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

IJDas (1) Wildflower, Panama

Small Field Dosimetry: Overview of AAPM TG-155 and the IAEA-AAPM Code of Practice (Therapy)

Indra J. Das, PhD, FAAPM, FACR, FASTRODepartment of Radiation OncologyIndiana University School of MedicineIndianapolis, Indiana, USA IJDas (2)

403530252015100.80

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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

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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%

IJDas (4)

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

IJDas (5)Das et al, Med. Phys. 35, 206-215, 2008

Definition of Small Fields

IJDas (6)

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

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

IAEA/AAPM Proposed Pathway

IJDas (10)

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

msrclinmsrclin

ffQQkM

MDD ,

,2

1

2

1

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Relative Dosimetry

rel

relf

Qf

Q

fQ

fQff

QQ adingOutput

MDMD

k msrclinmsrclin )(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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Meaning of k in Micro-Chambers

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

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Stopping Power Ratio

IJDas (14)Sanchez-Deblado et al, Phy. Med. Biol., 48, 2081, 2003

Radiological Parameters

IJDas (15) F. Araki, Med. Phys. 33, 2955-2963 (2006).

Cyber Knife Dosimetry

0.5%

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Correction Factors

Francescon, et al Med Phys 35, 504, 2008

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

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0.900.920.940.960.981.001.021.041.061.081.101.121.141.16

0 5 10 15 20 25 30 35

Corre

ctio

n fac

tor

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

IJDas (18) Med. Phys. 38 (12), 6992, 2011

of Linear Accelerators (Varian)

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

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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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CyberKnife data

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

1012

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

5 mm cone

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Med Phys, 41(4), 041711, 2014

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Newer Data on

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

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

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New Published data From IAEA

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Correction Factors

IJDas (27) IJDas (28)

Correction in Profile (OAR)

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

5 mm cone 25 mm cone

IJDas (29) Sham et al, Med Phys, 35, 3317-3330, 2008

Depth Dose & Source Size

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

Good News with Modern Microdetectors

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

IJDas (31) IJDas (32)

Machine, Cone & Depth?

Radiotherapy Oncology, 112, 442-446, 2014

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Validity of Proposed Method

Field Size (cm)

Rela

tive d

ose

at d

max

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Summary Understand the limit of small field Focal spot issues Detectors that are water equivalent like,

MicroLion, MicroDiamond and Plastic Scintillators are best suited

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

Watch for IAEA and TG-155 guidelines when they are published

IJDas (35)Thanks