New Dosimetry Measurements- New Developments

J. Seuntjens J. Gibbons

I. Das

WA-A-211A Continuing Education Session

Much of what we will say…

Clinical Dosimetry Measurements in Radiotherapy

(AAPM 2009 Summer School)

D. W. O. Rogers & J. Cygler Editors ISBN: 9781888340846 Published: 2009

1,128 pp Hardcover

Price: US $140.00

AAPM, July 29, 2009 2

Session Outline

• Part I: Measurement dosimetry principles and application to nonstandard beams - J. Seuntjens

• Part II: Practical aspects of measurement dosimetry - J. Gibbons

• Part III: Small field dosimetry - I. Das

3

Outline - Part I

• Background• Definitions and nomenclature• Fundamental issues in nonstandard beam

dosimetry• New formalism for nonstandard beam

reference dosimetry

Background

5

Background

• Developments in RT techniques have substantially increased the use of small fields and larger uniform or non-uniform fields, composed of small fields

• Beam modifiers shaping small and composed fields such as standard-, mini- and micro- MLCs have become common equipment on conventional linacs

• Radiation therapy procedures are shifting from traditional 3D conformal deliveries to:

– novel, dedicated treatment units specifically designed for stereotactic (GammaKnife, CyberKnife) or IMRT treatments (TomoTherapy)

– traditional units delivering dynamically composed fields (RapidArc, VMAT, IMRT, etc)

AAPM, July 29, 2009

6AAPM, July 29, 2009 7

In practice…Modality Typical

calibration field size (static)

S&S or dynamic capabilities?

IMRT, SRS 10 x 10 cm2 Yes

TomoTherapy 5 x 20 cm2 Yes

Cyberknife 6 cm diameter Yes

GammaKnife 1.6 cm / 1.8 cm diameter

Yes

8

Issues with nonstandard beams

• There are no primary standards that measure dose directly in nonstandard beams

• Delivery protocols in nonstandard beams are not standardized (and not easily “standardizable”)

• The path between a static open 10x10 cm2

field calibration and delivery conditions introduces significant uncertainties

9

Large differences in Output Factorsamong users/machines

Statistics of 45 Output Factors for 6 mm and 18 mm square fields Novalis, SSD = 100 cm, depth = 5 cm, various detectors)

From Wolfgang Ullrich, BrainLab

factor of 2 in dose determination!

Definitions and nomenclature

11

Small photon fields

broad photon field

volume volume

narrow photon field

A small field is defined as a field with a size smaller than the “lateral range” of charged particles

12

Nonstandard field: small and composite

• Non-standard fields are defined as:– Single small fields

• CPE or TCPE is distorted

– Composition of small fields either in step-and-shoot, arcs, or dynamic multiple fields

• CPE or TCPE may be (but is not necessarily and not intentionally) distorted

• Examples: TomoTherapy, Cyberknife, etc.

• Why are we worried about CPE or TCPE in measurement dosimetry?

Fundamental issues in nonstandard beam dosimetry

14

Classification of dosimeters and mechanisms

Dosimeter Mechanism

Gas-filled ionization chamber Ionization in gasses

Liquid ionization chamber Ionization in liquids

Semiconductors (diodes, diamond detectors, MOSFET)

Ionization in solids

TLD Luminescence

Scintillation counters Fluorescence

Film, gel, Fricke Chemical reactions

Calorimetry Heat

Measurement dosimetry in medium

where:

dose to medium at the point

raw signal, corrected for environmentalconditions as given by detector

detector cavity dose calibration coefficient(coupling constant)

dose conversion coefficient converts averagedetector dose into dose to medium at point

Dmed (r ) = cdetM(r )fmed (r )

Dmed (r )

M(r )

fmed (r )

cdet

Dosimeter dependent coefficients & coupling constants

Factor orcoefficient

Dosimetry techniqueCalorimetry Fricke

dosimetryIon chamberdosimetry

M T∆ ODLρ

∆ Q

cdet C 13( )Fe Gε +

1 Wgasm egas

fmed Unity ( )medD Fricke

smed,gaspQ

17

Why do we worry about CPE or TCPE?

unperturbed medium fluence (SA → CPE)

restricted collision s.p.

There is no theoretical reason why the SA evaluation cannot be performed using disequilibrium fluence.But, …, in this case we don’t expect smed,det to be a good representation of the ratio Dmed/Ddet

18

Stopping power ratios (SPRs)

• In regions of CPE and TCPE: SPR corrections accurately represent detector response (and are small for air-filled chambers in photon beams)

• In regions of non-CPE: SPR corrections DO NOT accurately reflect changes in detector response and additional, sometimes large, corrections are needed– small fields – build-up regions in any field, interface-proximal points in

heterogeneous phantoms (build-up and build-down)– sometimes: intensity modulated fields, etc

ICRU, September 2008 19

Narrow 1.5 mm field - extreme exampleRatio of avg. dose to water <Dw> to dose to cavity air

Off-axis distance (mm)

Model A14P chamberCollecting electrode diameter: 1.5 mmSeparation: 1 mm

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

0 2 4 6 8

<D

w>/D

air

Stopping power ratio w/air

averaged over cavity volume

Paskalev, Seuntjens, Podgorsak (2002) AAPM

Proc. Series 13, Med. Phys. Publishing,

Madison, Wi, 298 – 318.

Perturbation correction factors: traditional factorization for ionization chambers:

• Pwall: wall perturbation correction factor (pwall)• Pgr: correction for gradient effect (effective p. of meas., pdis)• Pfl: fluence perturbation correction factor (pcav)• Pcel: central electrode perturbation correction factor (pcel)

PQ = PwallPgrPflPcel

21

Perturbation correction factors

• depth, field size, and radiation quality dependent– for reference dosimetry using ionization chambers:

evaluation based on Monte Carlo calculations, measurements and relatively well documented

– for relative dosimetry: their variation relative to the reference point is traditionally ignored but can be very significant in non-CPE conditions.

ICRU, September 2008 22

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

IMRT is far from standard reference conditions

From Art Boyer, StanfordICRU, September 2008 23

sw,air = 1.1229

sw,air = 1.1201

Data for IMRT dosimetry can be determined very accurately

(Andreo, ESTRO 2008)but what is the use, …

Bouchard et al, 2009 (Med Phys, in print)

Extreme nonstandard fields

Exradin A12Exradin A14

Bouchard et al, 2009 (Med Phys, in print)

Exradin A14

ICRU, September 2008 26

From Roberto Capote, IAEA

27

Reference dose measurements in dynamic fields

Fraser, D. et al., (2009)

PinPointIC10NE2571 =

=0.973

( ) 0.022NE

NE

x

s x

==

0.963

( ) 0.024IC10

IC10

x

s x==

0.944

( ) 0.035PP

PP

x

s x

A new proposed formalism for reference dosimetry of

nonstandard beams

29

• Two new reference fields:– Small static field dosimetry: machine-specific-

reference field (msr) for treatment machines that cannot establish a conventional reference field.

– Composite (dynamic) field dosimetry: plan-class specific reference field (pcsr). Represents a class of dynamic or step-and-shoot delivery fields, or a combination of fields, such that full charged particle equilibrium (CPE) is achieved at the position of the detector.

Reference dosimetry protocol: the IAEA/AAPM proposal

30

1. Small Static FieldsReference Calibration, ref field fmsr

0 0

,, , , , ,

msr refmsr msrmsr msr msr

f ff fw Q Q D w Q Q Q Q QD M N k k= ⋅ ⋅ ⋅

is a factor which corrects for the differences between the conditions of field size, geometry, phantom material and beam quality of the conventional reference field fref and the machine-specific reference field fmsr

,,

msr ref

msr

f f

Q Qk

refref

msr

msr

msr

msrrefmsr

msr fQ

fQw

fQ

fQwff

QQMD

MDk

,

,,, =

Andreo, ESTRO 2008

31

1. Small Static Fields Relative dosimetry, clinical field fclin

is a field factor which converts the absorbed dose to water for the machine-specific reference field, fmsr to the absorbed dose to water for the clinical field fclin. In relative dosimetry of single static fields this factor is conventionally called a field output factor, which ought to be defined as a ratio of Dw.

,,

, ,clin ms clin msr

clin ms

r

clin m rsr

f fw Q

f fQ Qw QD D= ⋅Ω

,,

clin msr

clin msr

f fQ QΩ

msrclin

msrclinmsr

msr

clin

clinmsrclin

msrclin

ffQQf

Q

fQff

QQ kM

M ,,

,, ⋅=Ω

It can be calculated directly as a ratio of Dwusing Monte Carlo alone or it can be measured as a ratio of detector readings multiplied by a Monte Carlo calculated correction factor.

Andreo, ESTRO 2008

32

Monte Carlo D-ratios vs experimental M-ratios. 6 MV, 5cm depth

Data from S.Dobladoet al (2007)

0 0

,, , , , ,

msr refmsr msr

msr msr msr

f ff fw Q Q D w Q Q Q Q QD M N k k= msrclin

msrclin

msr

msr

clin

clin

ffQQ

fQw

fQw DD ,

,,, Ω=Machine specific

reference field fmsr

Clinicalfclin

e.g. a GammaKnifeclinical plan

msrclin

msrclin

ffQQ

,,Ω

Tomotherapy5cm x 20cm

REFERENCE DOSIMETRY RELATIVE DOSIMETRY

GammaKnife∅∅∅∅ 1.6/1.8 cm

CyberKnife0.6 cm

1

≡ Ionizationchamber

Broad beamreference field

fref

00 ,,, QQQwD kN

Hypothetical

reference field fref

BrainLABmicro MLC10cmx10cm

refmsr

msr

ffQQk,,

Radiosurgical collimators∅∅∅∅ 18 mm

refmsr

msr

ffQQk,,

Andreo, ESTRO 2008

34

2. Composite FieldsReference Calibration, ref field fpcsr

refpcsr

pcsr

pcsr

pcsr

pcsr

pcsr

ffQQQQQwD

fQ

fQw kkNMD ,

,,,,, 00⋅⋅⋅=

A plan-class specific reference field, pcsr, is a reference field for a class of dynamic or step-and-shoot delivery fields, or a class of combinations of fields in a configuration that is as close as possible to the final clinical delivery scheme, but delivers a homogeneous absorbed dose to an extended and geometrically simple target volume

Andreo, ESTRO 2008

35

2. Composite Fields Relative dosimetry, clinical field fclin

,,

, ,pcs clinrclin

c

pcs

lin

r

clip ncsr pcsr

f f

Q

ffw Q w Q QD D ⋅Ω=

has to be established for every treatment delivery, and can be determined using the same set-up and the same phantom as for calibration. E.g. for a patient specific clinical plan, the phantom should be the same as for the dosimetry in the pcsr field using a cross-calibrated chamber.

,,

clin pcsr

clin pcsr

f f

Q QΩ

Andreo, ESTRO 2008

refpcsr

pcsr

ffQQk ,,

refpcsr

pcsr

pcsr

pcsr

pcsr

pcsr

ff

QQQQQwD

f

Q

f

Qw kkNMD,,,,,, 00

= pcsrclin

pcsrclin

pcsr

pcsr

clin

clin

ff

QQ

f

Qwf

Qw DD,,,, Ω=

Broad beamreference field

frefPlan-class

specific reference fieldfpcsr

00 ,,, QQQwD kN

Clinicalfclin

msrpcsr

msrpcsr

ffQQk,,

Hypotheticalreference field e.g. 9-field prostate

pcsre.g. 9-field prostate

clinical plan

pcsrclin

pcsrclin

ffQQ

,,Ω

(e.g. IMRT Linac)

refmsr

msr

ffQQk,,

fmsr

(e.g. Tomotherapy5cm x 20cm)

REFERENCE DOSIMETRY RELATIVE DOSIMETRY

20º 60º

100º

140º180º220º

260º

300º 340º

2

≡ Ionizationchamber

Andreo, ESTRO 2008

0.9775

0.9800

0.9825

0.9850

0.9875

0.9900

0.9925

0.9950

0.9975

1.0000

1.0025

1.0050

1.0075

Exradin A12 NE2571 Exradin A1SL Exradin A14 PinPoint 31006

Ionization chamber

k_p

csr

Fully-rotated Delivery

Farmer-type 0.6 cm3

ionization chamberssmaller ionization chambers

Ionization

Chamber

Exradin

A12NE2571

Exradin

A1SL

Exradin

A14

PinPoint®

31006

Fully-rotated Delivery

0.9918

±0.0028

0.9915

±0.0028

0.9902

±0.0028

0.9932

±0.0029

0.9921

±0.0028

Collapsed Delivery

0.9899±0.0021

0.9907±0.0021

0.9982±0.0022

1.0025±0.0022

0.9937±0.0022

Chung et al, 2009

7 beam pcsr