Definition of phase-space (phsp) data
A phsp is defined as a collection of representative pseudo-particles
emerging from a radiation therapy treatment source along with their
properties that include energy, particle type, position, direction,
progeny and statistical weight. Every pseudo-particle in the phsp
should be scored in such a way that it is recorded only once during
the course of passage through the scoring surface (i.e., no further
recording of the same particle or descendents). A phsp can take the
form of a computer file containing the detailed description of the
phsp particles, i.e., phsp variables as generated via a Monte-Carlo
simulation of the treatment source, or the form of a computer code
(event generator) which simulates the treatment source using either
a full Monte-Carlo simulation or beam model of the radiation therapy
source.