IJDas (1)

Indra J. Das, PhD, FAAPM, FACR, FASTRODepartment of Radiation OncologyNew York University Langone Medicine CenterNew York, NY

Small-Field Dosimetry

IJDas (2)

Treatment Fields

Advance Therapy FieldsSRS/SRTGamma KnifeCyber-KnifeTomotherapyIMRT

4x4 cm2 0.3x0.3 cm2

Magna-FieldsTraditional Fields

40x40 cm2 4x4 cm2200x200 cm2

Small Field

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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

IJDas (4)

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)

Cone

Fac

tor (

St)

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

Dosimetric Variation with Detectors

14%

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Wrong detector used for BrainLab cone calibration

Misadministration Media Coverage

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Misadministration Media Coverage

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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

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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

IJDas (9) Das et al, Med. Phys. 35, 206-215, 2008

Definition of Small Fields

IJDas (10)

�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

alized

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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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 charged particle crossers in the cavity &Cavity should be non-perturbing

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12

34 1

32

Lateral Electronic Equilibrium, LEE

Large Field Small Field

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Density Effect

Water Vapor

Liquid Water

Dense Water

Bouchard et al, Med Phys, 42, 6063-6047, 2015

Bea

m o

n ca

vity

Bea

m o

ff ca

vity

IJDas (14)

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

Source Size, Modern Machines

IJDas (16) Alfonso, et al. Med Phys 35, 5179-5186 (2008)

IAEA/AAPM Proposed Pathway

IJDas (17)

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 1

2

2

1,, M

MDDk msrclin

msrclin

ffQQ

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

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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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Small Field Dosimetry Publications

IAEA-TRS 483, 2017

+ TG-155 (AAPM)Relative dosimetry

IJDas (20)

Meaning of k in Micro-Chambers

Kumar et al, PMB, 60, 8187, 2015

Bouchard et al, Med Phys, 36, 4654, 2009

IJDas (21)

Stopping Power Ratio

IJDas (22) Sanchez-Deblado et al, Phy. Med. Biol., 48, 2081, 2003

Radiological Parameters

IJDas (23) F. Araki, Med. Phys. 33, 2955-2963 (2006).

Cyber Knife Dosimetry

0.5%

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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

nsiti

vity

PFD

SFD

A-16 PinPoint Markus IC40.125cc

0.3cc

0.6 cc

Detector Response

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Correction Factors

Francescon, et al Med Phys 35, 504, 2008

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

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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

Corre

ction

facto

r

Field size (mm)

Siemens; PTW diode 60012Elekta; PTW diode 60012Siemens; Exradin A16Elekta; Exradin A16Siemens; Sun Nuclear Dedge"Elekta; Sun Nuclear Dedge"Siemens; PTW Pinpoint 31014Elekta; PTW Pinpoint 31014Siemens; PTW microLionElekta; PTW microLion

Francescon et al Med Phys,38, 6513, 2011

Published data on

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Monte Carlo Derived

Kumar et al, PMB, 60, 8187, 2015

IJDas (28) Med. Phys. 38 (12), 6992, 2011

of Linear Accelerators (Varian)

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Radiotherapy and Oncology 100 (2011) 429–435

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

of Linear Accelerators

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Cyber Knife

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

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Med Phys, 41(4), 041711, 2014

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Correction in Profile (OAR)

Francescon et al, Med Phys, 41(10), 101708, 2014

5 mm cone 25 mm cone

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Profile & Source Size

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

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Effect of Focal Spot

0.5

0.6

0.7

0.8

0.9

1.0

1.6 1.8 2.0 2.2 2.4 2.6 2.8

Out

put (

Mon

te C

arlo

)

Full Width at Half Maximum, FWHM (mm)

20 mm Cone10 mm cone7.5 mm cone5 mm cone

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

IJDas (36)

Effect of Beam Energy

0.5

0.6

0.7

0.8

0.9

1.0

6.4 6.6 6.8 7.0 7.2 7.4 7.6

Out

put (

Mon

te C

arlo

)

Effective Photon Energy (MeV)

20 mm Cone10 mm cone7.5 mm cone5 mm cone

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

IJDas (37) Papaconstadopoulos, et al, Phys. Med. Biol. 59 (2014) 5937–5952

Good News with Modern Microdetectors

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Machine, Cone & Depth?

Radiotherapy Oncology, 112, 442-446, 2014

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PTW-Pinpoint-

per

PTW-Pinpoint-

par

IBACC01-per

IBACC01-par PTW-LIC

PTW-Micro

DiamondSI PSD IBA-EFD IBA-SFD

PTW-Unshielded Diode

IBA-PFDPTW-

ShieldedDiode

Series1 14.7 10.2 8.1 5.0 1.1 0.2 0.0 -0.9 -2.0 -5.0 -5.3 -9.0

-14-12-10-8-6-4-202468

10121416

% D

iffer

ence

from

1.0

in k

Fac

tor

k Factor for Varian IX Machine, 5x5 mm2 Field

Benmakhlouf et al, Med Phys 41(4), 041711, 2014

IJDas (41)

k Factor of CyberKnife 5 mm Cone

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

PTWPinPoint

Diamond(old) SI-PSD EBT2 MicroTLD IBA-SFD

PTWunshielded

Diode

PTWshieldedDiode

SN-Edge

Series1 12.0 7.5 0.1 -1.4 -1.4 -3.0 -5.0 -6.0 -6.0

-14-12-10-8-6-4-202468

10121416

% D

iffer

ence

from

Mon

te C

arlo

in

k F

acto

r

IJDas (42)

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Cor

rect

ion

Fact

or

Field Size (cm x cm)

- MC-Francescon et al (2011)

- SFD-Azangwe et al (2014)

- MLIC-Azangwe et al (2014)

10.3%

1.5%0.2%

for Detectors msrclin

msrclin

ffQQk ,

,

IJDas (43)

for select detectors

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Cor

rect

ion

Fact

or

Field Size (cm x cm)

- microLion - Diode D1V - Scintillator, W1 (parallel) - Scintillator, W1 (perpendicular) -microDiamond (T-60019)

6.9%

0.8%1.8%

msrclin

msrclin

ffQQk ,

,

IJDas (44)

for Detectors and Devicesmsrclin

msrclin

ffQQk ,

,

Francescon, Beddar & Das, ESTRO, 2014

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Tomotherapy

MLC, Linac

Cyber Knife Field size (cm)

TRS-483, 2016

of Different Devices and Selected Detectorsmsrclin

msrclin

ffQQk ,

,

IJDas (46)

MicroDiamond-Multi-institution

Masi et al, Physica Medica,32, 1637-1643, 2016

CyberKnife

IJDas (47)

Plastic Scintillator-Multi-institution

Masi et al, Physica Medica,32, 1637-1643, 2016

CyberKnife

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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 do

se a

t dm

ax

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Summary Understand the limit of small field, in terms of focal

spot & spectral issues Detectors that are water equivalent like, MicroLion,

MicroDiamond, Plastic Scintillators and EBT are best suited

Use proper correction factors to correct detector response to correct for the dose

Use data from for IAEA TRS-483 and TG-155 guidelines when published

IJDas (50)

Thanks