Small field dosimetry, a clinical perspective, Indra J Das, AAPM-2012
Small Field Dosimetry: A Clinical Perspective
Indra J. Das, PhD, FAAPM, FACR, FASTRO
Department of Radiation Oncology
Indiana University School of Medicine
Indianapolis, IN, USA
Wrong detector used for
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BrainLab cone calibration
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TG-155 Approved Task
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
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Treatment Fields
What is a Small Field?
Magna-Fields
Lack of charged particle
Traditional Fields
200x200 cm2
40x40 cm2
Small Field
4x4 cm2
0.3x0.3 cm2
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.
Review and summarize literature on dosimetry of small fields irrespective of the
origin and treatment modality.
Provide overview of the issue of CPE for the small field dosimetry in
homogeneous and inhomogeneous media.
Provide meaningful information on the spectrum and shift in beam energy from
Monte Carlo.
Provide radiation parameters (men/r, S/r, etc) for small field dosimetry from
published literature from Monte Carlo.
Provide suitability of specific detectors with respect to perturbations and signal
to noise ratio.
Provide available information on the correction and perturbation factors in
detectors.
Provide guidelines in measurement methods for modeling the treatment planning
systems for small fields.
Provide suitability of algorithms based on measurement for beam modeling in
small fields especially in inhomogeneous medium.
Provide error analysis and limit of uncertainty in the measurements.
Provide guidelines and recommendations for accurate determination of
dosimetric data for small fields.
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■
4x4 cm2
Advance Therapy Fields
SRS/SRT
Gamma Knife
Cyber-Knife
Tomotherapy
IMRT
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■
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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Small field dosimetry, a clinical perspective, Indra J Das, AAPM-2012
Source Size
Definition of Small Fields
90%, 70%, 50%, 30%, 10% iso-intensity line
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Jaffray et al, Med Phys 20, 1417-1427 (1993)
Das et al, Med. Phys. 35, 206-215, 2008
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Small Field Dosimetry Problem
Dosimetry
Total scatter factor from different institutions
1.02
Absolute
1.00
0.98
Dose
■
Institutional
variability in 6 MV
Radionics SRS
dosimetry
Depth Dose
■
– [D(r,d)/D(r,dm)]
TMR
Profiles
Output, Scp (total scatter factor)
■
■
■
– [D(r)/D(ref)]
Cone Factor (St)
0.96
Relative
0.94
WEH
U-Arizona
Temple U
U Penn
Erlanger
Serago et al.
Fan et al.
Zhu et al.
0.92
0.90
0.88
0.86
12% diff
0.84
0.82
0.80
10
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Dosimetric Variation with Detectors
15
20
25
30
35
40
Cone Diameter (mm)
Das et al, J Radiosurgery, 3, 177-186, 2000
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Radiation Measurements
Total scatter factor with various detectors
Charged particle equilibrium or electronic
equilibrium
1.00
0.98
■
■
Cone Factor (St)
0.96
0.94
0.92
Photon energy and spectrum
Diamond
CEA (Film)
Kodak(Film)
TLD
Pinpoint(par)
Pinpoint(per)
0.125ion(par)
0.90
0.88
0.86
■
■
■
■
0.80
15
20
25
30
35
Field size
Off axis points like beamlets in IMRT
Changes men/r and S/r
Detector size
0.82
10
Change in spectrum
●
●
0.125ion(per)
MC(1mm)
MC(5mm)
14%
0.84
Range of secondary electrons
Medium (tissue, lung, bone)
40
Volume
Signal to noise ratio
Cone Diameter (mm)
Das et al, J Radiosurgery, 3, 177-186, 2000
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Small field dosimetry, a clinical perspective, Indra J Das, AAPM-2012
IAEA/AAPM proposed pathway
CPE & Electron Range
CPE, Charged Particle Equilibrium
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
■
■
dmax
Alfonso, et al. Med Phys 35, 5179-5186 (2008)
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Relative Dosimetry
Why So Much of Fuss?
msr , f ref
Dwf ,msrQmsr = M Qf msr
N DW ,Qo kQ,Qo kQf msr
,Q
msr
(
(
)(
)(
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
)
)
fclin
 Dwf clin
 M Qf clin
M Qf clin
,Qclin / M Qclin
clin
clin
clin , f msr
=
k Qfclin
,Qmsr
f msr 
f msr
f msr 
M Qmsr  Dw , Qmsr / M Qmsr  M Qf msr
msr
f msr
Ω Qfclin
=
clin Qmsr
(D
(D
f msr
w ,Qclin
f clin
w , Qmsr
, f msr
kQf clin
=
clin ,Qmsr
, f msr
kQfclin
=
clin ,Qmsr
)/ (M
)/ (M
f clin
w ,Qclin
f clin
w ,Qmsr
) (Output )
) = (Re ading )
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D1 M 1
≠
D2 M 2
rel
rel
[
D1 M 1
clin , f msr
=
• kQfclin
,Qmsr
D2 M 2
( S w,air ) fclin ⋅ Pfclin
( S w,air ) fmsr ⋅ Pmsr
]
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1.E+0
6 MV; Central Axis
AIR, at exit
1.1
0.8
0.7
Scanditronix-SFD
Scanditronix-PFD
Exradin-A16
PTW-Pinpoint
PTW-0.125cc
PTW-0.3cc
PTW-0.6cc
PTW-Markus 1.1
Wellhofer-IC4
0.5
0.4
0.3
0.2
0.1
1.E-2
0.5 cm
1.95
1.90
1.E-4
15 MV; Central Axis
0.0
1.0
2.0
3.0
4.0
E (MeV)
5.0
6.0
7.0
1.0
1.E+1
0.0
1
2
3
4
5
6
7
8
0.9
9
Field Size (cm)
WATER, at dmax
11
0.8
Relative dose at d max
Das et al, TG-106, Med Phys,
35, 4186, 2008
10
(a)
1.E+0
5 cm, primary
0.7
Photon Fluence
0
6 MV
2.00
1.E-3
Scanditronix-SFD
Scanditronix-PFD
Exradin-A16
PTW-Pinpoint
PTW-0.125cc
PTW-0.3cc
PTW-0.6cc
PTW-Markus
Wellhofer-IC4
0.6
0.5
0.4
0.3
0.2
0.1
1.E-1
2
3
4
5
6
7
8
9
Field Size (cm)
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1.80
1.75
1.70
1.65
1.60
0.5 cm, primary
1.55
1.E-2
1.50
0
1.E-3
1
2
3
4
5
6
Diameter of Cone (cm)
1.E-4
0.5 cm, scatter
Verhaegen, Das, Palmans, PMB, 43, 2755-2768, 1998
1.E-5
1
1.85
5 cm, scatter
0.0
0
Average Energy (MeV)
0.6
Spectra & Effective Energy
from SRS Cones (0.5-5 cm)
1.E-1
Photon Fluence
0.9
Relative dose at d max
(c)
5 cm
Field Size Limit for
Accurate Dose
Measurements with
Available Detectors
1.0
10
11
0.0
1.0
2.0
3.0
E (MeV)
4.0
5.0
6.0
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Small field dosimetry, a clinical perspective, Indra J Das, AAPM-2012
Radiological Parameters
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Cyber Knife Dosimetry
Sanchez-Deblado et al, Phy. Med. Biol., 48, 2081, 2003
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Francescon et al, Med. Phys. 25(4), 503, 1998
0.5%
MC
MC
MC
MC
MC
MC
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F. Araki, Med. Phys. 33, 2955-2963 (2006).
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Errors in Measured Reading
Correction Factors
Correction Factor depends on:
Field size
Source size (FWHM)
Detector type
Francescon, et al Med Phys 35, 504, 2008
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Kawachi et al, Med Phys, 35, 4591-4598, 2008
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Small field dosimetry, a clinical perspective, Indra J Das, AAPM-2012
f clin , f msr
Published data on kQclin ,Qmsr
Detector Density Effect
1.16
Siemens; PTW diode 60012
clin , f msr
kQf clin
, Qmsr
1.12
Correction factor
1.14
1.06
Elekta; PTW diode 60012
Siemens; Exradin A16
Elekta; Exradin A16
Siemens; Sun Nuclear Dedge
1.10
"Elekta; Sun Nuclear Dedge"
Siemens; PTW Pinpoint 31014
1.08
1.04
Elekta; PTW Pinpoint 31014
Siemens; PTW microLion
1.02
Elekta; PTW microLion
1.00
0.98
0.96
0.94
0.92
0.90
0
5
10
15
Francescon et al Med Phys,38, 6513, 2011
, f msr
kQf clin
clin ,Qmsr
20
25
30
Field size (mm)
35
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Scott et al, Phys Med Biol 57 4461–4476, 2012
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, f msr
kQf clin
of Linear Accelerators
clin ,Qmsr
Correction Factor vs Ion Chambers
Med. Phys. 38 (12), 6513-6527, 2011
Chung et al , Med Phys, 37, 2404-2413, 2010
, f msr
kQf clin
clin ,Qmsr
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Cyber Knife
Radiotherapy and Oncology 100 (2011) 429–435
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Tomotherapy kmsr Reference Dosimetry
Reference: 5x10 cm2, 85 cm SSD, 10 cm
Pantelis et al, Med Phy. 37, 2369-2379, 2010
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Sterpin et al, Med Phys, 39, 4066, 2012
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Small field dosimetry, a clinical perspective, Indra J Das, AAPM-2012
Depth Dose & Source Size
kQ is not Constant in Small Field
Kawachi et al, Med Phys, 35, 4591-4598, 2008
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Sham et al, Med Phys, 35, 3317-3330, 2008
Dose and Penumbra with Spot Size
Scott et al, Med Phys, 36, 3132, 2009
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Profile & Source Size
Sham et al, Med Phys, 35, 3317-3330, 2008
0.5 cm, 0.25 cm3 lung
0.9
Perturbations of the detector
Relative Dose
0.8
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).
■
Ho m o ge ne o us
M o nte C a rlo
EP L
TM R R a tio
P o we r La w TMR
C o nvo lutio n
C C C Wa te r
1.0
Range of secondary electrons
■
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1.1
Effect of Inhomogeneity
■
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0.7
0.6
0.5
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).
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0.4
0.3
0.2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Depth (cm)
Jones & Das, Med. Phys. 30, 296, 2003
Jones & Das, Med. Phys. 32, 766, 2005
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Small field dosimetry, a clinical perspective, Indra J Das, AAPM-2012
TG-155 Recommendation
Summary
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.
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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.
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-Summary
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.
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Thanks
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