S4 T1 Suentjens Small Field Dosimetry

Small Field Dosimetry for IMRT and Radiosurgery

Jan Seuntjens, Ph.D., FAAPM, FCCPMProfessor & Director Medical Physics

McGill UniversityCanada

SEAAPM 2011Thursday April 7 1

Overview – “Small field” radiotherapy

• Background– Issues and definitions

• Measurement physics of small fields

• Measurement physics of IMRT (composite) fields

• New dosimetry of small and IMRT fields –status report

• Conclusions

SEAAPM 2011Thursday April 7 2

Acknowledgments – small field committees

• IAEA committee– Palmans (Chair)– Andreo– Huq– Mackie – Ulrich– Kilby– Izewska– Capote– Alfonso– Seuntjens

• AAPM Committees– TG‐178 (Goetsch et al)– TG‐155 (Das et al)– WGDPCB

• ICRU Report Committee

Thursday April 7 SEAAPM 2011 3

Background

• Technological improvements in conventional Linacs have improved mechanical accuracy and stability as well as dosimetric control

• Increased availability of tertiary MLCs on conventional accelerators (Brainlab M3, NovalisTx)

• Specialized machines for IGRT using small fields or combination of small fields (GammaKnife, CyberKnife, TomoTherapy, Vero)

Has technology gotten ahead of comprehension of basic dosimetry principles?

Summary of some challenges in small‐field dosimetry

• Definition of field size?

• Difficulties in accurate measurements

• Modeling of small field dose calculations in TPSs

• Calibration protocol reference conditions cannot be achieved

IMRT versus SRS/SRT – some nomenclature

• nonstandard (a.k.a. non‐compliant) fields: radiation fields for which reference dosimetrycannot be reliably performed using the existing protocols (AAPM TG‐51 or IAEA TRS‐398)– Small fields (static)

• Reference conditions cannot be met (10 x 10 cm2 is not available)

– Composed fields (IMRT, step‐and‐shoot or dynamic)• Delivery conditions are far removed from calibration conditions

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!

Physics of small fields

What constitute small‐field conditions?

• Beam‐related small‐field conditions– the existence of lateral charged particle disequilibrium

– partial geometrical shielding of the primary photon source as seen from the point of measurement

• Detector‐related small‐field condition– detector size compared to field size

What constitute small‐field conditions?

ICRU, September 2008 11

Lateral charged particle loss

broad photon field

volume volume

narrow photon field

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

is a measure of the degree of equilibrium or transient equilibrium

Li et al. 1995 Med Phys 22: 1167‐70

Loss of lateral charged particle equilibrium

Concept of rLEE

Source occlusion

Large field conditions Small field conditions(IPEM Report 103)

Overlapping of beam penumbras

From Das et al. 2008 Med Phys 35: 206‐15

• definition of field size?

What constitute small field conditions?

Detector dependence of output factor

Thursday April 7 SEAAPM 2011 16From Doblado et al. 2007 Phys Med 23:58‐66

Detector issues in small field dosimetry

• Energy dependence of the response

• Perturbation effects– Central electrode

– Wall effects

– Fact that cavity is air‐filled instead of water

– Volume averaging

• Interaction of these effects with beam focal spot size

Detector‐related small field condition

One could claim that the GammaKnife 18 or 14 mm diameter fields are not small (quasi point source + electron equilibrium length about 6 mm)

Meltsner et al., Med Phys 36:339 (2009)

Exradin A16 inner diameter

Exradin A16 outer diameter

Measurements with small‐field detectors

Sauer & WilbertMed Phys 34, 1983‐88 (2007)IC = PTW 31010 (0.125 cm3)PiP = PTW 31006 (Pinpoint)

SES = size of equivalent square

Why do we worry about CPE or TCPE?

Valid for CPE or TCPE.Correction factor kQ well‐known for:

Q1 = Co‐60; Q2 = linac beam with field10 x 10 cm2; z = 10 cm; SSD or SAD 100 cm

For the measurement of absorbed dose:• In regions of CPE and TCPE: SPR conversion is accurate

– Dosimetry according to TG‐51 or TRS‐398

• In regions of non‐CPE: SPR conversion is not accurate and additional, sometimes large, corrections are needed, i.e., – Dosimetry in narrow stereotactic fields

• In regions of “recomposed‐CPE” (IMRT, Tomotherapy, etc): we don’t know (since composition of CPE may be disturbed by detector, or may not be perfect)– Dosimetry of intensity modulated fields

22Sanchez‐Doblado et al Phys. Med. Biol. 48 2081‐2099 (2003)

Small field dosimetry

0.3% effect0.3% effect

Stopping power ratio water to air

Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)Eklund and Ahnesjö, Phys Med Biol 53:4231 (2008)

Very small effects!

Narrow 1.5 mm fieldRatio of dose to water to dose to air averaged over cavity volume

Off‐axis distance (mm)

Collecting electrode diameter: 1.5 mmSeparation: 1 mm

0 2 4 6 8

Stopping power ratio w/air

Paskalev, Seuntjens, Podgorsak (2002) AAPM Proc. Series 13, Med. Phys. Publishing, Madison, Wi, 298 – 318.

Role of different perturbation factors in small fields

080915

Crop et al., Phys Med Biol 54:2951 (2009)

PP31006 and PP31016 chambers

Off‐axis behaviour of correction factors

080915

8 mm x 8 mm field, 10 cm depth (0.6 mm, 2 mm spot sizes)

Very small effects!

Off‐axis behaviour of correction factors

080915

8 mm x 8 mm field, 10 cm depth (0.6 mm, 2 mm spot sizes)

Very large effects!

Summary of measurement issues in small fields

• Beam dependent issues– Beam focal spot size plays a role

– Lateral disequilibrium

– How do we measure beam quality in practice?

• Detector effects– There is no ideal detector

– Volume averaging and perturbation effects

– Corrections depend on beam focal spot size

Physics of IMRT fields

Composite nonstandard fields

• Composed of multiple small fields

• Thus: same dosimetric issues as in small fields– Lateral charged particle disequilibrium

– Partial occlusion of source

– Detector response problems, volume averaging, perturbations

• How can dosimetric accuracy be affected?

Dose measurements in composite fields

0.973( ) 0.022NE

0.963( ) 0.024IC10

0.944( ) 0.035PP

Fraser et al (2009) JACMP 10 (4): 241‐51

For the measurement of absorbed dose:• In regions of CPE and TCPE: SPR conversion is accurate

– Dosimetry according to TG‐51 or TRS‐398

• In regions of non‐CPE: SPR conversion is not accurate and additional, sometimes large, corrections are needed, i.e., – Dosimetry in narrow stereotactic fields

• In regions of “recomposed‐CPE” (IMRT, Tomotherapy, etc): we don’t know (since composition of CPE may be disturbed by detector, or may not be perfect)– Dosimetry of intensity modulated fields

Field name CIMRT,6 MVmeasured CIMRT,6 MV

calculated DifferenceStatic #1 1.019 ± 4.0% 0.950 6.8%

Static #2 1.173 ± 6.1% 1.150 1.9%

Static #3 1.124 ± 3.2% 1.094 2.7%

Static #4 1.274 ± 6.0% 1.233 3.2%

Static #5 1.172 ± 2.5% 1.141 2.7%

Dynamic #1 1.139 ± 3.0% 1.143 -0.3%

Dynamic #2 1.169 ± 2.5% 1.161 0.7%

Dynamic #3 1.089 ± 3.9% 1.004 7.8%

Dynamic #4 1.007 ± 3.2% 1.031 -2.4%

Dynamic #5 0.920 ± 5.4% 0.885 3.8%

Dynamic #6 1.583 ± 5.9% 1.605 -1.4%

Dynamic #7 1.077 ± 5.7% 1.014 5.9%

Dynamic #8 1.079 ± 5.9% 1.005 6.9%

Bouchard &Seuntjens, Med Phys 31: 2453‐2464 (2004)

Exradin A12 Exradin A14

Gradient correction due to volume averaging is dominating

Issue with reference dosimetryconditions

Modality Most relevant static calibration

field size

S&S or dynamic capabilities?

IMRT, SRS/SRT

10 x 10 cm2 Yes

TomoTherapy 5 x 20 cm2 Yes

CyberKnife 6 cm diameter Yes

GammaKnife 16 mm / 18 mm diameter

New dosimetry of small and IMRT fields

Status Report

Several working groups and TG’s

• AAPM TG 155 – small field relative dosimetry

• AAPM TG 178 – GammaKnife dosimetry

• IAEA small field committee – liaised with AAPM WGDPCB

• ICRU report committee on prescribing and reporting of stereotactic radiation therapy

• IPEM – Report 103

• DIN – small field subcommittee

Upcoming guidelines and recommendations

• New formalism• Data for new formalism• Practical issues

– Small fields:• Beam quality• Suitable detectors• Correction factors• etc

– Composite fields IMRT:• Beam quality• Suitable detectors• Reference field criteria• Correction factors• etc

New formalism

• Two related routes for Dw in non‐conventional reference conditions, both requiring an extension of concept of reference field and modified reference conditions

• Small static field dosimetry– intermediate machine‐specific‐reference field (msr)

– Recommendations for correction factors to measured output factors

• Composite field – IMRT dosimetry– plan‐class specific reference field (pcsr)

– A pcsr field can be a 3‐D irradiated volume or a 4‐D delivery sequence.

– The pcsr should be as close as possible to a class of clinical plans of interest, and provide a uniform dose over a region exceeding thedimensions of a reference detector

Alfonso et al Med Phys 35: 5197 (2008)

Static small fields

,, , , , ,

msr refmsr msr

msr msr msr

f ff fwQ Q D wQ Q Q Q QD M N k k= msrclin

msrclin

fQw DD ,

,,, Ω=Machine specificreference field fmsr

Clinicalfclin

Tomotherapy5 cm x 20 cm

REFERENCE DOSIMETRY RELATIVE DOSIMETRY

GammaKnife∅ 1.6/1.8 cm

CyberKnife 6 cm

≡ Ionizationchamber

Broad beamreference field fref

00 ,,, QQQwD kNHypothetical

reference field fref

BrainLABmicro MLC10cmx10cm

refmsr

ffQQk ,,

Radiosurgical collimators∅ 1.8 cm

refmsr

ffQQk ,,

msrclin

msrclinmsr

clinmsrclin

msrclin

QQ kMM ,

,,, ⋅=Ω

How to specify beam quality?

Sauer (2009) Med. Phys. 36: 4168

Multiple beams from BJR Suppl 25

AAPM TG‐148 (Langen et al. 2010 Med Phys 37:4817‐53): “dd(10)x[HT‐ref]”

Beam quality specifier for Tomotherapy

Data for

• correction factors are small for the larger field msr

Data for (cont’d)

• correction factors are small for the larger field msr

Sterpin et al (2011 ‐ preliminary data)

Tomotherapy msr field

Field output factors

)1()2(

)2()2(

)1()1(

)2()1(

msrclin

msrclinmsr

clinmsrclin

msrclin

QQ kMM

,,, ⋅=

⎥⎥⎦

⎢⎢⎣

⎡⋅==Ω

clinmsrclin

msrclin fQ

k ⋅=,

080915 47/25

Field output factors ‐ CyberKnife: Pantelis et al. 2010 Med Phys 37:2369

0 5 10 15 20

diameter / mm

A16PinPointDiode 60008Diode 60012EDGEAlanineTLDEBT filmPolymer gel

0 5 10 15 20

diameter / mm

60) 2/(

PinPointDiode 60008Diode 60012EDGEAlanineTLDEBT filmPolymer gel

0 5 10 15 20

diameter / mm

) PinPoint

Diode 60008

Diode 60012

Alanine

)2()1(

, ,, ,

Diode 60008

Diode 60012

EBT film

Polymer gel

detector

(Mclin / M

ref )* kclin,m

ExrA16 PinPoint SHD USD EDGE alanine TLD EBT GEL

Getting output factor data for multiple detector types

Thursday April 7 SEAAPM 2011 48Sauer and Wilbert 2007 Med Phys 34:1983‐8

Output factors – CyberKnifecoupling of beam spot size and detector correction factors

Francescon et al 2008 Med Phys 35:504‐13

msrclin

ffQQ,,Ω

collimator mm 60

msrclin

msrffQQ

Different FWHM primary source

IMRT fields

New formalism

• Two related routes for Dw in non‐conventional reference conditions, both requiring an extension of concept of reference field and modified reference conditions

• Small static field dosimetry– intermediate machine‐specific‐reference field (msr)

– Recommendations for correction factors to measured output factors

• Composite field – IMRT dosimetry– plan‐class specific reference field (pcsr)

– A pcsr field can be a 3‐D irradiated volume or a 4‐D delivery sequence.

– The pcsr should be as close as possible to a class of clinical plans of interest, and provide a uniform dose over a region exceeding thedimensions of a reference detector

Alfonso et al Med Phys 35: 5197 (2008)

refpcsr

ffQQk,,

refpcsr

ffQQQQQwD

fQw kkNMD ,

,,,,, 00= pcsrclin

pcsrclin

fQw DD ,

,,, Ω=Broad beamreference field

frefPlan‐class

specific reference fieldfpcsr

00 ,,, QQQwD kN

Clinicalfclin

msrpcsr

ffQQk ,,

Hypotheticalreference field e.g. 9‐field prostate

(e.g. IMRT Linac)

refmsr

ffQQk ,,

(e.g. Tomotherapy5cm x 20cm)

REFERENCE DOSIMETRY RELATIVE DOSIMETRY

20º 60º

140º180º220º

300º 340º

≡ Ionizationchamber

pcsrclin

pcsrclinpcsr

clinpcsrclin

pcsrclin

,, ⋅=Ω

Plan class specific reference fields

Dynamic IMRT H&N – Chung et al. 2010 Med. Phys. 37:2404‐13

TomoTherapy – Bailat et al. 2009Med. Phys. 36:3891‐3896

VMAT – Rosser and Bedford 2009 Phys. Med. Biol. 54:7045‐7061

pcsr field

Bailat et al. 2009

Rosser and Bedford 2009

Chung et al. 2010

pcsr field correction factors

pcsr field

Bailat et al. 2009

Rosser and Bedford 2009

Chung et al. 2010

Suitable IMRT calibration fields?

Chung et al 2011

What are possible criteria for suitable IMRT calibration fields?

Chung et al 2011

Conclusions

• Small and IMRT field dosimetry can be complex– There are hefty perturbation effects that can have significant impact on reference dosimetry procedures and output factors

– Comparison between different detectors provides valuable information

• In small field dosimetry– Machine‐specific reference fields defined

– Data on correction factors is being collected

Conclusions (cont’d)

• In IMRT / composite field dosimetry– Measurement uncertainties, reference detectors– Practical criteria for the reference field definition are being developed

• New documents will be coming out providing guidelines on how to better deal with these issues

Continued education is needed and can prevent complacency

S4 T1 Suentjens Small Field Dosimetry

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