Advanced variance reduction techniques applied to Monte Carlo simulation of linacs

Transcription

1 MAESTRO Advanced variance reduction techniques applied to Monte Carlo simulation of linacs Llorenç Brualla, Francesc Salvat, Eric Franchisseur, Salvador García-Pareja, Antonio Lallena Institut Gustave Roussy April

2 Simulation of a Linac with PENELOPE and Penmain, PenCT, Penfast, Penlinac 2

3 The codes PENMAIN: Steering main program for simulation of clinical linear accelerators and other irradiation devices, detectors,... PENGEOM: Constructuve quadric geometry. Ability to handle mobile parts (MLCs, filters, targets,...) PENCT: Simulation of electron-photon transport in CT structures (patient, phantoms) PENFAST: Fast code for the generation of dose distributions in CT structures. Combines simplifications in the physics interaction models with a condensed tracking algorithm for electrons and positrons. PENLINAC: Prepares PENMAIN input files for the simulation of linacs in various operation modes 3

4 Penmain Phase space file Photo: Institute Gustave Roussy, France PenCT, Penfast 4

5 PENLINAC. Building linac simulation files PENLINAC is a Fortran code that prepares PENMAIN input files for the simulation of linacs in various operation modes (photon or electron beams, nominal energies,...) It creates the geometry, input and material files for PENMAIN Automatically incorporates variance reduction techniques adapted to each particular situation (off-axis, energy, operational mode,...) It uses a modular library of geometry elements (target, flattening filters, applicators, etc). It works only for a limited set of machines, whose detailed geometries have previously been defined (and validated) Required inputs: only those known to the medical physicist. Namely, brand and model, mode (electron or photon), nominal energy, applicator, field size, MLC position. 5

6 Primary collimator Beryllium window Flattening filter Ionization chamber Varian Clinac 2100 C/D 6MV, photon mode (Generated with Penlinac) Jaws MLC 6

7 Scattering foils Target Flattening filter Electron mode Photon mode Electron applicator 7

8 Scattering foils (Varian 2100C/D) 8

9 Electron applicator (Varian 2100C/D) 9

10 Electron applicators 6x6 cm 2 25x25 cm 2 10

11 Multileaf collimator Flattening filter Primary collimator Jaws Tongue and groove 11

12 Simulation of a multileaf collimator Screenshot courtesy of Institut Gustave Roussy 12

13 Variance reduction techniques Standard techniques: Russian roulette Interaction forcing Splitting Advanced techniques: Particle killing phase space planes Variable skins Symmetry splitting (rotational, 4-fold, spot) Ants colony (in development) 13

14 Particle killing phase space planes Particle killing PSPs 2D body-view of Varian 2100 C/D 14

15 Variable skins Skins Electron-absorbing bodies 2D body-view of Varian 2100 C/D 15

16 Lateral body-view of two closed leaves Skins Electron absorbing bodies Electron absorbing bodies Main beam direction 16

17 Symmetry splitting Rotational Splitting Original particle Splitted particle 4-fold Splitting 17

18 Ant colony method Low-energy High-energy electrons in air electrons in materials different from air 18

19 Variance reduction on MLCs Simplified geometry Variable skins One, two or three Compton events Penfast physics for Compton interactions (to be implemented) 19

20 MLC geometry simplifications Detailed MLC, material view 20

21 MLC geometry simplifications Detailed MLC, body view 1,100 bodies. 1,400 surfaces. 20,000 lines of code 20

22 MLC geometry simplifications Simplefied MLC, material view 20

23 MLC geometry simplifications Simplefied MLC, body view 250 bodies. 200 surfaces. 3,900 lines of code 20

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27 Results 23

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29 Results 24

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33 Results 27

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35 Results 28

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37 MLC simulation Detailed, full Simplified, full One Compton, No skin Two Compton, No skin Three Compton, No skin One Compton, Skin=5.0 mm One Compton, Skin=2.5 mm Two Compton, Skin=5.0 mm Efficiency % difference Simulation time Detailed full : 1.4 hours on a 2.8 GHz Intel Core 2 Duo Extreme. Compiler: Intel Fortran (option -fast). 1x10 7 primary photons. 30

38 Conclusions Penmain simulations of photon beams run 40 times faster when advanced variance reduction techniques were used (rotational splitting, particle killing PSPs, skins) Penmain simulations of electron beams run 30 times faster when ants colony method, particle killing PSPs and skins were used Penfast is about 20 times faster than PenCT both using the same absorption energies and interaction forcing parameters Most of the variance reduction techniques are problem dependent, but their usage can be automated by means of Penlinac 31