Mania AspradakisJohn ByrneHugo PalmansJohn Conway Jim WarringtonKaren Rosser Simon Duane

Why are we concerned with small MV photon fields?

SRS dosimetry

Das et al, J Radiosurgery, 3, 177-186, 2000

403530252015100.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

Pinpoint(par)Pinpoint(per)0.125ion(par)0.125ion(per)

Diamond

TLD

MC(1mm)MC(5mm)

CEA (Film)Kodak(Film)

Total scatter factor with various detectors

Cone Diameter (mm)

Con

e Fa

ctor

(St)

14%

Which detector and measurement methodology?

2

Incidents due to errors in dosimetry

•http://www.french-nuclearsafety.fr/index.php/content/download/15544/100847/Toulouse_ASN_report1.pdf

Why are we concerned with small MV photon fields?

Is there enough education and training to carry out dosimetric measurements in small fields?

3

Small MV photon fields on equipment originally designed and/or configured for treatments using broad photon fields

Why are we concerned with small MV photon fields?

E.g. source size modelling on TPS

Appropriate fluence and dose models on TPSs?Aspradakis Aspradakis et al et al Med Dos 30, 233, 2005Med Dos 30, 233, 2005

4

Use of specialised equipment and techniquesWhy are we concerned with small MV photon fields?

How to calibrate specialised equipment? 5

With decreasing field size:

6

With decreasing field size:

rmax

100 x 200 mm2

5 x 4 mm2

The dose at the inner part of the field is influenced by the lateral range of the electrons compared to the field size

7

Definition of small MV photon field• For the selected energy and medium, is

the field size large enough to ensure lateral CPE?

• Is the entire source in the detector’s-eye- view?

• Is the detector small enough not to perturb fluence significantly?

8

IPEM report 103: contentsChapter 1: IntroductionChapter 2: Physics and challenges in small field dosimetryChapter 3: DetectorsChapter 4: Machine QAChapter 5: General considerationsChapter 6: Reference dose measurementChapters 7, 8, 9: Relative dose measurementChapter 10: Monte CarloChapter 11: VerificationChapter 12: Summary and conclusions

9

Reference dose measurement with air-filled ionisation chambers

0

0

0

00

o

Qp,

Q

water

air

Qp,

Q

water

air

Qp,

Q

water

airQ

air

Qp,

Q

water

airQ

air

QQ,

kS

kS

kSeW

kSeW

k

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛

≈

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

=

ρ

ρ

ρ

ρ

Conclusion: Existing water to air ratios of Spencer-Attix restricted mass collision stopping powers published for broad (10cm x 10cm) fields can be used for dosimetry in small and composite fields

Challenge: derivation of perturbation factors for available small field (mini- and micro-) ionisation chambers

10

A chamber of cavity length of 24mm underestimates dose by 1.5% in the 6cm field on Cyberknife

Reference dose measurements in a 6cm diameter radiation field need to be carried out with an ionisation chamber of length not greater than 10mm at a source-to-chamber distance of 80cm

(Cyberknife).

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

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

Reference dose measurement with air-filled ionisation chambers - volume effect

11

Reference dosimetry1. Use of mini- or micro- air-filled ionisation chambers (BUT signal to noise

ratio?)2. The lack of perturbation factors increases uncertainty in the

measurement3. Need to consider:

– Chamber fully covered by the radiation field– Leakage– Cable effects– Polarity effects

4. Liquid-filled ion-chambers, diamonds and diodes not yet sufficiently characterised and commissioned for use in reference dosimetry

5. Specialised systems:– alternative reference conditions or adopt the proposed IAEA/AAPM formalism

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

– alternative procedures to determine beam quality (Sauer, (2009). Med Phys 36(9): 4168-72).12

Relative dosimetry: measurement of penumbra• To detect 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

13

Relative dosimetry Determination of depth functions (RDD/PDD/TPR) - 1

• Conversion between RDD/PDD and TPR/TMR → not recommended– How valid are existing conversion formulas at small fields and non-

standard SSDs? • BJR25 (1996), NCS 1998, Bjarngard et al, (1996), Med Phys 23(5):

629-34.

• Functional representation of TPR → not recommended– Xiao et al, (1998), Phys Med Biol 43(8): 2195-206, Sauer et al (2009)

Med Phys 36 (12)

14

Relative dosimetry Determination of depth functions (RDD/PDD/TPR) - 2

• Measurement

– Micro ionisation chambers (volume < 0.01cm3)

– Small Diode; for smallest field: SFD

– 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!)

15

Relative dosimetry: Field size factor, Scp

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 DIAre

lativ

e 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

( )( )

( ) ( ) ( ) ( ) detpEdetw

refw

wcp ,

,

DAkAkDAkAD

zADzADS

==

=

Field size dependence of• energy correction factor kE

for different detectors ?• perturbation correction factor kp ?• volume effect

0 2 4 6 8 10 12 14 16 180.0

0.2

0.4

0.6

0.8

1.06 MV corr.

IC PiP DiGre DiYe MOS1 MOS2 DIA

Dos

e / G

y/10

0MU

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 16

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

high perturbation?

Challenge: perturbation factors

A: field size (aperture)zref : reference depth

[ ]AA

frefairwater, conversion factor

( ) ( )( )

( )( ) [ ]A

A

A

A

kρS

zAMzAM

zADA,zDAS

ref

ref

detp,

w

airrefref

ref

refrefw

refwcp ,

,, ⎥

⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛==

17

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

high energy dependence

( ) ( )( )

( )( ) [ ] [ ]A

AAAkk

zAMA,zM

zADA,zDAS

refrefdetp,detE,

refref

ref

refref

refdiodecp ,,

==

≈1 for

detector Ø <ACharacterise the sensitivity of the diode

( ) bAaAk +=E

18

Relative dosimetry: Field size factor, Scp

• Cross-calibrate a Si-diode at medium field sizes with a small IC ('daisychain')

• Evaluate energy/field size dependence• Avoid volume effects (use a small detector)• Corroboration of data

Eklund & Ahnesjö PMB 2010

LEE

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

nal r

atio

s

SES / cmSauer & Wilbert MP 34, 2007, 1983-1988

Li et al MP 22, 1995, 1167-1170

19

Relative dosimetry: Field size factor, Sc in-air output ratio

• Mini-ionisation chamber or diode

• Mini-phantom design - high density material AAPM TG 74, Zhu et al 2009, MP 36(11)

• Careful alignment of detector with CAX

• Measuring at extended SSD problematic

20

IPEM report 103: supporting chaptersChapter 4: Machine acceptance and quality assurance

• machine alignment - better than 1mm/1°• calibration and QA of collimating jaw - better than 0.5mm

Chapter 5: General considerations with measurements• detector construction, leakage, cable effects etc

Chapter 10: Monte Carlo• use of the method for small field applications

Chapter 11: Verification• on methods for verifying plans comprising of small fields

21

IPEM report 103: main scope• Educate on the physics and challenges in the dosimetry of

small MV photon fields

• Review commercially available detectors and measurement methodologies suitable for implementation in the clinic

• Give recommendations of good practice in order to reduce uncertainty in the determination of dosimetric parameters

• Explain the need to commission TPSs specifically for small fields

• To point out directions along which future work and research efforts are required in this challenging field of dosimetry

22

Small field MV photon dosimetry Future requirements

• TPS and equipment manufacturers to share information on the definition of field size on their systems

• Machine QA procedures and tolerances to be extended to include checks at narrow collimated beams

• Validate (or develop) appropriate detector systems and detector response correction methods (Computer-Aided-Dosimetry?) for reference and relative dosimetry

• Extend current typical datasets (e.g BJR Suppl25, NCS12) to small fields

• Better fluence and dose engines on TPSs and MU calculators to reduce systematic errors in treatment planning calculations

23

Thank you for your attention!maria.aspradakis@ksl.ch

Acknowledgement

Dr Anders AhnesjöProf Otto Sauer

Mr Geoff BudgellProf Frank Verhaegen

Ms Marie Goodall, IPEM Office