Geometrical Particle Splitting

TOPAS variance reduction is further described in:

Ramos-Mendez et al, “Geometrical splitting technique to improve the computational efficiency in Monte Carlo calculations for proton therapy,” Med. Phys. 40, 041718 (2013)

This technique was designed for heavy charged particles. In this implementation, you must specify whether the beam entering into the sub-component has cylindrical symmetry or not. This is because the particles may or may not be randomly redistributed around the SplitAxis.

The Russian roulette is applied in a particular direction. That is, at the split plane and prior to being split, the particle is subject to the Russian roulette if its direction does not point towards a Region of Interest (ROI). Then the radius of the ROI and its position on the SplitAxis must to be defined too. Further, the Russian roulette can be turned on/off at specific surfaces between sub-components.

s:Vr/ParticleSplit/Type = "GeometricalParticleSplit"
s:Vr/ParticleSplit/SplitAxis = "zaxis"
d:Vr/ParticleSplit/RussianRoulette/ROIRadius = 7.8 cm
d:Vr/ParticleSplit/RussianRoulette/ROITrans = 10 cm
bv:Vr/ParticleSplit/RussianRoulette = 2 "false" "true"

To set whether the region at each sub-component is symmetric or not and to define the corresponding split number:

bv:Vr/ParticleSplit/Symmetric = 2 "false" "true"
uv:Vr/ParticleSplit/SplitNumber = 2 8 8

In addition for this technique, geometrical Russian roulette will be played if a particle leaves the component or the world in a scheme similar to the Importance Sampling technique.