  Purpose: The presence of respiratory motion during radiation treatment leads
to degradation of the expected dose distribution, both for target coverage and
healthy-tissue sparing, particularly for techniques like pencil-beam scanning
proton therapy which have dynamic delivery systems. While tools exist to
estimate this degraded four-dimensional (4D) dose, they typically have one or
more deficiencies such as ...
  Methods: To quickly compute the 4D-dose, the three main tasks of the
calculator were run on graphics processing units (GPUs). These tasks were:
simulating the delivery of the plan using measured delivery parameters to
distribute the plan amongst 4DCT phases characterizing the patient breathing,
using an in-house Monte Carlo simulation (MC) dose calculator to determine the
dose delivered to each breathing phase, and accumulating the doses from the
various breathing phases onto a single phase for evaluation. The accumulation
was performed by individually transferring the energy and mass of dose-grid
subvoxels, a technique models the transfer of dose in a more physically
realistic manner. The calculator was run ...
  Results: 4D doses were successfully computed for the three test cases with
computation times ranging from 4-6 min on a server with eight NVIDIA Titan X
graphics cards; the most time-consuming component was the MC dose engine. The
subvoxel-based dose-accumulation technique produced stable 4D-dose
distributions at subvoxel scales of 0.5-1.0 mm without impairing the total
computation time. The uncertainties in the beam-delivery simulation ...
  Conclusions: A MC-based and GPU-accelerated 4D-dose calculator was developed
to estimate the effects of respiratory motion on pencil-beam scanning proton
therapy treatments. The calculator can currently be used ...
