The TOPAS tool for particle simulation, a Monte Carlo simulation tool for physics, biology and clinical research

Abstract

Purpose: This paper covers recent developments and applications of the TOPAS TOol for PArticle Simulation and presents the approaches used to disseminate TOPAS.

Materials and methods: Fundamental understanding of radiotherapy and imaging is greatly facilitated through accurate and detailed simulation of the passage of ionizing radiation through apparatus and into a patient using Monte Carlo (MC). TOPAS brings Geant4, a reliable, experimentally validated MC tool mainly developed for high energy physics, within easy reach of medical physicists, radiobiologists and clinicians. Requiring no programming knowledge, TOPAS provides all of the flexibility of Geant4.

Results: After 5 years of development followed by its initial release, TOPAS was subsequently expanded from its focus on proton therapy physics to incorporate radiobiology modeling. Next, in 2018, the developers expanded their user support and code maintenance as well as the scope of TOPAS towards supporting X-ray and electron therapy and medical imaging. Improvements have been achieved in user enhancement through software engineering and a graphical user interface, calculational efficiency, validation through experimental benchmarks and QA measurements, and either newly available or recently published applications. A large and rapidly increasing user base demonstrates success in our approach to dissemination of this uniquely accessible and flexible MC research tool.

Conclusions: The TOPAS developers continue to make strides in addressing the needs of the medical community in applications of ionizing radiation to medicine, creating the only fully integrated platform for four-dimensional simulation of all forms of radiotherapy and imaging with ionizing radiation, with a design that promotes inter-institutional collaboration.

Figure 1

A portion of the C++ code that a user of native Geant4 would need to write to define a simple setup of a cylinder of water centered in a world of air (with some details removed for space). Inset shows the equivalent full set of TOPAS parameters for this setup.

Figure 2.

Left: Secondary neutron yield from low energy protons hitting a lithium thick target using uniform splitting VRT, with comparison to measured data. Middle: TOPAS simulation showing the electrons (red) produced by x-rays (not shown) from a lead target (in magenta) using directional bremsstrahlung VRT in the target and importance sampling in cells located downstream of the target (parallel lines). Right: Importance maps calculated using TOPAS for a geometry-based VRT applied to a generic electron treatment head.

Figure 3.

Examples of Independent dose calculations using the TOPAS DICOM interface, (A) Patient specific QA and (B) a patient dose recalculation. The interface sets up patient specific geometries, such as range shifter (RS), aperture (APT), water phantom (WC), and dose-grid (DG), including translational and rotational movements.

Figure 4

TOPAS brachytherapy example. Top: HDR brachytherapy treatment with a Varian source wire at one dwell position in a simulated gynecological applicator. Bottom: Close-up of the wire (blue) showing gamma-rays (green) emitted from randomly sampled positions within the Ir192 source capsules (white)