1. Improvement of the Monte Carlo Simulation Efficiency of a Proton Therapy Treatment Head Based on Proton Tracking Analysis and Geometry Simplifications Miguel A. Cortés-Giraldo*, José M. Quesada, M. Isabel Gallardo (Universidad de Sevilla) Harald Paganetti (Massachusetts General Hospital - Boston, MA, USA) 6th DITANET Topical Workshop on Particle Detection Techniques Seville (Spain) November 8th, 2011 (*) e-mail: miancortes@us.es

2. Contents  Introduction  Methods  Results  Conclusions

3.  Introduction  Methods  Results  Conclusions

4. Motivation  Monte Carlo (MC) simulations are:  A precise technique to calculate dose in patients…  but expensive in terms of CPU time.  The aim of this work is:  To decrease the CPU time needed to create a phase-space file in the MC simulation of a passive scattering proton therapy treatment head.  To develope techniques capable of increasing the computational efficiency in the simulation of nozzles with similar geometry.

5. The MC code (phase-space files) Geant4.9.0.p01 Only proton tracking is taken into account in detail in order to create a phase-space file as fast as possible. Secondary radiation is evaluated separately Monte Carlo treatment head model: Paganetti et al. Med. Phys. 31:2107-18 (2004) Physics settings (Geant4 physics list): Zacharatou and Paganetti IEEE-TNS 55:1018-25 (2008) Francis H Burr Proton Therapy Center (Boston, MA, USA)

6.  Introducton  Methods  Results  Conclusions

7. Methodology  The efficiency improvement is evaluated for various nozzle set-ups:  Covering the energy range of the proton beam.  Output efficiency: 25-cm (maximum) and 12-cm diameter snout (most typical case in proton therapy).  Validation with published results.  Identical computational conditions. (Paganetti et al. Med. Phys. 31:2107-18, 2004.)

8. Time spent along the nozzle IC2 2 nd scatterer RMW IC1

9. Proton tracking filtering  The basic idea is to terminate the tracking of protons which, very likely, will not reach the aperture

10. Proton tracking filtering  There is a strong correlation of the protons reaching the nozzle exit and their dynamical conditions at the exit of the scatterer. An example… A tolerance margin is taken into account. Open field conditions.

11. Simplifications of the monitor chambers  A detailed geometry model of the monitor chambers slows down the MC simulation.  Considering all the layers grouped together simplifies the tracking of particles, improving the efficiency.

12. Production cuts per region  Production cut: key parameter in Geant4 simulations.  The secondary production cut value is higher in regions filled by air (magnets, jaws…)  The scattering and modulation devices require a lower value of the production cuts.

13.  Introduction  Methods  Results  Conclusions

14. Proton tracking filtering The efficiency increases by about 30% with a 12 cm snout. In the worst case scenario (25 cm), it improves by about 5%.

15. Simplifications of the monitor chambers The efficiency improvement varies between 5% and 15%. The improvement increases with the proton beam range

16. Production cuts per region  0.2 mm for devices filled by air (jaws, aperture…); the CPU time decreases by about 5%.  For scatterers and modulators the production cut value is 0.05mm. Using a global production cut value too high may change the energy distribution at the exit of the nozzle. (Geant4.9.0.p01)

17. Output fluence verification 12 cm diameter snout Range = 12.00 cm Modulation width = 4.0 cm

18. Output fluence verification 12 cm diameter snout Range = 17.19 cm Modulation width = 6.78 cm

19. New time profiles 12 cm diameter snout25 cm diameter snout

20.  Introduction  Methods  Results  Conclusions

21. Conclusions  We have developed techniques to increase the computational efficiency of Geant4 simulations to obtain phase-space files of a passive scattering proton therapy nozzle.  For the most typical case in the facility, the efficiency increases by about 35%; in the worst case scenario, it improves by about 15%.  These techniques can be applied to other treatment heads, simulated either with Geant4 or another MC transport code.

22. Acknowledgements  Ministerio de Ciencia e Innovación (P07-FQM-02894 y FIS2008-04189).  Junta de Andalucía (FPA2008-04972-C03-02).  PO1 Grant.  Physics Research group at Dep. Radiation Oncology (Massachusetts General Hospital, Boston, MA, USA).