tg-155: small fields and non-equilibrium condition photon beam … · 2011. 8. 2. · tg-155...
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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
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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)]
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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%
IJD/AAPM-2011
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
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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,
,
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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
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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
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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
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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
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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
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IJD/AAPM-2011
ThanksIJD/AAPM-2011