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Evangelos Matsinos, Barbara Schaffner, Wolfgang Kaissl Varian Medical Systems Baden, Switzerland Technology for people Better technology. Better outcomes.

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Presentation on theme: "Evangelos Matsinos, Barbara Schaffner, Wolfgang Kaissl Varian Medical Systems Baden, Switzerland Technology for people Better technology. Better outcomes."— Presentation transcript:

1 Evangelos Matsinos, Barbara Schaffner, Wolfgang Kaissl Varian Medical Systems Baden, Switzerland Technology for people Better technology. Better outcomes. Optimisation in proton scanning beams First results…

2 Better technology. Better outcomes. EM, May 2002 Page 2 Basic steps Calculation of the dose-deposition coefficients (ddcs) Optimisation of the spot weights

3 Better technology. Better outcomes. EM, May 2002 Page 3

4 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)

5 Better technology. Better outcomes. EM, May 2002 Page 5 beamlet spot depends on the density and on z S x and S y depend on z

6 Better technology. Better outcomes. EM, May 2002 Page 6 Optimisation: a closer look Desired dose at point i: p i Dose delivered at point i: d i = a ij x j (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: F obj = (d i - p i ) 2 (sum over all sources j)

7 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)

8 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%

9 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

10 Better technology. Better outcomes. EM, May 2002 Page 10 Target: 2,412 points, cm 3 Distal Organ: 2,166 points, cm 3 Proximal Organ: 683 points, cm 3 Number of points: 5,261

11 Better technology. Better outcomes. EM, May 2002 Page 11 Number of parameters: 4,798

12 Better technology. Better outcomes. EM, May 2002 Page 12 ProtonHelios

13 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

14 Better technology. Better outcomes. EM, May 2002 Page 14 Dose distribution (exclusive fit to the target)

15 Better technology. Better outcomes. EM, May 2002 Page 15 Dose distribution (fit to all structures)

16 Better technology. Better outcomes. EM, May 2002 Page 16 Comparison of a few numbers Method Minimal F obj Relative time Target dose (Gy) Maximal weight CG36, SA35, PSI37, GSPM

17 Better technology. Better outcomes. EM, May 2002 Page 17 Weight distribution PSI method

18 Better technology. Better outcomes. EM, May 2002 Page 18 Weight distribution SA method

19 Better technology. Better outcomes. EM, May 2002 Page 19 A head tumour

20 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.

21 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?


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