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Small Field Dosimetry: An Overview of Reference and Relative Dosimetry in

Non-Equilibrium Condition

Indra J. Das, PhD, FAAPM, FACR, FASTRODepartment of Radiation Oncology

Indiana University School of Medicine Indianapolis, IN

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Lecture OrganizationOverview of Reference and Relative

DosimetryIndra J Das

SRS: Detector SelectionSonja Dieterich

An Independent Audit and Quality Assurance David Followill

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Outline of Presentation Definition of Small Field

o Electron rangeo Electronic Equilibriumo Transient Equilibriumo Lateral Equilibrium

Source Occlusiono Variation is shape, sizeo Machine specific, age, vintageo Source size and doseo Size of detector

Detector Issueso Volume averagingo Perturbation

IAEA/AAPM Frameworko Nomenclatureo Reference Dosimetry

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TG-155: Small Fields and Non-Equilibrium ConditionPhoton Beam DosimetryIndra J. Das (Chair)

Indiana University School of Medicine, Indianapolis, IN 46202Paolo Francescon (Co-chair)

Ospedale Di Vicenza, Viale Rodolfi, Vicenza 36100, ItalyAnders Ahnesjö

Uppsala University & Nucletron Scandinavia AB, 751 47 Uppsala, SwedenMaria M. Aspradakis

Department of Radiation Oncology, Kantonsspital Lucerne, Lucerne, SwitzerlandChee-Wai Cheng

University Hospitals Case Medical Center, Cleveland, OH,George X. Ding

Vanderbilt University Medical Center, Nashville, TN 37232 John Fenwick

Oxford University, Oxford, EnglandGeoffrey S. Ibbott

Radiological Physics Center, MD Anderson Cancer Center, Houston, TX 77030Mark Oldham

Duke University Medical Center, Durham, NC 27710M. Saiful Huq

University of Pittsburgh Cancer Institute, Pittsburgh, PA 15232Chester S. Reft

University of Chicago, Chicago, IL 60637Otto A Sauer

University of Wuerzburg, Wuerzburg, Germany

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Treatment FieldsMagna-Fields

Traditional Fields

Advance Therapy FieldsSRS/SRTGamma KnifeCyber-KnifeTomotherapyIMRT

40x40 cm2 4x4 cm2

4x4 cm2 0.3x0.3 cm2

200x200 cm2

Small Field

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Meaning of Small Field? Lack of charged particle

o Dependent on the range of secondary electrons

o Photon energyCollimator setting that obstructs the

source size Detector is comparable to the field

size

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CPE & Electron RangeCPE, Charged Particle EquilibriumElectron range= dmax in forward

directionElectron range in lateral direction

o Nearly energy independento Nearly equal to penumbra (8-10 mm)o rLEE (g/cm2)=5.973[TPR(20,10)]-2.688

Field size needed for CPEo Lateral rangeo 16-20 mm

dmax

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1

2

3

4

3

2

Electronic Equilibrium, CPE, TCPE1

23

33

33

33

32

10

0

Dose = KERMACPE

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Cavity

Bragg-Gray Cavity: Range and Cavity Size

Dose = .S/ (number/cm2)(MeVcm2/g) = MeV/g= J/kg (Gy)

Range

Dose is calculated only by crossers in the cavity &Cavity should be non-perturbing

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1

23

4 132

Lateral Electronic Equilibrium, LEE Large Field Small Field

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Large Range ElectronsSmall Range Electrons Lateral Electron Range

LEE No LEE

IJDas [12] Das et al, Med. Phys. 35, 206-215,

Definition of Small Fields

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�1.5�1.0�0.5 0.0 0.5 1.0 1.50.0

0.2

0.4

0.6

0.8

1.0

x �cm�2.0x2.0 cm2

Source sizescm

---- 0.01---- 0.35---- 0.50

�6.5�6.0�5.5�5.0�4.5�4.0�3.50.0

0.2

0.4

0.6

0.8

1.0

x �cm�

Ener

gyFl

uenc

eSc�

norm

aliz

ed

Energy Fluence Penumbra/Output

�1.5�1.0�0.5 0.0 0.5 1.0 1.50.0

0.2

0.4

0.6

0.8

1.0

x �cm�1.0x1.0 cm2

�1.5�1.0�0.5 0.0 0.5 1.0 1.50.0

0.2

0.4

0.6

0.8

1.0

x �cm�0.5x0.5 cm2

Large field

Nyholm et al Radiother Oncol 78, 347, 2006 and Phys Med Biol 51,6245, 2006

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Understanding Profiles

Large Field Small Field

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Distance

Dos

e

Large Detector

Small Detector

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Source Size

Jaffray et al, Med Phys 20, 1417-1427 (1993)

90%, 70%, 50%, 30%, 10% iso-intensity line

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Varian TrueBeam

Sawkey et al, Med Phys, 40, 332, 2013

Dimensionsx, y (mm)

0.9, 0.7

0.8, 0.7

0.9, 0.9

0.9, 0.8

T2 T3T1

6 MV

6 MVFFF

10 MV

10 MVFFF

IJDas [18] Scott et al, Med Phys, 36, 3132, 2009

Dose and Penumbra with Spot Size

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DosimetryAbsolute

oDoseRelative

oDepth Dose [D(r,d)/D(r,dm)]

oTMRoProfilesoOutput, Scp (total scatter factor) [D(r)/D(ref)]

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403530252015100.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

WEHU-ArizonaTemple UU PennErlanger Serago et al.Fan et al.Zhu et al.

Total scatter factor from different institutions

Cone Diameter (mm)

Cone

Fac

tor (

St)

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

Small Field Dosimetry Problem

Institutional variability in 6 MV Radionics SRS dosimetry

12% diff

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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)

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

Dosimetric Variation with Detectors

14%

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Dose to Water For Small Fields: Volume Averaging

Capote et al, Med Phys, 31, 2416-2422, 2004

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CyberKnife data

-12

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

PinPoint Diamond(old) EBT2 MicroTLD IBA-SFD PTW Diode SN-Edge

Unc

orre

cted

out

put d

iffer

ence

(%)

with

MC

Moignier et al, Med Phys 41(7), 071702, 2014

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Ratio of Readings?ax

0

( ) ( ) ( ) ( )( )

mh

abd h hD E h d hd h

( ) ( )rr m

ref ref aref ref

D r Q r W SD Q e

m

ma

Q W SDm e

1 2( )

refQF Fr CQ C

Q(E,r) = Qr Pion Prepl Pwall Pcec Ppcf

( ) ( )rr m

ref ref aref ref

D r Q r W SD Q e

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Radiation MeasurementsCharged particle equilibrium

o Range of secondary electronso Medium (tissue, lung, bone)

Photon energy and spectrum o Change in spectrum

Field sizeOff axis points like beamlets in IMRT

o Changes in (en/) from reference field, fref

o Change in (L/) from reference field, frefDetector

o Volumeo Densityo Signal to noise ratio

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Alfonso, et al. Med Phys 35, 5179-5186 (2008)

IAEA/AAPM proposed pathway

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Relative Dosimetry

rel

relf

Qf

Q

fQ

fQff

QQ adingOutput

MDMD

k msrclin

msrclin )(Re)(

//

clin

msr

clin

msr

clin

clin

msr

clin

,w,w

,w,w,,

refmsr

msr

msr

msr

msr

msr

,,,,,ffQQQoQQoDW

fQ

fQw kkNMD

msrclin

msrclinmsr

msr

clin

clin

msr

msr

msr

msr

clin

clin

clin

clin

msr

msr

clin

clinmsrclin

msrclin

,,

,w

,w

// ff

QQfQ

fQ

fQ

fQ

fQ

fQ

fQ

fQff

QQ kMM

MDMD

MM

msrfmsrairw

fclinfclinairwffQQ PS

PSk msrclin

msrclin

)()(

,

,,,

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msr

clinmsrclin

msrclin

f

wallcellstemgradfl

w

air

f

wallcellstemgradfl

w

airffQQ

PPPPPL

PPPPPL

k

,,

Meaning of k Defined in IAEA/AAPM

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kQ is not Constant in Small Field

Kawachi et al, Med Phys, 35, 4591-4598, 2008

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Why So Much of Fuss? Reference (ref) conditions cannot be

achieved for most SRS devices (cyberknife, gammaknife, tomotherapy etc)

Machine Specific reference (msr) needs to be linked to ref

Ratio of reading (PDD, TMR, Output etc) is not the same as ratio of dose

2

1

2

1

MM

DD

msrclin

msrclin

ffQQk

MM

DD ,

,2

1

2

1

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6 MV; Central Axis

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 1 2 3 4 5 6 7 8 9 10 11

Field Size (cm)

Rel

ativ

e do

se a

t dm

ax

Scanditronix-SFDScanditronix-PFDExradin-A16PTW-PinpointPTW-0.125ccPTW-0.3ccPTW-0.6ccPTW-MarkusWellhofer-IC4

15 MV; Central Axis

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 1 2 3 4 5 6 7 8 9 10 11

Field Size (cm)

Rel

ativ

e do

se a

t dm

axScanditronix-SFDScanditronix-PFDExradin-A16PTW-PinpointPTW-0.125ccPTW-0.3ccPTW-0.6ccPTW-MarkusWellhofer-IC4

Das et al, TG-106, Med Phys, 35, 4186, 2008

Field Size Limit for Accurate Dose Measurements with Standard Detectors

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0.00.10.20.30.40.50.60.70.80.91.01.1

0 1 2 3 4 5 60.00.10.20.30.40.50.60.70.80.91.01.1

0 1 2 3 4 5 60.00.10.20.30.40.50.60.70.80.91.01.1

0 1 2 3 4 5 60.00.10.20.30.40.50.60.70.80.91.01.1

0 1 2 3 4 5 6

Validity of Proposed Method

Field Size (cm)

Rel

ativ

e D

ose

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SummarySmall field is a condition where dosimetry is

usually difficult due to electronic disequilibriumUse of IAEA/AAPM protocol for reference and

relative dosimetry based on Alfonso et al (2008) is recommended

Focal spot of a machine is a critical parameter in small field dosimetry

Absolute dosimetry protocol for small field is forthcoming

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Small field is defined as a condition of ?

9%85%2%0%4% 1. Lateral electronic disequilibrium

2. Range of electron beams in lateral direction3. Collimator size that obstructs the primary radiation source4. All of above5. None of above, it is field ≤3x3 cm2

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Directions

Answer: 4 - All of above

Ref: “Das et al, Small fields: Non-equilibrium radiation dosimetry," Med Phys 35, 206-215 (2008)

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IAEA/AAPM have provided a frame work for small field dosimetry in terms of the correction factor, which is dependent on? msrclin

msrclin

,,ffQQk

88%2%1%8%2% 1. Beam energy

2. Type of detector (microChambers, Diode, Mosfet etc)3. Type of machine (IMRT, CyberKnife, GammaKnife etc)4. Focal spot size5. All of above

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Answer: 5 – all of above

Ref: Alfonso et al, "A new formalism for referenece dosimetry of small and nonstandard fields," Med Phys 35, 5179-5186 (2008)

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Thanks