Small-Field Dosimetry: Its Importance In Clinic

Indra J. Das/ Small Fields (Page-1)
Misadministration Media Coverage
Small-Field Dosimetry:
Its Importance In Clinic
Indra J. Das, PhD, FAAPM, FACR, FASTRO
Department of Radiation Oncology
Indiana University School of Medicine &
IU Health Proton Therapy Center
IJDas (1) Indianapolis, IN, USA
AAPM-2013
IJDas (2)
AAPM-2013
TG-155: Small Fields and Non-Equilibrium Condition
Photon Beam Dosimetry
Misadministration Media Coverage
Indra 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
IJDas (4) Otto A Sauer
AAPM-2013 University of Wuerzburg, Wuerzburg, Germany
IJDas (3)
AAPM-2013
What is a Small Field?
Treatment Fields
Magna-Fields
 Lack of charged particle
Traditional Fields
200x200 cm2
40x40 cm2
Small Field
4x4 cm2
IJDas (5)
AAPM-2013
Wrong detector used for
BrainLab cone calibration
0.3x0.3 cm2
 Dependent on the range of secondary
electrons
 Photon energy
4x4 cm2
Advance Therapy Fields
SRS/SRT
Gamma Knife
Cyber-Knife
Tomotherapy
IMRT
 Collimator setting that obstructs the
source size
 Detector is comparable to the field size
IJDas (6)
AAPM-2013
Indra J. Das/ Small Fields (Page-2)
Source Size
Calculation of Fluence Map Elements
Components Critical for Small Fields
Direct beam source size
Open beam off axis distribution
Fluence modulation
Step&shot
Dynamic
Wedges
Head scatter sources
flattening filter
collimators
wedges
Monitor back scatter
Processes to
include
Collimator leakage, including
MLC interleaf leakage
shape of MLC leaf ends
MLC positioning
Beam spectra
Spectral changes
Electron contamination
90%, 70%, 50%, 30%, 10% iso-intensity line
IJDas (7)
AAPM-2013
IJDas (8)
AAPM-2013
Jaffray et al, Med Phys 20, 1417-1427 (1993)
Definition of Small Fields
Courtesy Anders Ahnesjö
Calculation of Fluence Map Elements
Raytrace to calculate attenuation
or use approximate penumbra shape
POI-eye-view of the source!
IJDas (9)
AAPM-2013
Das et al, Med. Phys. 35, 206-215, 2008
For each element, find the contributions
from the relevant sources
Courtesy Anders Ahnesjö
IJDas (10)
AAPM-2013
Energy Fluence Penumbra/Output
Profile and Leaf Tip Positions & Location
Source plane
xHVL - xw
Energy FluenceSc normalized
Source sizes
Nyholm
IJDas (11)
AAPM-2013
1.0
1.0
cm
0.8
0.8
---- 0.01
---- 0.35
---- 0.50
0.6
0.6
0.4
Large field
cm
x
cm
0.4
isocenter plane
2.0x2.0
1.0x1.0cm
cm22
0.5x0.5 cm2
0.2
0.2
0.0
6.5 6.0 5.5 5.0 4.5 4.0 3.5
x
cm
0.0
1.5 1.0 0.5 0.0 0.5 1.0 1.5
x
cm
et al Radiother Oncol 78, 347, 2006 and Phys Med Biol 51,6245, 2006
xHVL xw
x
Potential inconsistency dose calculations ↔ delivery
Delivery: uses of lookup table or parameterized correction
TPS:
- “ - or direct calculation of 50% transmission
IJDas (12)
AAPM-2013
Courtesy Anders Ahnesjö
Indra J. Das/ Small Fields (Page-3)
Dose and Penumbra with Spot Size
Dosimetry
 Absolute
 Dose
 Relative
 Depth Dose
o
[D(r,d)/D(r,dm)]
 TMR
 Profiles
 Output, Scp (total scatter factor)
o
IJDas (14)
AAPM-2013
Scott et al, Med Phys, 36, 3132, 2009
Dosimetric Variation with Detectors
Small Field Dosimetry Problem
Total scatter factor from different institutions
1.02
Total scatter factor with various detectors
1.00
1.00
0.98
0.98
Institutional
variability in 6 MV
Radionics SRS
dosimetry
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
0.96
Cone Factor (St)
Cone Factor (St)
0.96
0.94
0.92
0.88
0.86
14%
0.84
0.82
0.82
0.80
10
15
20
25
30
35
0.80
40
10
Cone Diameter (mm)
IJDas (15)
AAPM-2013
15
20
25
40
Das et al, J Radiosurgery, 3, 177-186, 2000
Dose profile of a 12.5 mm cone
110
Film
Diamond
Ion (Pinp oint )
Ion (Parallel Plate)
100
0.8
90
Normalized Dose (%)
0.7
Monte Carlo
Diamond
Diode
LAC+film
Film
0.6
0.5
0.4
PTW 31014 (Pinpoint)
PTW 31010 (Semiflex)
PTW 30006 (Farmer)
0.3
80
Dose profiles for a 40 mm cone with different detectors
70
110
60
100
50
90
40
30
20
10
0
0
1.0
1.5
2.0
2.5
2
4
6
8
10
Distance (mm)
3.0
12
14
16
18
20
Film
Diamomd
Ion (Pinp oint)
Ion (parallel-plate)
Ion (0.125 cc)
80
70
60
50
40
30
20
10
Side of Square Field (cm)
0
10
IJDas (17)
AAPM-2013
35
Profiles with different detectors
0.9
0.2
0.5
30
Cone Diameter (mm)
IJDas (16)
AAPM-2013
Das et al, J Radiosurgery, 3, 177-186, 2000
Dose to Water For Small Fields: Volume Averaging
Output Factor
Diamond
CEA (Film)
Kodak(Film)
TLD
Pinpoint (par)
Pinpoint (per)
0.125ion(par)
0.125ion(per)
M C(1mm)
M C(5mm)
0.90
Normalized Dose (%)
IJDas (13)
AAPM-2013
[D(r)/D(ref)]
From Roberto Capote, IAEA
IJDas (18)
AAPM-2013
Hedrian, Hoban & Beddoe, PMB, 41, 93-110, 1996
Das et al, J Radiosurgery, 3, 177-186, 2000
12
14
16
18
20
Distance (mm)
22
24
26
28
30
Indra J. Das/ Small Fields (Page-4)
 
D   en 
  
Ratio of Readings?
D
h m ax
0
Radiation Measurements
d (h )   (h ) 

 Eab (h )d (h )
d ( h )   
 Charged particle equilibrium or electronic
equilibrium
 Range of secondary electrons
 Medium (tissue, lung, bone)
 Q W  S 
Dm       
 m   e    a
m
D ( r )  Q( r )   W 

 
Dref  Qref   e ref
r
 Photon energy and spectrum
 S 
 
   a

m
 Change in spectrum
r



ref
 Field size
 Off axis points like beamlets in IMRT
 Changes en/ and S/
 Detector size
Q(E,r) = Qr Pion Prepl Pwall Pcec Ppcf,
D (r )  Q( r )   W 

 
Dref  Qref   e ref
r
IJDas (19)
AAPM-2013
r
  S m 
  
   a 

ref
 Q (r )
 
 Q ref


 C F1  C F 2


 Volume
 Signal to noise ratio
IJDas (20)
AAPM-2013
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
IJDas (21)
AAPM-2013
 Lateral range
 16-20 mm
dmax
IJDas (22) Alfonso,
AAPM-2013
Relative Dosimetry
msr
msr , f ref
Dwf ,msrQmsr  M Qf msr
N DW , QokQ ,QokQf msr
,Q
f msr
 Qfclin

clinQmsr
, f msr

kQf clin
clin , Qmsr


D
D
clin , f msr
kQfclin
,Qmsr 
IJDas (23)
AAPM-2013




f clin
 Dwf clin
 M Qfclin
M Qfclin
,Qclin / M Qclin
clin
clin
clin , f msr

kQfclin
,Qmsr
f msr 
f msr
f msr 
M Qmsr  Dw ,Q msr / M Qmsr  M Qf msr
msr
f msr
w ,Qclin
f clin
w ,Qmsr
/ M
/ M
f clin
w ,Qclin
f clin
w ,Qmsr
  (Output )
 (Re ading )
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
D1 M 1

D2 M 2
rel
rel

D1 M 1
clin , f msr

 kQf clin
,Qmsr
D2 M 2
( S w,air ) fclin  Pfclin
( S w,air ) fmsr  Pmsr
et al. Med Phys 35, 5179-5186 (2008)
IJDas (24)
AAPM-2013

Indra J. Das/ Small Fields (Page-5)
6 MV; Central Axis
Detectors
1.1
Field Size Limit for
Accurate Dose
Measurements with
Available Detectors
1.0
0.9
Relative dose at d max
0.8
0.7
0.6
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
Detector
15 MV; Central Axis
1.0
0.0
0
1
2
3
4
5
6
7
8
0.9
9
10
11
0.8
Relative dose at d max
Field Size (cm)
0.7
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
Das et al, TG-106, Med Phys,
35, 4186, 2008
0.2
0.1
0.0
0
1
2
3
4
5
6
7
8
9
10
Field Size (cm)
IJDas (25)
AAPM-2013
Manufacturer
Type
volume
SFD
Scanditronix
Photon diode 1.7x10-5cm3
PFD
Scanditronix
Photon diode 1.9x10-4cm3
Exradin A-16 Standard Imaging Ion chamber 0.007cm3
Wellhofer-IC4 Scanditronix
Ion chamber 0.40 cm3
Pinpoint
PTW
Ion chamber 0.015cm3
0.125cc
PTW
Ion chamber 0.125cm3
0.3cc
PTW
Ion chamber 0.3 cm3
0.6cc
PTW
Ion chamber 0.6 cm3
Diamond
PTW
Diamond
0.003cm3
Markus
PTW
Parallel plate 0.055cm3
Edge Detector Sun Nuclear
Diode
10-5cm3
Other
11
IJDas (26)
AAPM-2013
Radiation Detectors
PTW Choice
for small Fields
www.PTW.de
IJDas (27)
AAPM-2013
IJDas (28)
AAPM-2013
Understand Detector, Connector & Cable
Sensitivity vs Volume of Detectors
140
PFD
130
120
Relative sensitivity
110
100
90
0.6 cc
80
70
60
50
40
30
SFD
0.3cc
20
10
A-16
0
1.0E-04
1.0E-03
PinPoint
1.0E-02
Markus
Srivastava et al, SU-GG-T-270, 2010
0.125cc
IC4
1.0E-01
1.0E+00
3
Volume (cm )
IJDas (29)
AAPM-2013
IJDas (30)
AAPM-2013
Indra J. Das/ Small Fields (Page-6)
1.E+0
Photon Fluence
Ion Chamber Issues in Small Fields
 Volume averaging
 Poor signal: signal to noise ratio
 Pion: Ion recombination
AIR, at exit
(c)
5 cm
Spectra & Effective Energy
from SRS Cones (0.5-5 cm)
1.E-1
1.E-2
0.5 cm
 300 volts may not be needed
 May be in proportional counter mode or gas
multiplication
 Two voltage method may not be applicable
1.95
0.0
1.0
2.0
3.0
4.0
E (MeV)
5.0
6.0
7.0
1.E+1
WATER, at dmax
(a)
Photon Fluence
Current
Large volume Ion chamber
5 cm, primary
1.E-1
IJDas (31)
AAPM-2013
200
300
400
600
Voltage
1.75
1.70
1.65
1.50
0
1.E-3
1
2
3
4
Verhaegen, Das, Palmans, PMB, 43, 2755-2768, 1998
0.0
IJDas (32)
AAPM-2013
1.0
2.0
3.0
4.0
5.0
6.0
E (MeV)
Sanchez-Deblado et al, Phy. Med. Biol., 48, 2081, 2003
IJDas (34)
AAPM-2013
Cyber Knife Dosimetry
Francescon et al, Med. Phys. 25(4), 503, 1998
0.5%
MC
MC
MC
MC
MC
MC
IJDas (35)
AAPM-2013
F. Araki, Med. Phys. 33, 2955-2963 (2006).
6
0.5 cm, scatter
Radiological Parameters
IJDas (33)
AAPM-2013
5
Diameter of Cone (cm)
5 cm, scatter
500
1.80
1.55
1.E-2
1.E-5
100
1.85
1.60
0.5 cm, primary
1.E-4
0
Average Energy (MeV)
1.90
1.E-4
1.E+0
Small volume Ion chamber
6 MV
2.00
1.E-3
IJDas (36)
AAPM-2013
Indra J. Das/ Small Fields (Page-7)
Errors in Measured Reading
Correction Factors
Correction Factor depends on:
Field size
Source size (FWHM)
Detector type
IJDas (37)
AAPM-2013
IJDas (38)
AAPM-2013
Francescon, et al Med Phys 35, 504, 2008
Kawachi et al, Med Phys, 35, 4591-4598, 2008
Detector Density Effect
Published data on
1.16
Siemens; PTW diode 60012
1.14
1.12
Elekta; PTW diode 60012
Siemens; Exradin A16
1.10
Elekta; Exradin A16
Siemens; Sun Nuclear Dedge
"Elekta; Sun Nuclear Dedge"
Siemens; PTW Pinpoint 31014
Correction factor
1.08
1.06
Elekta; PTW Pinpoint 31014
Siemens; PTW microLion
1.04
Elekta; PTW microLion
1.02
1.00
0.98
0.96
0.94
0.92
0.90
0
IJDas (39)
AAPM-2013
5
10
15
20
25
30
Field size (mm)
Francescon et al Med Phys,38, 6513, 2011
35
IJDas (40)
AAPM-2013
Correction Factor vs Ion Chambers
IJDas (41)
AAPM-2013
Chung et al , Med Phys, 37, 2404-2413, 2010
Scott et al, Phys Med Biol 57 4461–4476, 2012
of Linear Accelerators (Varian)
IJDas (42)
AAPM-2013
Med. Phys. 38 (12), 6992, 2011
Indra J. Das/ Small Fields (Page-8)
of Linear Accelerators
Cyber Knife
Med. Phys. 38 (12), 6513-6527, 2011
Radiotherapy and Oncology 100 (2011) 429–435
IJDas (43)
AAPM-2013
IJDas (44)
AAPM-2013
Pantelis et al, Med Phy. 37, 2369-2379, 2010
kQ is not Constant in Small Field
Tomotherapy kmsr Reference Dosimetry
Reference: 5x10 cm2, 85 cm SSD, 10 cm
IJDas (45)
AAPM-2013
Sterpin et al, Med Phys, 39, 4066, 2012
IJDas (46)
AAPM-2013
Depth Dose & Source Size
IJDas (47)
AAPM-2013
Sham et al, Med Phys, 35, 3317-3330, 2008
Kawachi et al, Med Phys, 35, 4591-4598, 2008
Profile & Source Size
IJDas (48)
AAPM-2013
Sham et al, Med Phys, 35, 3317-3330, 2008
Indra J. Das/ Small Fields (Page-9)
Effect of Inhomogeneity
Electron range & 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).
IJDas (49)
AAPM-2013
IJDas (50)
AAPM-2013
Inhomogeneity Corrections
1.1
0.5 cm, 0.25 cm3 lung
1.0
Homogen eou s
Mon te Carlo
EPL
TMR Rati o
Power Law TMR
Convolut ion
CCC Water
0.9
0.8
Relative Dose
TG-155 Recommendation
0.7






0.6

0.5
0.4

0.3
0.2
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14
Depth (cm)
IJDas (51)
AAPM-2013
Jones & Das, Med. Phys. 30, 296, 2003
Jones & Das, Med. Phys. 32, 766, 2005

IJDas (52)
AAPM-2013
Summary
 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.
IJDas (53)
AAPM-2013
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.
-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.
IJDas (54)
AAPM-2013
Indra J. Das/ Small Fields (Page-10)
Thanks
IJDas (55)
AAPM-2013