8/2/2012
Chihray Liu,
University of Florida
and
Hansen Chen,
New York-Presbyterian Hospital
Introduction
Previous talk already describe the data collection
and the clinical validation
Elekta FFF Beam Characteristics
Pinnacle Treatment Planning commissioning Tips
Discussion of Clinic Implementation and the Future
Directions
Elekta Linac
Electron beam
Target
Extra-focal
radiation
Primary collimator
2 mm steel plate
Flattening
filter
Ion Chamber
e-
Currently FFF is not available.
approximate release date : early 2013
6 and 10 MV dose rate ~1400 MU/min (open field)
Flattened beam
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8/2/2012
Flattening filter function
Aim is to produce a uniform intensity
But in doing so it
Attenuates central axis dose to ~50%
Creates the largest source of scattered radiation in the head
Changes the beam spectrum across the field
Driving force for FFF: SBRT – hypofractionation
treatment delivery to reduce the treatment time
Relative dose
100
6MV
beam
50
0
-300
-200
-100
0
100
200
300
Distance from central axis
FFF Beam Characteristics
Simpler LINAC physics
Higher dose rate (easy for LINAC to produce high dose output)
Output calibration (chamber response)?
Non-flat beam
Same spectrum at the different off-axis point=> slow variation for
the depth dependent profiles
Easy to obtain symmetry profiles
What about the field size definition?
Beam quality reduced after remove the FF
No extra-focal radiation contribution due to the flattening filter
(ie. Minimum head scatter variation for larger field size)
Reduced head leakage radiation
sharper field edge
Dose rate effects and Calibration
140
Dose rate DOUBLED
1.8 times wedge
100
Reading
2.1 times open
Treatment time halved
80
60
40
20
0
0
100
200
300
400
mu
1.6
SSD dependent Output
1.4
Normailsed response
for open field
Measured dose still
linear & reproducible
Output calibration: Pion
increase, but not much
MU Linearity
120
1.2
1
0.8
0.6
0.4
FF in
0.2
FF out
0
50
100
150
200
SSD (cm)
2
8/2/2012
Open field profiles at d= 10 cm
100
Relative dose
80
60
40
20
0
0
50
100
150
Distance from central axis (mm)
200
Small fields – 4x4 cm
FF
in
FF
out
FF vs. FFF profiles, depth dependent
FS=20x20 cm2
80%FS
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8/2/2012
Wedge Profiles
Open field profiles: symmetry
2R
vari
atio
n
Δ
Φ
Bf
vari
atio
n
Δ
r
Linear regression: abs(Lslope) = abs(Rslope)
Relative dose
100
50
0
-300
-200
-100
0
100
200
300
Distance from central axis (mm)
Field size definition
Derivative of profile
Relative profile
8
6
4
2
0
-25
-15
-5
5
15
25
-2
-4
-6
-8
Field width
4
8/2/2012
Beam quality reduced
Surface Dose
1.30
Normalised Surface Dose
1.25
1.20
1.15
1.10
1.05
1.00
0.95
0.90
FF out
FF in
0.85
0.80
0
10
20
30
40
Field Size (cm )
Sc
Scp
1.05
1.03
Sc
1.01
0.99
FF out
0.97
FF in
0.95
0.93
0
5
10
15
20
25
30
35
40
45
Field Size
40x40 cm2 Sp = 1.03 (FFF),
1.045(FF) due to profile diff.
100 cm SSD, d=dmax
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8/2/2012
Collimator exchange effect
Leakage
Penumbra
Film results 100SSD, dmax
(20-80%)
FF in
4.1
3.7
3.6
4.2
FF out
2.8
2.8
3.0
5.8
Diff
-1.3
-0.9
-0.6
+1.6
100
80
% D ep t h D o se
ES
5
10
15
20
60
40
Non-flat
Flat
20
Beam edge is sharper
Dose outside field lower
0
-50
-40
-30
-20
-10
0
Distance off-axis (mm)
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8/2/2012
MLC transmission
FFF TPS benefits
azimuthal symmetry profiles -> beam model accuracy
increase
Less variation of output factor -> beam model
accuracy increase
Same spectrum ->simplify beam model -> dose
calculation accuracy increase
TPS modeling tips
Spectrum
Spectral off-axis softening factor
Off-axis modeling
extra focal radiation
Calculated output factor
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8/2/2012
FDD comparison
160
140
FDD
120
6x10
6x30
100
fff6x10
fff6x30
80
60
It40
is possible to adjust the beam quality the same as with FF for
10x10 cm2 ; however the 30x30 cm2 PDD will not be the same due
to20the profile differences between FF vs. FFF.
0
0
10
20
30
40
Depth (cm)
Spectrum
Spectrum should not expect to be the same as
FF beam even with the same QI for 10x10 fields
FF
FFF
Spectral off-axis softening Factor
FF
FFF
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8/2/2012
Flattening Filter Attenuation
off-axis profile modeling
FF
FFF
Flattening Filter Scatter Source
extra-focal radiation
Minimum contribution from extra focal radiation
FF
FFF
Scp TPS correction factors
FF
FFF
Calculated output factor is largely depend on off-axis
intensity and the extra-focal radiation source
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8/2/2012
FFF Clinical implementation pro and con
Pro:
Dose rate x2+
SBRT (fIMRT, Dynamic Arc, conformal arc)-> fast delivery
Standard dose (2Gy or less) slightly better performance
Better normal tissue sparing due to th FFF profiles
Head leakage radiation reduce dramatically
Con:
Emergency patient treatment with large field length, a simple
point calculation will not be adequate
What is the future?
FFF + FF photon beam Linac: FF beam for the
emergency case and FFF for everything else.
Or FFF only machine with preplan for emergency
patient care provided by TPS vender, similar to
dynamic wedged concept.
Acknowledgement
Work courtesy of
Jason Cashmore, University Hospital Birmingham
Kevin Brown, Global VP Scientific Research,Elekta
Limited Crawley
10