Small Field Dosimetry: An Overview of
Reference and Relative Dosimetry in
Non-Equilibrium Condition
Indra J. Das, PhD, FAAPM, FACR, FASTRO
Department of Radiation Oncology
Indiana University School of Medicine
Indianapolis, IN
IJDas [1]
Lecture Organization
Overview of Reference and Relative
Dosimetry
Indra J Das
SRS: Detector Selection
Sonja Dieterich
An Independent Audit and Quality
Assurance
David Followill
IJDas [2]
Outline of Presentation
 Definition of Small Field
o
o
o
o
Electron range
Electronic Equilibrium
Transient Equilibrium
Lateral Equilibrium
 Source Occlusion
o
o
o
o
Variation is shape, size
Machine specific, age, vintage
Source size and dose
Size of detector
 Detector Issues
o Volume averaging
o Perturbation
 IAEA/AAPM Framework
o Nomenclature
o Reference Dosimetry
IJDas [3]
TG-155: Small Fields and Non-Equilibrium Condition
Photon Beam Dosimetry
IJDas [4]
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
University Hospitals Case Medical Center, Cleveland, OH,
George X. Ding
Vanderbilt University Medical Center, Nashville, TN 37232
John Fenwick
Oxford University, Oxford, England
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
Treatment Fields
Magna-Fields
200x200 cm2
Traditional Fields
40x40 cm
2
Small Field
4x4 cm2
IJDas [5]
0.3x0.3 cm2
4x4 cm2
Advance Therapy Fields
SRS/SRT
Gamma Knife
Cyber-Knife
Tomotherapy
IMRT
Meaning of Small Field?
 Lack of charged particle
o Dependent on the range of secondary
electrons
o Photon energy
 Collimator setting that obstructs the
source size
 Detector is comparable to the field
size
IJDas [6]
CPE & Electron Range
CPE, Charged Particle Equilibrium
Electron range= dmax in forward
direction
Electron range in lateral direction
o Nearly energy independent
o Nearly equal to penumbra (8-10 mm)
o rLEE (g/cm2)=5.973[TPR(20,10)]-2.688
Field size needed for CPE
o Lateral range
o 16-20 mm
IJDas [7]
dmax
Electronic Equilibrium, CPE, TCPE
012 33 3333 332 10
1
2
3
4
3
CPE
2
IJDas [8]
Dose = KERMA
Bragg-Gray Cavity: Range and Cavity Size
Dose is calculated only by crossers in the cavity &
Cavity should be non-perturbing
Range
Cavity
Dose = .S/ (number/cm2)(MeVcm2/g) = MeV/g= J/kg (Gy)
IJDas [9]
Lateral Electronic Equilibrium, LEE
Large Field
Small Field
1
4
IJDas [10]
2
3
1
2
3
Lateral Electron Range
Small Range Electrons
LEE
IJDas [11]
Large Range Electrons
No LEE
Definition of Small Fields
IJDas [12]
Das et al, Med. Phys. 35, 206-215,
Energy Fluence Penumbra/Output
Source
sizes
1.0
1.0
Energy Fluence Sc normalized
cm
IJDas [13]
0.8
0.8
0.6
0.6
0.4
Large field
---- 0.01
---- 0.35
---- 0.50
0.4
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
Nyholm et al Radiother Oncol 78, 347, 2006 and Phys Med Biol 51,6245, 2006
Understanding Profiles
Large Field
IJDas [14]
Small Field
Small Detector
Dose
Large Detector
Distance
IJDas [15]
Source Size
90%, 70%, 50%, 30%, 10% iso-intensity line
IJDas [16]
Jaffray et al, Med Phys 20, 1417-1427 (1993)
Varian TrueBeam
T1
6 MV
IJDas [17]
T2
T3
Dimensions
x, y (mm)
0.9, 0.7
6 MV
FFF
0.8, 0.7
10 MV
0.9, 0.9
10 MV
FFF
0.9, 0.8
Sawkey et al, Med Phys, 40, 332, 2013
Dose and Penumbra with Spot Size
IJDas [18]
Scott et al, Med Phys, 36, 3132, 2009
Dosimetry
Absolute
o Dose
Relative
o Depth Dose
 [D(r,d)/D(r,dm)]
o TMR
o Profiles
o Output, Scp (total scatter factor)
 [D(r)/D(ref)]
IJDas [19]
Small Field
Dosimetry
Problem
Total scatter factor from different institutions
1.02
1.00
0.98
Cone Factor (St)
0.96
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
0.84
12% diff
0.82
0.80
10
15
20
25
30
35
40
Cone Diameter (mm)
IJDas [20]
Das et al, J Radiosurgery, 3, 177-186, 2000
Dosimetric Variation with Detectors
Total scatter factor with various detectors
1.00
0.98
Cone Factor (St)
0.96
0.94
0.92
Diamond
CEA (Film)
Kodak(Film)
TLD
Pinpoint(par)
Pinpoint(per)
0.125ion(par)
0.90
0.88
0.86
14%
0.84
0.125ion(per)
MC(1mm)
MC(5mm)
0.82
0.80
10
15
20
25
30
35
40
Cone Diameter (mm)
IJDas [21]
Das et al, J Radiosurgery, 3, 177-186, 2000
Dose to Water For Small Fields: Volume Averaging
IJDas [22]
Capote et al, Med Phys, 31, 2416-2422, 2004
CyberKnife data
12
Uncorrected output difference (%)
with MC
10
8
6
4
2
0
-2
PinPoint
Diamond(old)
EBT2
MicroTLD
IBA-SFD
PTW Diode
SN-Edge
-4
-6
-8
-10
-12
IJDas [23]
Moignier et al, Med Phys 41(7), 071702, 2014
Ratio of Readings?
D
h m ax
0
d(h )  (h ) 

 Eab (h )d (h )
d (h )   
 Q  W  S 
Dm       
 m   e    a
m
r
m





S 
D(r) Q(r) W

      

Dref  Qref   e ref    a 
ref
r
Q(E,r) = Qr Pion Prepl Pwall Pcec Ppcf
m r
D(r)  Q(r)   W    S  

      

Dref  Qref   e ref    a 
r
ref
IJDas [24]
 Q (r ) 

 C F1  C F2
 Q ref 


Radiation Measurements
Charged particle equilibrium
o Range of secondary electrons
o Medium (tissue, lung, bone)
Photon energy and spectrum
o Change in spectrum
 Field size
 Off axis points like beamlets in IMRT
o Changes in (en/) from reference field,
fref
o Change in (L/) from reference field, fref
Detector
IJDas [25]
o Volume
o Density
o Signal to noise ratio
IAEA/AAPM proposed pathway
Alfonso, et al. Med Phys 35, 5179-5186 (2008)
IJDas [26]
Relative Dosimetry
f msr
w, Qmsr
D

k
f clin , f msr
Qclin ,Qmsr
k
IJDas [27]
f clin f msr
QclinQmsr

M
M Qfclin
clin
M

D

D
f clin , f msr
Qclin ,Qmsr
f msr
Qmsr
f msr , f ref
DW , Qo Q , Qo Qmsr , Q
N
k k
 D
/M   M


/M  M
 D
f msr
w ,Qclin
f clin
w ,Qmsr

f msr
Qmsr
f clin
w ,Qclin
f msr
w , Q msr
/ M
/ M
f clin
w ,Qclin
f clin
w ,Qmsr
f clin
Qclin
f msr
Qmsr
f clin
Qclin
f msr
Qmsr
  (Output )
 (Re ading )
( S w,air ) fclin  Pfclin
( S w,air ) fmsr  Pmsr
rel
rel
k
f clin , f msr
Qclin ,Qmsr
Meaning of k Defined in IAEA/AAPM
 L 

   Pfl  Pgrad  Pstem  Pcell  Pwall
  air

fclin
fclin, f msr
kQclin,Qmsr 
 L w

   Pfl  Pgrad  Pstem  Pcell  Pwall
  air

fmsr
w
IJDas [28]
kQ is not Constant in Small Field
IJDas [29]
Kawachi et al, Med Phys, 35, 4591-4598, 2008
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
IJDas [30]

D1 M 1
clin , f msr

 kQf clin
,Qmsr
D2 M 2

Field Size Limit for
Accurate Dose
Measurements with
Standard Detectors
6 MV; Central Axis
1.1
1.0
0.9
0.7
0.6
Scanditronix-SFD
Scanditronix-PFD
Exradin-A16
PTW-Pinpoint
PTW-0.125cc
PTW-0.3cc
PTW-0.6cc
PTW-Markus
Wellhofer-IC4
0.5
0.4
0.3
0.2
0.1
0.0
0
1
2
3
4
5
6
7
8
Field Size (cm)
9
10
15 MV; Central Axis
1.1
1.0
0.9
0.8
Relative dose at d max
R ela tive d o se at d m a x
0.8
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
0.511
0.4
0.3
0.2
0.1
Das et al, TG-106, Med Phys, 35, 4186, 2008
0.0
0
1
2
3
4
5
6
Field Size (cm)
IJDas [31]
7
8
9
10
11
Relative Dose
Validity of Proposed Method
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
1
2
3
4
Field Size (cm)
IJDas [32]
5
6
IJDas [33]
Summary
Small field is a condition where dosimetry is
usually difficult due to electronic disequilibrium
Use of IAEA/AAPM protocol for reference and
relative dosimetry based on Alfonso et al
(2008) is recommended
Focal spot of a machine is a critical parameter
in small field dosimetry
Absolute dosimetry protocol for small field is
forthcoming
IJDas [34]
Small field is defined as a condition of ?
4% 1.
0% 2.
2% 3.
85%4.
9% 5.
IJDas [35]
Lateral electronic disequilibrium
Range of electron beams in lateral direction
Collimator size that obstructs the primary radiation source
All of above
None of above, it is field ≤3x3 cm2
Directions
Answer: 4 - All of above
Ref: “Das et al, Small fields: Non-equilibrium radiation dosimetry,"
Med Phys 35, 206-215 (2008)
IJDas [36]
IAEA/AAPM have provided a frame work for small field dosimetry in
f ,f
terms of the correction factor, kQ ,Q which is dependent on?
2% 1.
8% 2.
1% 3.
2% 4.
88% 5.
IJDas [37]
clin
msr
clin
msr
Beam energy
Type of detector (microChambers, Diode, Mosfet etc)
Type of machine (IMRT, CyberKnife, GammaKnife etc)
Focal spot size
All of above
Answer:
5 – all of above
Ref: Alfonso et al, "A new formalism for referenece dosimetry of
small and nonstandard fields," Med Phys 35, 5179-5186 (2008)
IJDas [38]
Thanks
IJDas [39]