How accurately can we measure dose clinically? · Dosimetric and positional accuracy of TPA: acceptability criteria 4 mm 4 mm 4 mm largedose gradient 4% 3% 3% high dose region low

1

Accurate clinical dosimetry

Part I:Fundamentals and

New developments in reference dosimetry

Jan SeuntjensMcGill University

Montreal, Canada, H3G [email protected]

Accurate clinical dosimetry, Part I, 2008 1

Dosimetric accuracy requirement

• effect of doseuncertainty oncomplication-freetumour control (“utilityfunction”, Schultheiss &Boyer, 1988)

• ICRU Report 24 (1976)• Mijnheer et al (1987)• Brahme (1988)

1% higher accuracy

→ 2% increase of cure

5%

3.5%

3%The structures these numbers are referring to are targets volumes


Dosimetric and positional accuracy of TPA: acceptability criteria

4 mm

4 mm

4 mm

large dosegradient

4%

3%

3%

high doseregion

low dosegradient

3% (50%loc.dose)

antropomorphicphantom and/orcomplex beams

3% (50%loc.dose)3%Stack of tissue

slabs simple fields

3% (50%loc.dose)2%

Homogeneouswater slab -simple fields

low doseregion

low dosegradient

centralaxis

Van Dyk (1993)


How accurately can we measure dose clinically?

• Which accuracy do we expect for a given clinicalsituation?

• How should we measure dose accurately?– when is a situation “well-behaved”?– when do we have non-equilibrium conditions?

• What is a suitable detector?• How do we handle complication areas?

– build-up region– narrow stereotactic beams– modulated fields and dynamic time-dependent measurement

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Outline

• Principles of measurement dosimetry• Detectors and phantoms• Reference dosimetry in TG51 compliant

beams• Issues in non-equilibrium dosimetry

– Relative dosimetry in non-equilibrium situations– Reference dosimetry in TG-51 non-compliant

beams


Classification of dosimetersDosimeter 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

3


Interpretation of a dosimeter measurement (measurement at point r)

Ddet

• smed,det is the stopping power ratio medium to cavity material• pQ is an overall perturbation correction factor that accounts for everything acavity theory does not account for.

fmed

fmed is the ratio of dose to medium to dose to cavity; factorization of fmed isquestionable in regions of strong disequilibrium

Dmed (r ) = cdetM(r )smed,det(r )pQ(r )

• cdet is the detector dose calibration factor and ties the clinical dosemeasurement to the chamber/detector calibration; for ionization chamber:“cavity gas calibration coefficient”

• M is the result of a measurement corrected for “technical” influencequantities to ensure that what we actually measure a quantity proportional tothe dose to the detector material


Reference dosimetry:TG-51 calibration protocol

• Cavity gas calibration coefficient derived froman absorbed dose to water calibration at 60Co

cdet =ND,w

Co

sw,air pQ( )Co

( =a.k.a

ND,air = Ngas )

Dw =ND,w

Co

sw,air pQ( )Co

Msw,air pQ

= ND,wCo MkQ kQ =

sw,air pQ

sw,air pQ( )Co

= fw( )CoQ

with


calibration beamair kermacalibrationcoefficient

cavity gascalibrationcoefficient

absorbeddose calibrationcoefficient

clinical beamabsorbeddose calibrationcoefficient

Calibration chain for ionization chambers


,D air gasN or N

Qkor

50' * *R ecal grk k P

,CoD wN

TG-51 or TRS 398

K XN or NTG-21 or TRS 277

,D wN

( )/ waterwall replairL P Pρ

4


Reference dosimetry:TG-51 calibration protocol (photons)

Dw = MND,wCo kQ

- Measurement at 10cm depth in water- 10 x 10 cm2 field- beam quality specifier:%ddx(10 cm)

Almond et al, 1999


Reference dosimetry:TG-51 calibration protocol (electrons)

Dw = MND,wCo kecalk 'R50

Pgr- Measurement at dref in water- 10 x 10 cm2 or 20 x 20 cm2 field- beam quality specifier: R50


Reference DosimetryNew developments - WGTG51

• A number of more recent chamber types arenot listed in TG-51

• Extensive experimental data on kQ factors isavailable that allows to estimate uncertaintieson kQ as well as the overall referencedosimetry calibration process.

→ AAPM Work Group (WGTG51, chaired by M.McEwen) to supply this information (See importantpresentation: McEwen and Rogers, TH-D-352-2)


Accuracy of clinical dosimetry

• Stability, linearity, reproducibility of the detector and themeasurement setup

• Energy dependence - what is the accuracy with which one canconvert detector dose to medium dose?

– CPE or TCPE and detector is small compared to the range of secondaryelectrons:

• fmed can be factorized as a stopping power ratio + correction factors (and thecorrections are small or constant)

– If these conditions are not fulfilled:• fmed cannot be factorized or very large correction and situation dependent

correction factors are needed (unpractical)

• Monte Carlo calculations (assumed accurate within the constraints of thealgorithm and basic cross-section datasets)

• Choose a more suitable detector (perturbation free, medium equivalent, etc)

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Why do we worry about CPE or TCPE?For the measurement of a dose distribution or for

measurement of output:• In regions of CPE and TCPE: SPR corrections

accurately represent detector response (and aresmall for air-filled chambers in photon beams)

• In regions of non-CPE: SPR corrections DO NOTaccurately reflect changes in detector response andadditional, sometimes large, corrections are needed– build-up regions in any field, interface-proximal points in

heterogeneous phantoms (build-up and build-down)– narrow stereotactic fields– intensity modulated fields


Different measurement issues in open photon fields in water

• in-field, CPE or TCPE– standard measurement, SPR is nearly constant and represents

detector response well; detector perturbations are small (point-of-measurement shift is 0.6*rinner upstream)

• in-field, non-CPE (build-up)– non standard measurement, SPR varies modestly but does not

represent variation in detector response well; detector perturbationsare large, detector dependent.

• penumbra (non-CPE)– detector size must be small relative to field-size and penumbra

width, SPR does not represent detector response well, lateralfluence perturbations, detector dependent.

• out-of-field (CPE or TCPE)– detector size must be sufficiently large to collect sufficiently large

signal. SPR represents detector response well.


Stopping power ratios (SPR’s)• SPR’s are depth, field size, and radiation quality

dependent– for reference dosimetry: values have been calculated with

Monte Carlo techniques and are well documented instandard dosimetry protocols

– for relative dosimetry in TCPE: their variation relative to thereference point can be calculated using Monte Carlotechniques

– for relative dosimetry in non-equilibrium situations: theirvariation relative to the reference point can still be calculatedbut no longer represents an accurate dose conversion.

Andreo and Brahme, Phys. Med. Biol. , 31, 839 (1986)

sw,air(z) for plane parallel bremsstrahlung beams- no e - contamination, central axis

10 MV

20 MV

30 MV

60Co

6


Sanchez-Doblado et alPhys. Med.Biol. 48 2081-2099 (2003)

Smallfields


Narrow 1.5 mm fieldRatio of dose to water to dose to air

Off-axis distance (mm)

Collecting electrodediameter: 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

Dw/D

air

stopping powerratio 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


Perturbation correction factors

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

based on Monte Carlo calculations,measurements and relatively well documented

– for relative dosimetry: their variation relative to thereference point is traditionally ignored but can bevery significant in non-CPE conditions.

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Buckley and Rogers, Med. Phys. 33 1788 - 1796 (2006)

NACP chamber in 6 MeV and 20 MeV electrons

Verhaegen et al, Phys. Med. Biol. 51 1221-1235 (2006)

NACP-02

Partialcompensation of wall andfluenceperturbationeffects


Build-up dose measurements• Measurements are needed to ensure a

treatment planning system accuratelycalculates dose in the build-up region.

• The behaviour of ionization chambers in thebuild-up region is questionable.


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Accurate clinical dosimetry, Part I, 2008 28 Accurate clinical dosimetry, Part I, 2008 29

NACP chamber response in buildup region

cavair air

Q WD

m e

=

max max

( ) ( )

( ) ( )

cav

cav

D d Q d

D d Q d=

NACPmeasurementaccurate DOSRZnrc cavity

simulations

69.0 ± 0.3 %67 ± 2 %40x40

47 ± 2 %45 ± 1 %10x10

Measured relative surfaceionization

Calculated relativesurface cavity dose

Field size(cm2)


Perturbation correction factors – 10x10 cm2

( )( ) ( ) ( )water

watercavwater airair

L dD d D d dρ

= Φ

0.78 ± 0.020.0

0.99± 0.0081.5

Perturbation correctionDepth (cm)


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Kawrakow (2006) Med. Phys. 33, 1829

Reconcilingmeasurements andcalculations in thebuild-up region byadjusting EPOM

→simple if it works;but is chamberdependent!

Effective point of measurement in build-up region isnot the same as in CPE conditions

18 MV

McEwen et al (Med. Phys. 35, 950 (2008) )

Experimental shifts of point of measurement

Inflection point variabilityMatch @ inflection point

Ververs, Siebers and Kawrakow 2008 - TU-C-AUD B-1Accurate clinical dosimetry, Part I, 2008 35

Reference dosimetry in TG-51/TRS-398 non-compliant beams

• Many radiation treatment machines cannot producereference field sizes or SSDs according to therecommendations of TG-51 or TRS-398 protocols(e.g., GammaKnife, CyberKnife, and TomoTherapy).

• IMRT/IMRS and SRS/SRT/SBRT treatment plans aredesigned to treat from multiple beam directions.

• Delivery of these plans use small fields that are notvery similar to the reference conditions used forbeam calibration.→ Reference conditions for beam calibration should be as

close as possible to the patient irradiation conditions.

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Reference dose measurements in dynamic fields

Fraser, D. et al.,(2007)

0.85

0.90

0.95

1.00

1.05

1.10

0 10 20 30 40 50Plan No.

(DC/D

mea

s)IM

RT

Pin PointIC10NE2571 =

=0.973

( ) 0.022NE

NE

x

s x

==

0.963

( ) 0.024IC10

IC10

x

s x==

0.944

( ) 0.035PP

PP

x

s x


6.9%1.0051.079 ± 5.9%Dynamic #8

5.9%1.0141.077 ± 5.7%Dynamic #7

-1.4%1.6051.583 ± 5.9%Dynamic #6

3.8%0.8850.920 ± 5.4%Dynamic #5

-2.4%1.0311.007 ± 3.2%Dynamic #4

7.8%1.0041.089 ± 3.9%Dynamic #3

0.7%1.1611.169 ± 2.5%Dynamic #2

-0.3%1.1431.139 ± 3.0%Dynamic #1

2.7%1.1411.172 ± 2.5%Static #5

3.2%1.2331.274 ± 6.0%Static #4

2.7%1.0941.124 ± 3.2%Static #3

1.9%1.1501.173 ± 6.1%Static #2

6.8%0.9501.019 ± 4.0%Static #1

Dif ferenceCIMRT,6 MVcalculatedCIMRT,6 MV

measuredField name

Correction to chamber measurements in IMRT fields

Bouchard and Seuntjens, MedPhys 31, 2453-2464 (2004)


• IAEA - AAPM joint initiative– IAEA has had two consultancy meetings to

develop the protocol (Dec 07, May 08)– AAPM Workgroup on non-compliant beam

dosimetry– IPEM endorsement expected– Other national organizations to follow

• An international protocol that forms asupplement to existing protocols

Reference dosimetry protocol


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

reference field (msr) for treatment machines thatcannot establish a conventional reference field.

– Composite (dynamic) field dosimetry: plan-classspecific reference field (pcsr). Represents a classof dynamic or step-and-shoot delivery fields, or acombination of fields, such that full chargedparticle equilibrium (CPE) is achieved at theposition of the detector.

Reference dosimetry protocol

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1.8 cm

6 cm



Physics behind the conversion factors

from measurements using aprimary absorbed dose standard

from measurementsusing a “suitable”detector

from Monte Carlocalculations or othermodel


Example: Tomotherapy calibration

0.997Helical, 1 cm0.9902 cm x 2 cm

1.000Helical, 2.5 cm0.99310 cm x 2 cm

1.000Helical, 5 cm0.99710 cm x 5 cm

fpcsr (route2)

Duaneet al, 2006

fmsr (route1)

Jeraj et al, 2005

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Discussion: defining a pcsr field

• minimize disequilibrium effects near detector• delivery of pcsr must resemble delivery of actual plans

corresponding to the same class• measurements in solid phantoms

→ Design treatment plan for pcsr :→ must preserve the essence of the modulation→ conversion factors can be accurately measured or calculated

→ Patient-specific QA procedure is much closer to pcsr than to reffield.


Summary• Reviewed

– basics of dosimetry– TG-51 and current extensions– applicability of cavity theory to non-reference conditions

illustrated via small fields, build-up dose and non-compliantbeams

• accurate measurements require accurateunderstanding of the detector response

• conventional cavity theory for detector response isnot accurate in non-equilibrium conditions

• this has, in some cases, significant clinical impact ona dose measurement

• “suitable detectors” or “corrections” are needed

Documents

How accurately can we measure dose clinically? · Dosimetric and positional accuracy of TPA: acceptability criteria 4 mm 4 mm 4 mm largedose gradient 4% 3% 3% high dose region low