  We have used the TOPAS Geant4-based package to create a parametric simulation
of a new TOF-PET scanner design based on a low atomic number scintillator.
TOPAS was extended to provide complete information on individual particle
position, energy, and identity, allowing access to all particle interactions in
the simulation. The data were then used as the 'truth' information of particle
kinematics for the simulation of the TOF-PET scanner, as described in a
separate paper. The Derenzo and XCAT phantoms were added to the simulation with
in-patient scattering, positron emission placement, and positron diffusion.
Running Geant4 simulations through TOPAS requires substantially smaller amounts
of user training than would be needed in raw Geant4.
  This paper describes the methods used to simulate the low-Z whole-body
scanner at reduced dose. Specifically, this paper covers the simulation of the
FDG decay energy spectrum; the simulation of tissue densities, atomic
compositions, and radiotracer doses within imaging phantoms with doses; the
custom TOPAS extension used to record particle interaction data; computational
performance; and required Geant4 libraries used in TOPAS.
