Dosimetry of Small IMRT Fields
Parham Alaei, Ph.D.
Department of Radiation Oncology
University of Minnesota
NCCAAPM Symposium-October 10, 2013
1
IMRT and Small Fields
• IMRT beams typically contain “small” fields or
segments
• How “small” depends on the planner/planning
system/institutional policy
• Minimum segment size used in planning
should be equal or larger than that for which
beam data was collected
2
Beam data for planning systems
• Without IMRT:
• With IMRT:
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4x4 cm2 40x40 cm2
40x40 cm22x2 cm2
Or smaller
Small field data challenges
• Detector size (affects profile and output factor
measurements)
• Alignment of water tank, detector, etc.
• Jaw/MLC positioning errors
• Beam modeling in TPS
• IAEA and AAPM guidelines for absolute dosimetry of
small fields
• TG-155: “Small fields and non-equilibrium condition
photon beam dosimetry”
4
Focus on: Output Factors
• Output factors needed for treatment planning system
commissioning
• Measurement field sizes/depths depend on the
planning system used:– Pinnacle: Range of field sizes (small->large), 10 cm depth, 100 cm SSD
– Eclipse: Range of field sizes (small->large), 10 cm depth, 100 cm SSD
– Xio: Range of field sizes (4x4-40x40), 10 cm depth
– Monaco: Range of field sizes (1x1-40x40), 10 cm depth, 100 cm SSD
– MasterPlan: Range of field sizes (5x5-20x20, smaller fields if needed),
10 cm depth, 100 cm SAD
5
TG 120 Recommendations
6
Med. Phys. 38(3): 131-1338 (2011)
ASTRO Recommendations
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Practical Radiation Oncology (2011) 1, 190–195
8
9
10
Das, Ding, and Ahnesjö: Small fields: Nonequilibrium radiation dosimetry Med. Phys. 35: 206-215 (2008)
Total scatter factor Scp versus
field size measured with
various available
radiation detectors for (a) 6
MV and (b) 15 MV beams.
11
S. Dieterich and G. W. Sherouse: Comparison of seven commercial dosimetry diodes for SRS Med. Phys. 38: 4166-4173 (2011)
Focus on: Output Factors and IMRT
• If the output factors measured for input into
the treatment planning system are wrong …
• What is the consequence for “small field”
plans (IMRT, SBRT, SRS, …)?
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Small fields and IMRT
• A typical IMRT plan has 5-10 segments per beam
• There are at least ~2-3 large segments per beam
• So there may be ~ 0-7 “small” segments
• A fraction of these are < 4 cm2
– Assuming 4 cm2 is the cut-off for potential erroneous
measurements
• So the error introduced by inaccurate small field
beam data should be minimal
• This also depends on the planning system and/or
delivery system (linear accelerator)
13
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Medical Physics, Vol. 39, No. 8, August 2012 pp. 4691-4694
Pinnacle treatment planning system
Elekta and Varian accelerators
Results may be different for other planning systems!
Method
• Two five-field IMRT plans created on TomoTherapy
“cheese” phantom (one for each linac)
• Plans optimized to deliver 200 cGy to a cylindrical
target of 2 cm diameter and 2 cm length
• Four structures used as organs-at-risk
• DMPO, minimum segment area: 2 cm2 , minimum
segment MU: 1
• Ion chamber location contoured
15
16
Output factor vs. field size (meas. @ 10 cm depth)
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0.70
0.75
0.80
0.85
0.90
0.95
1.00
2x2 3x3 4x4 5x5 6x6 8x8 10x10
Re
lati
ve
Ou
tpu
t F
ac
tors
Field Size (cm)
Elekta
Varian
Method (Cont’d)
• Small field output factors (2x2 and 3x3 cm2) altered
by ±5, 10, and 20% and machine re-commissioned
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Altering small field output factors
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0.70
0.75
0.80
0.85
0.90
0.95
1.00
2x2 3x3 4x4 5x5 6x6 8x8 10x10
Re
lati
ve
Ou
tpu
t F
ac
tors
Field Size (cm)
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
2x2 3x3 4x4 5x5 6x6 8x8 10x10
Re
lati
ve
Ou
tpu
t F
ac
tors
Field Size (cm)
Measured
-20%
-10%
-5%
5%
10%
20%
Method (Cont’d)
• Plan re-optimized every time
• Small change in segments between plans (0-2)
• All plans were delivered to the phantom with the
ionization chamber positioned at isocenter
• In order to confirm the variations in output factors in
the planning system, single static 2×2 cm2 fields were
created for each re-commissioned machine and
delivered to the phantom
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Summary of plans
• Elekta • Varian
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Plan Name No. of segments Monitor Units
20% Decrease 538
10% Decrease 530
5% Decrease 515
Base 56 508
5% Increase 486
10% Increase 487
20% Increase 468
X: 2.0-4.5 cm
Y: 2.0-4.0 cm
40% of segments 3x3 cm2 or smaller
Plan Name No. of segments Monitor Units
20% Decrease 48 562
10% Decrease 51
5% Decrease 50
Base 53
5% Increase 53
10% Increase 50
20% Increase 55
X: 4.0-5.9 cm
Y: 4.0 cm
55% of segments 3x3 cm2 or smaller
Measured vs. Calculated dose-Elekta
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-10.00
-8.00
-6.00
-4.00
-2.00
0.00
2.00
4.00
6.00
8.00
10.00
20% decrease 10% decrease 5% decrease Base 5% increase 10% increase 20% increase
Perc
en
tag
e D
ose D
iffe
ren
ce (
cG
y)
Percent Output Factor Variation
Measured vs. Calculated dose-Varian
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-5.00
-3.00
-1.00
1.00
3.00
5.00
20% decrease 10% decrease 5% decrease Base 5% increase 10% increase 20% increase
Perc
en
tag
e D
ose D
iffe
ren
ce (
cG
y)
Percent Output Factor Variation
Measured vs. Calculated dose-Summary
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Elekta
% Difference
(Meas. vs. Calc.)
Plan Name
20% Decrease 8.7
10% Decrease 3.6
5% Decrease 0.8
Base 0
5% Increase -2.9
10% Increase -5.4
20% Increase -8.3
Varian
% Difference
(Meas. vs. Calc.)
Plan Name
20% Decrease -0.1
10% Decrease -0.2
5% Decrease -0.1
Base 0
5% Increase -0.5
10% Increase -0.4
20% Increase 0.1
40% of segments 3x3 cm2 or smaller
Why the difference?
• Elekta
– X-diaphragm traces MLCs,
Y-jaws conform to each
segment
• Varian
– X & Y jaws fixed for the
delivery of all segments,
encompassing the largest
segment
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Why the difference?
• Elekta
– Handled as a conformal
jaw machine in Pinnacle
– MUs calculated by
employing small field
output factors as a
consequence of
conforming the collimator
jaws to each of the
segments
• Varian
– Handled as a non-conformal
jaw machine in Pinnacle
– MUs calculated by
employing output factors of
the larger-encompassing jaw
size, and modeling blocked-
field effects
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Varian segment MUs not sensitive to small field OF changes
Verifying the results using static fields
• Single, 6 MV, 2 x 2 cm2 field incident on phantom
• Same machines with modified small field OF used to
compute the dose with the TPS
• Dose calculated and measured at 3 cm depth for all
plans
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Verifying the results using static fields
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Correlation between OF change and dose/MU delivered
Single jaw/MLC-defined 2x2 cm2 field
Elekta % Difference
Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.)
20% Decrease 41 28.8 26.2
10% Decrease 36 28.4 11.5
5% Decrease 34 28.8 5.6
Base 33 28.3 0.0
5% Increase 31 28.2 -4.9
10% Increase 30 28.3 -9.5
20% Increase 27 28.6 -17.2
Single jaw-defined 2x2 cm2 field
Varian % Difference
Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.)
20% Decrease 41 28.8 25.2
10% Decrease 36 28.4 12.3
5% Decrease 34 28.8 5.5
Base 33 28.3 0.0
5% Increase 31 28.2 -4.9
10% Increase 30 28.2 -14.6
20% Increase 27 28.6 -16.6
Verifying the results using static fields
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No correlation between OF change and dose/MU delivered
Single MLC-defined 2x2 cm2 field (jaws at 10x10)
Varian % Difference
Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.)
20% Decrease 32 28.7 0.0
10% Decrease 0.4
5% Decrease 0.0
Base 0.0
5% Increase 0.4
10% Increase 0.0
20% Increase 0.2
Results
• Incorrect output factors do affect the dose
delivered for a conformal jaw machine (Elekta,
Siemens, Varian TrueBeam with jaw tracking)
• But they do not affect the dose delivered for a
non-conformal jaw machine (Varian)
• As applicable to Pinnacle TPS!
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Conclusion
• It is very important to have accurate output
factor data in the TPS, as it is to have accurate
beam profiles
• Inaccurate data could lead to miscalculation of
dose
• If you don’t have reliable data for small
fields/segments, don’t use them!
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Questions?
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