IJD/AAPM-2011

Small-Field Dosimetry

Indra J. Das, PhD, FACRDepartment of Radiation OncologyIndiana University School of MedicineIndianapolis, IN, USA

IJD/AAPM-2011

TG-155: Small Fields and Non-Equilibrium Condition

Photon Beam DosimetryIndra J. Das (Chair)

Indiana University School of Medicine, Indianapolis, IN 46202

Paolo Francescon (Co-chair)

Ospedale Di Vicenza, Viale Rodolfi, Vicenza 36100, Italy

Anders Ahnesjö

Uppsala University & Nucletron Scandinavia AB, 751 47 Uppsala, Sweden

Maria M. Aspradakis

Department of Radiation Oncology, Kantonsspital Lucerne, Lucerne Switzerland

Chee-Wai Cheng

Midwest Proton Radiotherapy Institute, Bloomington, IN,

George X. Ding

Vanderbilt University Medical Center, Nashville, TN 37232

Geoffrey S. Ibbott

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

Mark Oldham

Duke University Medical Center, Durham, NC 27710

M. Saiful Huq

University of Pittsburgh Cancer Institute, Pittsburgh, PA 15232

Chester S. Reft

University of Chicago, Chicago, IL 60637

Otto A Sauer

University of Wuerzburg, Wuerzburg, Germany

IJD/AAPM-2011

TG-155 Approved Task1. Collaborate with the new task group (Non-compliant/IAEA) on absolute

dosimetry to ensure that there is no overlap between the two task groups, but rather are complementary to each other.

2. Review and summarize literature on dosimetry of small fields irrespective of the origin and treatment modality.

3. Provide overview of the issue of CPE for the small field dosimetry in homogeneous and inhomogeneous media.

4. Provide meaningful information on the spectrum and shift in beam energy from Monte Carlo.

5. Provide radiation parameters (men/r, S/r, etc) for small field dosimetry from published literature from Monte Carlo.

6. Provide suitability of specific detectors with respect to perturbations and signal to noise ratio.

7. Provide available information on the correction and perturbation factors in detectors.

8. Provide guidelines in measurement methods for modeling the treatment planning systems for small fields.

9. Provide suitability of algorithms based on measurement for beam modeling in small fields especially in inhomogeneous medium.

10. Provide error analysis and limit of uncertainty in the measurements.

11. Provide guidelines and recommendations for accurate determination of dosimetric data for small fields.

IJD/AAPM-2011

Treatment Fields

Magna-FieldsTraditional Fields

Advance Therapy Fields

SRS/SRT

Gamma Knife

Cyber-Knife

Tomotherapy

IMRT

40x40 cm2 4x4 cm2

4x4 cm2 0.3x0.3 cm2

200x200 cm2

Small Field

IJD/AAPM-2011

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

IJD/AAPM-2011

Source Size

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

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

IJD/AAPM-2011Das et al, Med. Phys. 35, 206-215, 2008

Definition of Small Fields

IJD/AAPM-2011

Dosimetry

Absolute

■ Dose

Relative

■ Depth Dose– [D(r,d)/D(r,dm)]

■ TMR

■ Profiles

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

IJD/AAPM-2011

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

WEH

U-Arizona

Temple U

U Penn

Erlanger

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

IJD/AAPM-2011

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 F

acto

r (S

t)

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

Dosimetric Variation with Detectors

14%

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3028262422201816141210

0

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 (

%)

20181614121086420

0

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, 1996

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

Profiles with different detectors

IJD/AAPM-2011

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

( )

refQ

F Fr

CQ

C

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

( ) ( )

rr m

ref ref aref ref

D r Q r W S

D Q e

IJD/AAPM-2011

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

IJD/AAPM-2011

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

IJD/AAPM-2011Alfonso, et al. Med Phys 35, 5179-5186 (2008)

IAEA/AAPM proposed pathway

IJD/AAPM-2011

Relative Dosimetry

refmsr

msr

msr

msr

msr

msr

,

,,,,

ff

QQQoQQoDW

f

Q

f

QwkkNMD

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

QQk

M

M

MD

MD

M

M

msrfmsrairw

fclinfclinairwff

QQPS

PSk msrclin

msrclin

)(

)(

,

,,

,

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,

,

IJD/AAPM-2011

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

ati

ve

do

se a

t d

ma

x

Scanditronix-SFD

Scanditronix-PFD

Exradin-A16

PTW-Pinpoint

PTW-0.125cc

PTW-0.3cc

PTW-0.6cc

PTW-Markus

Wellhofer-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

ma

x

Scanditronix-SFD

Scanditronix-PFD

Exradin-A16

PTW-Pinpoint

PTW-0.125cc

PTW-0.3cc

PTW-0.6cc

PTW-Markus

Wellhofer-IC4

Field Size Limit for

Accurate Dose

Measurements with

Available Detectors

Das et al, TG-106, Med Phys,

35, 4186, 2008

IJD/AAPM-2011

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 cm

AIR, 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

(MeV

)

Spectra & Effective Energy

from SRS Cones (0.5-5 cm)

IJD/AAPM-2011 IJD/AAPM-2011Sanchez-Deblado et al, Phy. Med. Biol., 48, 2081, 2003

Radiological Parameters

IJD/AAPM-2011F. Araki, Med. Phys. 33, 2955-2963 (2006).

Cyber Knife Dosimetry

0.5%

IJD/AAPM-2011

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

MC

MC

MC

MC

MC

MC

IJD/AAPM-2011

Correction Factors

Francescon, et al Med Phys 35, 504, 2008

Correction Factor depends on:Field size

Source size (FWHM)

Detector type

IJD/AAPM-2011Kawachi et al, Med Phys, 35, 4591-4598, 2008

Errors in Measured Reading

IJD/AAPM-2011

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)

Co

rrec

tio

n f

acto

r

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

IJD/AAPM-2011Chung et al , Med Phys, 37, 2404-2413, 2010

Correction Factor vs Ion Chambers

IJD/AAPM-2011

Cyber Knife

Pantelis et al, Med Phy. 37, 2369-2379, 2010 IJD/AAPM-2011

kQ is not Constant in Small Field

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

IJD/AAPM-2011Sham et al, Med Phys, 35, 3317-3330, 2008

Depth Dose & Source Size

IJD/AAPM-2011

Profile & Source Size

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

IJD/AAPM-2011

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).

IJD/AAPM-2011

Electron range & inhomogeneity

IJD/AAPM-2011

0.5 cm, 0.25 cm3 lung

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

Depth (cm)

Rel

ativ

e D

ose

Ho mo geneo us

Mo nte Carlo

EP L

TMR Ratio

P o wer Law TMR

Co nvo lutio n

CCC Water

Jones & Das, Med. Phys. 30, 296, 2003Jones & Das, Med. Phys. 30, 296, 2003

Jones & Das, Med. Phys. 32, 766, 2005 IJD/AAPM-2011

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.

IJD/AAPM-2011

Conclusions

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.IJD/AAPM-2011

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.

-Conclusions

IJD/AAPM-2011

ThanksIJD/AAPM-2011