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, FASTRO Department of Radiation Oncology Indiana University School of Medicine

Indianapolis, Indiana, USA

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40353025201510

0.80

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 Penn

Erlanger 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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40353025201510

0.80

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

Fa

cto

r (S

t)

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

Dosimetric Variation with Detectors

14%

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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

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

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 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

Ener

gy

Flu

ence

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

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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

Dimensions

x, y (mm)

0.9, 0.7

0.8, 0.7

0.9, 0.9

0.9, 0.8

T2 T3 T1

6 MV

6 MV

FFF

10 MV

10 MV

FFF

Source Size, Modern Machines

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

IAEA/AAPM Proposed Pathway

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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

M

M

D

D

msrclinmsrclin

ff

QQkM

M

D

D ,,

2

1

2

1

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Relative Dosimetry

rel

rel

f

Q

f

Q

f

Q

f

Qff

QQading

Output

MD

MDk msrclin

msrclin )(Re

)(

/

/

clin

msr

clin

msr

clin

clin

msr

clin

,w,w

,w,w,

,

re fm s r

m s r

m s r

m s r

m s r

m s r

,

,,,,

ff

QQQ oQQ oD W

f

Q

f

Qw kkNMD

msrclin

msrclinmsr

msr

clin

clin

msr

msr

msr

msr

clin

clin

clin

clin

msr

msr

clin

clinmsrclin

msrclin

,

,

,w

,w

/

/ff

QQf

Q

f

Q

f

Q

f

Q

f

Q

f

Q

f

Q

f

Qff

QQ kM

M

MD

MD

M

M

msrfmsrairw

fclinfclinairwff

QQPS

PSk msrclin

msrclin

)(

)(

,

,,

,

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Meaning of k in Micro-Chambers

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Stopping Power Ratio

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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 size

Source size (FWHM)

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

Co

rrec

tio

n fa

cto

r

Field size (mm)

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 (18) 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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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

Unco

rrec

ted o

utp

ut

dif

fere

nce

(%

)

wit

h M

C

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

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New Published data From IAEA

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Correction Factors

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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

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Machine, Cone & Depth?

Radiotherapy Oncology, 112, 442-446, 2014

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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

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

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

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

Validity of Proposed Method

Field Size (cm)

Rel

ativ

e dose

at

dm

ax

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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

AAPM-2015/

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What is a “small field” in photon beam dosimetry?

Sam Question # 1

63%

4%

2%

25%

6% A. Less than 3x3 cm2

B. Less than 2x2 cm2

C. Depends on type of machine (CyberKnife, Gamma Knife, etc)

D. Depends on detector (pinpoint, microdiamond, micro-diode etc)

E. Depends on beam energy and source size

AAPM-2015/

IJDas (36)

Sam- Answer

Answer: E - Depends on Beam Energy and Source Size

Ref: “Das et al, Small fields: Non-equilibrium radiation

dosimetry," Med Phys 35, 206-215 (2008)

AAPM-2015/

IJDas (37)

Sam Question # 2

The source size of a linear accelerator is typically?

78%

15%

3%

4%

0% A. 20 mm

B. 10 mm

C. 5 mm

D. 2 mm

E. 1-10 mm, depending on the machine

AAPM-2015/

IJDas (38)

Answer: E – Depending on machine

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

Sam- Answer 2

AAPM-2015/

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Sam Question # 3

Dosimetry of small field should be performed with;

A. Small water-equivalent

detectors (MicroLion, MicroDiamond, Plastic Scintillators)

B. Edge detector

C. Pinpoint

D. MicroMosfet

E. Any micro-detector

A. B. C. D. E.

89%

2%

9%

0%1%

AAPM-2015/

IJDas (40)

Answer: A – The detectors that are water equivalent

Ref: Azangwe et al, Med Phys 41, 072103 (2014)

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

Sam- Answer #3

IJDas (41) Thanks