potential

16

Yin et al Phys Med Biol 49(16) (2004)

Photon energy fluence spectra in water

variation with field size and depth in water

17

at depth of maximum build-up at 150mm depth

6MV

Yin et al Phys Med Biol 49(16) (2004)

Particle fluence spectra in water variation with field size, 6MV 50mm depth

18 Eklund and Ahnesjö, PMB 53(16) 2008

Particle fluence spectra in water variation with field size, 6MV 50mm depth

19

Eklund and Ahnesjö, PMB 53(16) 2008

Source occlusion & drop in output

20

IPEM report 103, 2010

source occlusion, overlapping penumbrae, drop in

output & apparent widening of field

21

point source

used in

calculation

A Ahnesjö, 2000

finite source

size used in

calculation

Dose

calculations

in small

fields

Source occlusion, drop in output

22

Treuer et al PMB 38(12) 1992

point source

used in

calculation

finite source size used in

calculation

Dose

calculations

in small

fields

Drop in output: detector dependence

23 Sanchez-Doblado et al , Physica Medica, 23, 58-66, 2007

Small MV photon fields challenges

• Absolute dosimetry

• Reference dosimetry

• Relative dosimetry

• Modelling fluence and dose on treatment

planning systems (TPS)

24

Small MV photon fields - challenges absolute dosimetry

• Absolute dosimetry is the basis of reference dosimetry carried

out in the clinic.

• Water or graphite calorimetry involves highly specialised and

time-consuming procedures at NMI and its use at small

radiation fields is limited by the physical properties of graphite

and water (specific heat capacity and thermal diffusivity).

• Work is in progress and PTB and NPL to measure doses from

field sizes less 40mm.

25 IPEM Report number 103, December 2010

Small MV photon fields – challenges fluence and dose modeling on planning systems

• Modelling of the direct source occlusion is most critical – but results

very sensitive to measurement errors

• For modelling the dose distribution:

– treatment head geometry

– MLC positions, field sizes

– handling/positioning of block collimators

– resoluton of energy fluence matrix incident on patient surface

• The calculation of monitor units (MU) from small field segments:

– full head scatter (flattening filter scatter, monitor backscatter, etc)

– collimator leakage

26

Small MV photon fields - challenges reference dosimetry with air-filled ionisation chambers

det

water

airair

airwater p

ρ

S

m

1

e

WMD

replcelwallflgrcelwalldet pppppppp

To account for perturbations from B-G cavity theory:

27

under the reference conditions (field and depth) as defined in dosimetry codes of practice

oo

refref

QQQwD

f

Q

f

Qw kNMD ,,,,

UK MV Dosimetry code of practice (IPSM 1990) Measurement of reference dose

DNRDcorrected

instrument

response [C]

UK MV dosimetry CoP, PMB, 35, 10, 1355-, 1990

oo QQQwDQwD kNN ,,,,,

Qo = reference beam quality

CGy Chamber types 2561, 2611

28

0

0

0

00

o

Q

Q

water

air

Q

Q

water

air

Q

Q

water

airQ

air

Q

Q

water

airQ

air

QQ,

pS

pS

pS

e

W

pS

e

W

k

Small MV photon fields - challenges reference dosimetry with air-filled ionisation chambers

29

Influence of field size on Spenser-Attix stopping power ratios MC simulations on CAX at 5cm depth in water

Sanchez-Doblado et al (2003), PMB 48: 2081

0.3%

0.5%

< 0.2%

30

Reference dosimetry with air-filled ionisation chambers

31

0

0

0

00

oQQ,

Q

Q

water

air

Q

Q

water

air

Q

Q

water

airQ

air

Q

Q

water

airQ

air

pS

pS

pS

e

W

pS

e

W

k

Sanchez-Doblado et al (2003), PMB 48: 2081

Ratios for small and composite fields differ no more than 0.5% from values for

a 10cm x 10cm field Existing water to air ratios of Spencer-Attix restricted

mass collision stopping powers published for broad fields can be used for

dosimetry in small and composite fields

Ding et al (2012), PMB 57(17): 5509

oo

refref

QQ,Qw,D,QQw, kNMDff

Beams with flattening filter (wFF)

Xiong and Rogers (2008), Med Phys 35(5) 2104-

FFF beam: profile not flat & spectrum is softer

Flattening Filter Free beams (FFF)


Xiong and Rogers (2008), Med Phys 35(5) 2104-

TPR20,10 decreases for a

given accelerating potential

(MV)

SA water to air stopping

power ratios increase

For CoP using TPR20,10 as

beam quality specifier, the

values of existing kQ,Qo

correction factors need to be

reduced by about 0.5%

Flattening Filter Free beams (FFF)



34

0

0

0

00

oQQ,

Q

Q

water

air

Q

Q

water

air

Q

Q

water

airQ

air

Q

Q

water

airQ

air

pS

pS

pS

e

W

pS

e

W

k

oo

refref

QQ,Qw,D,QQw, kNMDff

Example: Perturbation factors for ionisation chambers

replwalldet ppp

0.3%

Araki (2006), Med Phys 33(8)

35

10cm x 10cm 6cm

6.6 mm

16.25 mm

5 mm

23 mm

5.8 mm

2.5 mm

Detector size – the effect of volume averaging

Pantelis el al (2010), Med Phys 37 (5)

36

0.3%

1.4%

Small MV photon fields – reference dosimetry

37

Corrects for the difference between reference beam quality Qo and beam quality at

reference field fref (collimator setting 10 cm x 10cm)

oo

refref

QQQwD

f

Q

f

Qw kNMD ,,,,

What if the machine cannot set to the conventional

reference conditions?

•How to determine dose in specialised systems?

•How to determine dose in composite fields?

reff

Hypothetical reference field

refmsr

,m

, ff

QQ srk

of

cm10cm10

reference field

msrf20,10TPR10TPR20,10

msrf

IAEA/AAPM formalism for reference dosimetry small static MV photon fields – under non conventional reference conditions

Alfonso el al (2008), Med Phys 35 (11)

refmsr

msroo

msr

msr

msr

msr

,

Q,QQQ,Qw,D,QQw,

ffffkkNMD

40

Detector correction factor in a static field

msr

msr

ref

ref

msr

msrmsrrefmsr

msr f

Q

f

Q

f

Qw

f

Qw

QwD

QwDff

QQM

M

D

D

N

Nk

,

,

,,

,,,

,

Determined though:

•Experiment: by a primary standard

•Experiment: using dosimeters that can measure reference dose traceable

to a primary standard and which have sufficiently low uncertainty (alanine,

radiochromic film, diamond, liquid ion-chambers ...)

•Calculation: Monte Carlo simulations

IAEA/AAPM formalism for reference dosimetry small static MV photon fields – under non conventional reference conditions

Small fields: relative dosimetry measurement of penumbra

• To determine the penumbra correctly use a small diode (consider directional dependence)

• Check the detector response outside the geometrical field

• Correct for over/under-response or use an appropriate detector [e.g. (shielded) diode or

radiochromic film]

Heydarian et al PMB 41

(1996) 93–110

Ø7 mm Ø23 mm

43

Small fields: relative dosimetry measurement of depth functions (RDD/PDD/TPR)

• Micro ionisation chambers (volume < 0.01cm3)

• Small Diode; or smallest availlable detector

• Radiochromic film (e.g Gafchromic EBT, MD-55)

• Careful alignment of detector with CAX

• Estimation of the volume effect with changing

depth

• Direct measurement of TPR/TMRs (source-

detector-distance constant!)

44 IPEM Report 103, 2010

Relative dosimetry: Field size factor, Scp

• energy correction

• perturbation correction

• volume effect

45

det

water

airair

airwater p

ρ

S

m

1

e

WMD

A

A

A

A

pρ

S

zAM

zAM

zAD

A,zDAS

ref

ref

det

w

airrefref

ref

refrefw

refwcp

,

,

,

A: field size (aperture)

zref: reference depth

Relative dosimetry: Field size factor, Scp measurement with an ionisation chamber high perturbation?

Challenge: perturbation factors

A: field size (aperture)

zref: reference depth

A

A

A

A

pρ

S

zAM

zAM

zAD

A,zDAS

ref

ref

det

w

airrefref

ref

refrefw

refwcp

,

,

,

46

A

A

A

Apk

zAM

A,zM

zAD

A,zDAS

refrefdetdetE,

refref

ref

refref

refdiode

cp,,

Relative dosimetry: Field size factor, Scp measurement with a diode high energy dependence

1

for

detector Ø <A

(?)

47

Characterise the sensitivity of the diode


0 2 4 6 8 10 12 14 16 180.0

0.2

0.4

0.6

0.8

1.06 MV meas.

IC

PiP

DiGre

DiYe

MOS1

MOS2

DIArela

tive D

ose

SES / cm

0.0 0.5 1.0 1.5 2.0 2.50.0

0.2

0.4

0.6

0.8

Sauer & Wilbert MP 34, 2007, 1983-1988

48


0 2 4 6 8 10 12 14 16 18

0.96

0.98

1.00

1.02

1.04

DiYe

MOS1

MOS2

DIA

6 MV

IC

PiP

DiGre

Sig

na

l ra

tio

s

SES / cm

Sauer & Wilbert MP 34, 2007, 1983-1988

49

Characterise the sensitivity of the diode bAaAkE

Unshielded diodes under-respond at small fields?

watertoDose

dettoDose

Normalised

response ratios


50

Characterise the sensitivity of the diode MC simulation of normalised response of unshielded diode PTW 60012

detailed model of diode

simplified model of diode

Franscescon et al MP 38(12), 2011

Unshielded diode

over-responds at

small fields

correction factor to

ratio of readings < 1

Fluence perturbation!

Energy dependence


51

MC simulation of ‘output factor’ corrections

factors

Franscescon et al MP 38(12), 2011

6MV

Siemens

Elekta

Correction

to ratio of

readings


refrefrefw

wcp

AMAk

AMAk

AD

ADAS

detD the absorbed dose to the detector volume (reading of the detector)

refA 10cm 10cm

A arbitrary clinical field

On-going research to determine overall correction

factors for detectors available in the clinic; using the

IAEA/AAPM formalism of Alfonso et al 2008 new

CoP

The volume averaging

correction factor can be

defined as the ratio of the

detector response in its

central part to the

detector response over

its whole volume

Correction for volume averaging

Georg, et al, 2nd ESTRO Forum, Pre-meeting

workshop 2013

53


Defined as the inverse of the detector signal integrated

over its area and weighted by the off-axis ratio from film

measurements (film profiles)

averageF1

54 Morin et al MP, 40(1), 2013


Morin et al MP, 40(1), 2013

55

Ralston et al PMB, 57, 2012

Minimise energy/field size dependence

An approximation to account for the influence of spectral

changes between the 10cm 10cm and a smaller field (e.g..

4cm 4cm) on detector response would be to cross-calibrate

the small detector against a medium size detector in an

intermediate field (smaller than the reference field of 10cm ×

10cm);

This is referred to as ‘daisy-chaining’ by Dietrich & Sherouse MedPhys 38(7), 2011

refIC

IC

diode

diode

AM

AM

AM

AMAS int

int

cp

‘Daisy-chaining’ the normalisation of output factors through an intermediate field

Dietrich & Sherouse MedPhys 38(7), 2011

Normalisation to value at 10cm ×10cm

Normalisation to value at 4cm × 4cm

Summary of lecture • In a small field there is lack of lateral CPE, source occlusion and

detectors perturb charged particle fluence

• The consensus so far on current good practice for reference and

relative dosimetry in static small MV photon fields is:

• Choice of suitably small detector which is known to minimally perturb fluence

• Careful experimental setup

• Approximately correct for volume averaging and energy dependence of

detectors (new CoP not ready with detector perturbation correction factors for

use in small static fields)

• Corroboration of data

58

Thank you for your attention

[email protected]

MV Photon Dosimetry - National Physical Laboratory · MV Photon Dosimetry Small fields Maria Mania Aspradakis Cantonal Hospital of Lucerne, Switzerland [email protected] 1

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