Small-Field Dosimetry: Its Importance In Clinicamos3.aapm.org/abstracts/pdf/72-20252-243393-90753.pdfIndra J. Das/ Small Fields (Page-1) IJDas (1) AAPM-2013 Small-Field Dosimetry:

Indra J. Das/ Small Fields (Page-1)

IJDas (1)AAPM-2013

Small-Field Dosimetry: Its Importance In Clinic

Indra J. Das, PhD, FAAPM, FACR, FASTRODepartment of Radiation OncologyIndiana University School of Medicine &IU Health Proton Therapy CenterIndianapolis, IN, USA IJDas (2)

AAPM-2013

Wrong detector used for BrainLab cone calibration

Misadministration Media Coverage

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Misadministration Media Coverage

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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 AhnesjöUppsala University & Nucletron Scandinavia AB, 751 47 Uppsala, SwedenMaria M. AspradakisDepartment of Radiation Oncology, Kantonsspital Lucerne, Lucerne SwitzerlandChee-Wai ChengMidwest Proton Radiotherapy Institute, Bloomington, IN, George X. DingVanderbilt University Medical Center, Nashville, TN 37232 Geoffrey S. IbbottRadiological Physics Center, MD Anderson Cancer Center, Houston, TX 77030Mark OldhamDuke University Medical Center, Durham, NC 27710M. Saiful HuqUniversity of Pittsburgh Cancer Institute, Pittsburgh, PA 15232Chester S. ReftUniversity of Chicago, Chicago, IL 60637Otto A SauerUniversity of Wuerzburg, Wuerzburg, Germany

IJDas (5)AAPM-2013

Treatment Fields

Magna-FieldsTraditional Fields

Advance Therapy FieldsSRS/SRTGamma KnifeCyber-KnifeTomotherapyIMRT

40x40 cm2 4x4 cm2

4x4 cm2 0.3x0.3 cm2

200x200 cm2

Small Field

IJDas (6)AAPM-2013

What is a Small Field?

Lack of charged particle Dependent on the range of secondary

electrons

Photon energy

Collimator setting that obstructs the source size

Detector is comparable to the field size

Indra J. Das/ Small Fields (Page-2)

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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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Calculation of Fluence Map Elements

Direct beam source size Open beam off axis distribution

Fluence modulationStep&shot

Dynamic

Wedges

Head scatter sources

flattening filter

collimators

wedges

Monitor back scatter

Collimator leakage, includingMLC interleaf leakage

shape of MLC leaf ends

MLC positioningBeam spectra

Spectral changes

Electron contamination

Processes to include

Components Critical for Small Fields

Courtesy Anders Ahnesjö

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Das et al, Med. Phys. 35, 206-215, 2008

Definition of Small Fields

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For each element, find the contributions from the relevant sources

Calculation of Fluence Map Elements

Raytrace to calculate attenuationor use approximate penumbra shape

POI-eye-view of the source!

Courtesy Anders Ahnesjö

IJDas (11)AAPM-2013

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 sizes

cm

---- 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�

Ene

rgy

Flue

nce

Sc

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 IJDas (12)AAPM-2013

Potential inconsistency dose calculations ↔ delivery

Profile and Leaf Tip Positions & Location

isocenter plane

Source plane

xxHVL xw

xcm

cm

xHVL - xw

Delivery: uses of lookup table or parameterized correctionTPS: - “ - or direct calculation of 50% transmission

Courtesy Anders Ahnesjö

Indra J. Das/ Small Fields (Page-3)

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Scott et al, Med Phys, 36, 3132, 2009

Dose and Penumbra with Spot Size

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Dosimetry

Absolute Dose

Relative Depth Dose

o [D(r,d)/D(r,dm)]

TMR

Profiles

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

IJDas (15)AAPM-2013

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)

Con

e F

acto

r (S

t)

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

Small Field Dosimetry Problem

Institutional variability in 6 MV Radionics SRS dosimetry

12% diff

IJDas (16)AAPM-2013

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)

Co

ne F

acto

r (S

t)

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

Dosimetric Variation with Detectors

14%

IJDas (17)AAPM-2013

0.5 1.0 1.5 2.0 2.5 3.00.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Out

put F

acto

r

Side of Square Field (cm)

Dose to Water For Small Fields: Volume Averaging

PTW 31014 (Pinpoint)PTW 31010 (Semiflex)PTW 30006 (Farmer)

Monte CarloDiamondDiodeLAC+filmFilm

From Roberto Capote, IAEAIJDas (18)

AAPM-2013

30282624222018161412100

10

20

30

40

50

60

70

80

90

100

110

Film

Ion (0.125 cc)Ion (parallel-plate)Ion (Pinpoint)Diamomd

Dose profiles for a 40 mm cone with different detectors

Distance (mm)

No

rmal

ized

Do

se (

%)

201816141210864200

10

20

30

40

50

60

70

80

90

100

110

Diamond

Ion (Parallel Plate)

Film

Ion (Pinpoint )

Dose profile of a 12.5 mm cone

Distance (mm)

Nor

mal

ized

Dos

e (%

)

Hedrian, Hoban & Beddoe, PMB, 41, 93-110, 1996Das et al, J Radiosurgery, 3, 177-186, 2000

Profiles with different detectors

Indra J. Das/ Small Fields (Page-4)

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Ratio of Readings?

enD

enD

enD

ax

0

( ) ( )( ) ( )

( )mh

ab

d h hD E h d h

d h

( ) ( )rr m

ref ref aref ref

D r Q r W S

D Q e

m

m

a

Q W SD

m e

1 2

( )

re fQF F

rC

QC

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

( ) ( )rr m

ref ref aref ref

D r Q r W S

D Q e

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AAPM-2013

Radiation Measurements

Charged particle equilibrium or electronic equilibrium Range of secondary electrons Medium (tissue, lung, bone)

Photon energy and spectrum Change in spectrum

Field size Off axis points like beamlets in IMRT

Changes en/ and S/ Detector size

Volume Signal to noise ratio

IJDas (21)AAPM-2013

CPE & Electron Range

Electron range= dmax in forward direction

Electron range in lateral direction

Nearly energy independent

Nearly equal to penumbra (8-10 mm)

Field size needed for CPE Lateral range

16-20 mm

CPE, Charged Particle Equilibrium

dmax

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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 ading

Output

MD

MDk 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

/

/ ffQQf

Q

fQ

fQ

fQ

fQ

fQ

fQ

fQff

QQ kM

M

MD

MD

M

M

msrfmsrairw

fclinfclinairwffQQ PS

PSk msrclin

msrclin

)(

)(

,

,,,

IJDas (24)AAPM-2013

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

M

M

D

D

msrclin

msrclin

ffQQk

M

M

D

D ,,

2

1

2

1

Indra J. Das/ Small Fields (Page-5)

IJDas (25)AAPM-2013

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 d

ose

at d

max

Scanditronix-SFDScanditronix-PFDExradin-A16PTW-PinpointPTW-0.125ccPTW-0.3cc

PTW-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 d

ose

at d

max

Scanditronix-SFD

Scanditronix-PFDExradin-A16

PTW-PinpointPTW-0.125cc

PTW-0.3ccPTW-0.6cc

PTW-MarkusWellhofer-IC4

Field Size Limit for Accurate Dose Measurements with Available Detectors

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

IJDas (26)AAPM-2013

Detector Manufacturer Type volumeSFD Scanditronix Photon diode 1.7x10-5cm3

PFD Scanditronix Photon diode 1.9x10-4cm3

Exradin A-16 Standard Imaging Ion chamber 0.007cm3

Wellhofer-IC4 Scanditronix Ion chamber 0.40 cm3

Pinpoint PTW Ion chamber 0.015cm3

0.125cc PTW Ion chamber 0.125cm3

0.3cc PTW Ion chamber 0.3 cm3

0.6cc PTW Ion chamber 0.6 cm3

Diamond PTW Diamond 0.003cm3

Markus PTW Parallel plate 0.055cm3

Edge Detector Sun Nuclear Diode 10-5cm3

Other

Detectors

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Radiation Detectors

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PTW Choice for small Fields

www.PTW.de

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Sensitivity vs Volume of Detectors

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00

Volume (cm3)

Rel

ativ

e se

nsi

tivi

ty

PFD

SFD

A-16 PinPointMarkus

IC4

0.125cc

0.3cc

0.6 cc

IJDas (30)AAPM-2013

Understand Detector, Connector & Cable

Srivastava et al, SU-GG-T-270, 2010

Indra J. Das/ Small Fields (Page-6)

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Ion Chamber Issues in Small Fields Volume averaging Poor signal: signal to noise ratio Pion: Ion recombination

300 volts may not be needed May be in proportional counter mode or gas

multiplication Two voltage method may not be applicable

Voltage

Cur

rent

Large volume Ion chamber

Small volume Ion chamber

100 200 300 400 500 6000

IJDas (32)AAPM-2013

1.E-5

1.E-4

1.E-3

1.E-2

1.E-1

1.E+0

1.E+1

0.0 1.0 2.0 3.0 4.0 5.0 6.0

E (MeV)

Ph

oto

n F

lue

nc

e

0.5 cm, primary

5 cm, primary

0.5 cm, scatter

5 cm, scatter

WATER, at dmax (a)

Verhaegen, Das, Palmans, PMB, 43, 2755-2768, 1998

1.E-4

1.E-3

1.E-2

1.E-1

1.E+0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

E (MeV)

Ph

oto

n F

lue

nc

e

0.5 cm

5 cmAIR, at exit (c)

6 MV

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

0 1 2 3 4 5 6

Diameter of Cone (cm)

Ave

rag

e E

ner

gy

(M

eV

)

Spectra & Effective Energy from SRS Cones (0.5-5 cm)

IJDas (33)AAPM-2013

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Sanchez-Deblado et al, Phy. Med. Biol., 48, 2081, 2003

Radiological Parameters

IJDas (35)AAPM-2013 F. Araki, Med. Phys. 33, 2955-2963 (2006).

Cyber Knife Dosimetry

0.5%

IJDas (36)AAPM-2013

Francescon et al, Med. Phys. 25(4), 503, 1998

MC

MC

MC

MC

MC

MC

Indra J. Das/ Small Fields (Page-7)

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Correction Factors

Francescon, et al Med Phys 35, 504, 2008

Correction Factor depends on:Field sizeSource size (FWHM)Detector type

IJDas (38)AAPM-2013Kawachi et al, Med Phys, 35, 4591-4598, 2008

Errors in Measured Reading

IJDas (39)AAPM-2013

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

0 5 10 15 20 25 30 35

Field size (mm)

Cor

rect

ion

fact

or

Siemens; PTW diode 60012

Elekta; PTW diode 60012

Siemens; Exradin A16

Elekta; Exradin A16

Siemens; Sun Nuclear Dedge

"Elekta; Sun Nuclear Dedge"

Siemens; PTW Pinpoint 31014

Elekta; PTW Pinpoint 31014

Siemens; PTW microLion

Elekta; PTW microLion

Francescon et al Med Phys,38, 6513, 2011

Published data on

IJDas (40)AAPM-2013 Scott et al, Phys Med Biol 57 4461–4476, 2012

Detector Density Effect

IJDas (41)AAPM-2013 Chung et al , Med Phys, 37, 2404-2413, 2010

Correction Factor vs Ion Chambers

IJDas (42)AAPM-2013 Med. Phys. 38 (12), 6992, 2011

of Linear Accelerators (Varian)

Indra J. Das/ Small Fields (Page-8)

IJDas (43)AAPM-2013

Radiotherapy and Oncology 100 (2011) 429–435

Med. Phys. 38 (12), 6513-6527, 2011

of Linear Accelerators

IJDas (44)AAPM-2013

Cyber Knife

Pantelis et al, Med Phy. 37, 2369-2379, 2010

IJDas (45)AAPM-2013 Sterpin et al, Med Phys, 39, 4066, 2012

Tomotherapy kmsr Reference Dosimetry

Reference: 5x10 cm2, 85 cm SSD, 10 cm

IJDas (46)AAPM-2013

kQ is not Constant in Small Field

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

IJDas (47)AAPM-2013 Sham et al, Med Phys, 35, 3317-3330, 2008

Depth Dose & Source Size

IJDas (48)AAPM-2013

Profile & Source Size

Sham et al, Med Phys, 35, 3317-3330, 2008

Indra J. Das/ Small Fields (Page-9)

IJDas (49)AAPM-2013

Effect of Inhomogeneity

Range of secondary electrons Simple scaling based on density

M. K. Woo, and J. R. Cunningham, "The valididty of density scaling method in primary electron transport for photon and electron beams," Med. Phys. 17, 187-194 (1990).

Perturbations of the detector T. Mauceri, and K. R. Kase, "Effects of ionization chamber

construction on dose measurements in heterogeneity," Medical Physics 14, 653-656 (1987).

R. K. Rice, J. L. Hansen, L. M. Chin, B. J. Mijnheer, and B. E. Bjarngard, "The influence of ionization chamber and phantom design on the measurement of lung dose in photon beam," Medical Physics 15, 884-890 (1988).

IJDas (50)AAPM-2013

Electron range & inhomogeneity

IJDas (51)AAPM-2013

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 12 13 14

Rel

ativ

e D

ose

Depth (cm)

0.5 cm, 0.25 cm3 lungHomogeneous

Mon te Carlo

EPL

TMR Ratio

Power Law TMR

Convolut ion

CCC Water

Jones & Das, Med. Phys. 30, 296, 2003 Jones & Das, Med. Phys. 32, 766, 2005

Inhomogeneity Corrections

IJDas (52)AAPM-2013

TG-155 Recommendation Dosimetric measurements should be carried out with more than one detector system. Small volume detector should be used that has minimum energy, dose and dose rate

dependence as discussed in TG-120 and Report No103 should be used. Stereotactic diodes or electron diodes are recommended for field sizes < 1x1cm2 Micro chambers are best suited for dosimetric measurements for field sizes > 1x1

cm2 however, signal to noise as well as polarity effect should be evaluated. The quality of electrometer and triaxial cable as well as any connector and cables

need to be of high quality. Stereotactic diode with micron size sensitive volume should be the detector of choice

for measurements in beams in radiosurgery. The energy spectrum does vary in small fields such as SRS, and IMRT but these

changes result in insignificant variations in stopping power ratios when compared to those of the reference field used in dosimetry codes of practice.

The treatment planning system performance should be carefully validated when used for the treatment planning incorporating small fields. Although pencil beam and convolution/superposition dose engines are expected to perform well in small field treatment geometries and in almost homogeneous media, dose engines based on the Monte Carlo method are the most accurate method for modelling dose from small fields in heterogeneous media. The calculation grid size should be significantly smaller (~1/10) compared to the field size.

Small field dosimetry should have an independent audit by a different physicist either internal or external like Radiological Physics Center verification.

IJDas (53)AAPM-2013

Summary

Small volume detector should be used that has minimum energy, dose and dose rate dependence.

Micro-ion chambers are best suited for small field dosimetry; however, signal to noise should be evaluated.

Stereotactic diode are ideally suited for radiosurgery beams.

If field size is small compared to detector measurements should be performed at a greater source to surface distance with proper correction.

IJDas (54)AAPM-2013

-Summary

Energy spectrum does vary in small fields such as SRS, and IMRT, however, its impact is not significant.

Stopping power ratio in small fields for most ion chambers is relatively same as the reference field.

Spot check and verification of smaller fields should be carried out with at least another independent method (TLD, film, MC, etc).

Stay tuned to newer data and IAEA and AAPM TG guidelines.

Indra J. Das/ Small Fields (Page-10)

IJDas (55)AAPM-2013

Thanks