Biomedical Engineering Reference
In-Depth Information
Interaction forcing is particularly useful when a better statistics is required on
particles travelling through thin material layers or low density materials. For
example, water is nearly transparent to photons on the order of a few MeV, so lower
uncertainties are obtained when Compton interactions are forced.
Range rejection: Charged particles that have travelled far away from the zone of
interest and whose chance of contributing to the tallied quantities is negligible
are discarded so to avoid loosing computation time while simulating them. The
technique can be combined with Russian roulette (see below) in order to keep the
simulation unbiased.
Russian roulette: It is about discarding particles whose probability to contributing
to the tallied quantities is low, or whose contribution to the tallied quantities will be
low due to a low statistical weight. Particles are eliminated from the simulation with
probability
. In order to keep the simulation unbiased, particles that survive a
Russian roulette have their weight increased by a factor
K<1
1=.1 K/
.
Splitting: A particle which has a large probability of contributing to the final scored
quantity is split, that is,
copies of it are made and then each of this copies is
independently simulated. The statistical weight of each split particle is multiplied by
a factor
n
in order to keep the simulation unbiased. Figure
19.2
shows a graphical
representation of splitting applied to the simulation of a Varian Clinac 2100 C/D.
Rotational splitting: It is applied to particles travelling through geometries
with cylindrical symmetry when the primary source has also the same kind of
symmetry. This is the case of Varian Clinacs from the primary source downstream
to the ionisation chamber. It is a kind of splitting in which each split particle is
rotated about the central beam axis an azimuthal angle [
19
]. The azimuthal angle
between two neighbouring split particles is given by
1=n
. The particle
direction cosines of each replica are transformed in order to keep the direction of
the original particle relative to the central beam axis. See Fig.
19.2
.
Fan splitting: It is a particular case of rotational splitting in which the particles that
have travelled through a cylindrically symmetric geometry are split in a preferred
direction. Fan splitting is suited for off-axis fields [
26
]. The rationale is to split
particles in the direction of the off-axis field and hence increase the speed at which
the final score quantity is tallied. Figure
19.2
shows how particles are split in a
preferred direction covering the off-axis field.
Movable skins: The movable-skin method consists of defining relevant zones of
the geometry in which an accurate transport of radiation is performed, whereas in
less relevant zones the transport of some particles is discontinued [
16
]. By dividing
collimating structures, such as, collimators, jaws, multileaf collimators, etc. in zones
located near and away from the beam axis it is possible to simulate particles that will
contribute to the penumbra and, at the same time, simplify the transport of particles
that have a small chance of leaving the corresponding body. The thickness of the
zone in which accurate simulation is performed, that is the skin, depends on the
kind of particle and the energy. The surfaces dividing skin from non-skin regions
can be moved in order to adapt the thickness to a given configuration of the linac.
D 2=n
