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Incorporating Prior Knowledge into IMRT Beam Orientation Optimization
Lei Xing, Ph.D. and Andrei Pugachev, M.S. Department of Radiation OncologyStanford University School of MedicineStanford, CA 94305-5304
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180o
300o
340o
20o
60o
100o
140o
260o
220o
9-field head and neck Treatment
Fixed gantry IMRT
C e n t e r
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• Number and incident directions of beams• Beam modality • Beamlet weights
Things need to be optimized
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Beam orientation optimization
• Clinically, beam orientations are selected empirically.
• Excessive computational time is required for full beam orientation optimization.
20c
Nn
1n
(n)]D(n)[DrF −= ∑=
=σ
Dc(n) --- function of beamlet weights, beam orientation.
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Beam orientation optimization
• More intelligent and less computationally intensive methods of beam orientation selection are needed.
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Beam’s-Eye-View Dosemetrics (BEVD) for IMRT Beam Selection
• BEV volumetrics in conventional radiotherapy.
BE
V V
olum
etric
s
Gantry Angle
Only prior geometric information of the system is used.
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• We extend the original BEV to IMRT with consideration of
1. prior geometric and dosimetric information of the system, and
2. beam intensity modulation.
BEVD
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Beam’s-Eye-View Dosemetrics (BEVD) for IMRT Beam Selection
• BEVD for IMRT.
0°
90°270°
A Pugachev & L Xing, IJROPB 51, 1361-1370 (2001).
Maximum beam profile
Both prior geometric & dosimetric knowledge of the system are used.
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BEVD
• For each organ at risk (OAR), assign a tolerance dose.
• For every possible beam orientation, calculate single beam dose distribution that can be achieved without exceeding the tolerances of OARs and normal tissue.
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BEVD Calculation
• Divide a beam into bixel map.
• Introduce an empirical score function to evaluate the goodnessof each beamlet/beam.
• Obtain BEVD score for each incident beam direction.
• Select peaks of the BEVD score function, taking into account the principle of maximum beam separation.
• Perform beam intensity profile optimization.
m1 cm
1 cm
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BEVD ranking for an IMRT treatment of paraspinal tumor(Coplanar beam)
• Five beams with high BEVD score.• Selected gantry angles: 225°, 175°, 140°, 85°, 260°
Selectedbeams
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• Selected beams (gantry, table):(180,20) (180,-20) (230,20) (230,-20) (130,0)
Selectedbeams
BEVD ranking for an IMRT treatment of paraspinal tumor(Non-coplanar beam)
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Liver
KidneyGTV
35%50%65%80%95%
PBEV Selected Beam Orientations
Beams(gantry,table): (180,20), (180,-20)(230,20), (230,-20) (10,0)
0°
90°
180°
270°
Equiangular Beam Orientations
Gantry angles:40,110,180,255, 325
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Beam orientation selection
225°(15MV), 180°(6MV), 140°(6MV), 85°(15MV), 260°(15MV)
Selectedbeams
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Results
• Nasopharyngeal cancer– BEVD method applied to the selection of coplanar
beams
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Cord
GTV35%50%65%80%95%
Dose distribution of two-stage optimization0°
90°
180°
270°
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(b)
(a)
Fig. 2
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Results III
• Model cases – 2D.– Simplified dose model.– beam orientations are selected using BEVD
method.
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OAR3 OAR3
OAR1
OAR2
0°
0°
90°PTV
OAR1
OAR2
90°PTV
Dose distribution of five IMRT beams selected with BEVD
Tolerance: 0.8
0.1
0.3
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Dose
Vol
ume,
%
PTVOAR3
EquiangularBEVD selected
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OAR
0° 0°
90°
PTV
OAR90°
PTV
Dose distribution of five IMRT beams selected with BEVD
Equally spaced five beams
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115 145 180 245215
Gantry angle
iS
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Dose
Vol
ume,
%
PTV
OAR
EquiangularBEVD-selected
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BEVD-guided beam optimization for IMRT
• In IMRT, the quality of a beam direction can be measured by a BEVD score.
• Select beams with the highest BEVD score, taking into account the angular separation of the beams.
• BEVD is a computer assisting tool, not an automated tool. It can, however, be used as a priori knowledge of the system to improve the beam orientation calculation.
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(b)
(a)
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Initial set of beam orientations
Vary a gantry angle randomly
Beam intensity profile optimization
Temperature low enough?
Reduce temperature
Optimized plan
No
Yes
Accept or reject the trial beam configuration based on SA probability
A. Simulated annealing (SA) optimization
Initial set of beam orientations
Vary a gantry angle randomly
Calculate probability of acceptance based on BEVD score
Optimized plan
Accept
RejectAccept or reject trial beam
configuration
B. BEVD-guided simulated annealing optimization
Beam intensity profile optimization
Accept or reject the trial beam configuration based on SA probability
Temperature low enough?
Reduce temperature
No
Yes
BEVD-Guided Beam Orientation Optimization
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95
Liver
GTV
70
Spinal cord Kidney
5030
0°
270°
180°
90°
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BEVD-Guided Beam Orientation Optimization
T=0 With annealing
• Formalism is applicable for functional imaging-guided IMRT.
• BEVD-guided optimization improves convergence behavior of the system.
• Number of iterations is reduced by a factor of 10.
• Results are consistent with BEVD ranking.
• Speed depends weakly on the cooling schedule.
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Conclusions
• A practical BEVD tool has been developed for beam orientation selection in IMRT.
• The technique allows one to select beam orientations without paying the excessive computing overhead of computer beam orientation optimization.
• The approach has considerable potential for simplifying the IMRT planning process to maximize the potential of IMRT.
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Conclusions
• Incorporation of prior knowledge significantly improves the calculation: speed and convergence.
• The methodology also applies to the optimization of other system parameters, like beamlet weights.
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ACKNOWLEDGEMENTS
S. Crooks, C. Cotrutz, S. Hunjan, J. Lian, D.Y. Yang, A Boyer, Q.T. Le, S.S. Donaldson, S. Hancock, C. King, A. Koong
This work was partially supported by grants from the American Cancer Society, Whitaker Foundation and Department of Defense.