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Optimisation in proton scanning beams. First results…. Basic steps. Calculation of the dose-deposition coefficients (ddc’s) Optimisation of the spot weights. Spots and beamlets. Beam beamlets or pencil beams (defined by the resolution of the calculation grid) - PowerPoint PPT Presentation
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Evangelos Matsinos, Barbara Schaffner, Wolfgang Kaissl
Varian Medical SystemsBaden, Switzerland
Technology for peopleBetter technology. Better outcomes.
Optimisation in proton scanning beams
First results…
Better technology. Better outcomes. EM, May 2002 Page 2
Basic steps
Calculation of the dose-deposition coefficients (ddc’s)
Optimisation of the spot weights
Better technology. Better outcomes. EM, May 2002 Page 3
Better technology. Better outcomes. EM, May 2002 Page 4
Spots and beamlets
Beam beamlets or pencil beams (defined by the resolution of the calculation grid)
The dose from each beamlet is evaluated (at the vertices of the calculation grid)
The spot dose is calculated (as the sum of the dose contributions of the corresponding beamlets, weighted for the position of each beamlet within the spot)
Better technology. Better outcomes. EM, May 2002 Page 5
beamlet
spot
depends on the density and on z
Sx and Sy depend on z
Better technology. Better outcomes. EM, May 2002 Page 6
Optimisation: a closer look
Desired dose at point i: pi
Dose delivered at point i: di = aij xj
(sum over target points)
+ contribution due to the violation of dose-limit constraints (for targets and
organs)+ contribution due to the violation of dose-volume constraints (for organs)
Objective function: Fobj = (di - pi)2
(sum over all sources j)
Better technology. Better outcomes. EM, May 2002 Page 7
Optimisation methods
Conjugate Gradient (CG)
Simulated Annealing (SA)*
‘Simultaneous’ optimisation (PSI)
Generalised Sampled Pattern Matching (GSPM)*
(* = under development)
Better technology. Better outcomes. EM, May 2002 Page 8
Strategy in the optimisation Pre-optimisation Reasonable initial ‘guess’ for the weights Convergence two consecutive iterations yield improvement below 5% Main optimisation Full implementation of a method Convergence two consecutive iterations yield improvement below 0.1%
Better technology. Better outcomes. EM, May 2002 Page 9
Toy example
A phantom has been created with three important structures: one target and two organs; some inhomogeneity has been introduced (an additional structure simulating the presence of a bone)
Pixel size: 2.5mm Spot advance in y (scanning direction): 2.5mm Spot advance in x: 5mm Cut-off for dose contributions: 3 standard
deviations
Better technology. Better outcomes. EM, May 2002 Page 10
Target: 2,412 points, 57.27 cm3
Distal Organ: 2,166 points, 48.34 cm3
Proximal Organ: 683 points, 15.49 cm3
Number of points: 5,261
Better technology. Better outcomes. EM, May 2002 Page 11
Number of parameters: 4,798
Better technology. Better outcomes. EM, May 2002 Page 12
ProtonHelios
Better technology. Better outcomes. EM, May 2002 Page 13
Dose-Volume Histograms
Prescription dose: 50 Gy ( 2%)Organ constraints: 25 Gy in 10% of the distal organ;15 Gy in the
proximal organ
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Dose distribution (exclusive fit to the target)
Better technology. Better outcomes. EM, May 2002 Page 15
Dose distribution (fit to all structures)
Better technology. Better outcomes. EM, May 2002 Page 16
Comparison of a few numbers
Method
Minimal Fobj
Relative time
Target dose (Gy)
Maximal
weight
CG 36,902 3.8749.6
2.316.03
SA 35,621 4.9249.6
2.317.15
PSI 37,440 1.0049.7
2.351.97
GSPM --- 1.3349.9
1.96.74
Better technology. Better outcomes. EM, May 2002 Page 17
Weight distribution
PSI method
Better technology. Better outcomes. EM, May 2002 Page 18
Weight distribution
SA method
Better technology. Better outcomes. EM, May 2002 Page 19
A head tumour
Better technology. Better outcomes. EM, May 2002 Page 20
Conclusions
As far as the dose distribution is concerned, three optimisation methods (CG, SA, and PSI) yield results which seem to be in good agreement. Very similar dose distributions may be obtained on the basis of very different weight distributions.
The use of raw (unfiltered) weights does not seem to create cold/hot spots within the irradiated volume. It remains to be seen whether, in some occasions, filtering will be called for.
Better technology. Better outcomes. EM, May 2002 Page 21
Under consideration…
Other forms of the objective function to be tried?
Strategy in the optimisation: an improvement of about 25% was found in the execution time in case that the target dose is firstly optimised (with vanishing dose everywhere else)
Other optimisation methods to be tried?