  In this study, cone-beam single projection and axial CT scans are modeled
with a software package - DOCTORS, which solves the linear Boltzmann equation
using the discrete ordinates method. Phantoms include a uniform 35 cm diameter
water cylinder and a non-uniform abdomen phantom. Series simulations were
performed with different simulation parameters, including the number of
quadrature angles, the order of Legendre polynomial expansions, and coarse and
fine mesh grid. Monte Carlo simulations were also performed to benchmark
DOCTORS simulations. A quantitative comparison was made between the simulation
results obtained using DOCTORS and Monte Carlo methods. The deterministic
simulation was in good agreement with the Monte Carlo simulation on dose
estimation, with a root-mean-square-deviation (RMSD) difference of around 2.87
percent. It was found that the contribution of uncollided photon fluence
directly from the source dominates the local absorbed dose in the diagnostic
X-ray energy range. The uncollided photon fluence can be calculated accurately
using a ray-tracing algorithm. The accuracy of collided photon fluence
estimation is largely affected by the pre-calculated multigroup cross-sections.
The primary benefit of DOCTORS lies in its rapid computation speed. Using
DOCTORS, parallel computing using GPU enables the cone-beam CT dose estimation
nearly in real-time.
