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Four-Dimensional Monte Carlo Using the TOPAS TOol for PArticle Simulation


J Perl

J Perl1*, J Shin2, J Schuemann3, H Paganetti4, B Faddegon5, (1) SLAC National Accelerator Laboratory, Menlo Park, CA, (2) St. Jude Children's Research Hospital, Memphis, TN, (3) Massachusetts General Hospital, Boston, MA, (4) Massachusetts General Hospital, Boston, MA, (5) UC San Francisco, San Francisco, CA

TU-A-108-1 Tuesday 8:00AM - 9:55AM Room: 108

Purpose:
As the number and diversity of 4D radiation therapy treatment modalities increases, with techniques from IMRT, VMAT, SBRT, IGRT and IMPT to proton and ion beam scanning, the value of a comprehensive 4D Monte Carlo dose calculation tool has become evident. In silico experiments provide a powerful platform to study sensitivity of dose calculation techniques to motion errors, beam instabilities and interplay effects. We present a comprehensive system to meet this growing need.

Methods:
We have provided rich 4D capabilities within the TOPAS TOol for PArticle Simulation. TOPAS allows any one or more simulation parameter to vary over time, such as a beam current, the geometrical position of a treatment head component, a magnetic field, a 4D CT time slice or a gating signal for scoring. In use at 25 research institutions, TOPAS has been brought to bear on a wide range of 4D problems. We present 4D capabilities, including modulator wheel rotation during beam current modulation, beam scanning, MLC leaf motion, gantry rotation and 4D CT.

Results:
It is imperative that 4D simulation tools provide 4D-scoring capabilities, such as scorers that can be gated by time, or phase space output that can associate time to each saved particle. Only with such tools can one confirm that the simulation is accurate in not just three dimensions but four. We show such results that confirm TOPAS 4D performance, both as time-sliced histograms and through simulated 4D radiography (simulated films darkening as dose accumulates over time).

Conclusion:
TOPAS has proven to be a comprehensive 4D simulation tool. While current users are mainly in proton therapy, TOPAS is being extended for use in ion, x-ray, electron and brachytherapy. While there are many excellent MC tools already for some of these modalities, TOPAS is unique in the 4D domain.

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