  Online adaptive radiation therapy (ART) is an attractive concept that
promises the ability to deliver an optimal treatment in response to the
inter-fraction variability in patient anatomy. However, it has yet to be
realized due to technical limitations. Fast dose deposit coefficient
calculation is a critical component of the online planning process that is
required for plan optimization of intensity modulated radiation therapy (IMRT).
Computer graphics processing units (GPUs) are well-suited to provide the
requisite fast performance for the data-parallel nature of dose calculation. In
this work, we develop a dose calculation engine based on a finite-size pencil
beam (FSPB) algorithm and a GPU parallel computing framework. The developed
framework can accommodate any FSPB model. We test our implementation on a case
of a water phantom and a case of a prostate cancer patient with varying beamlet
and voxel sizes. All testing scenarios achieved speedup ranging from 200~400
times when using a NVIDIA Tesla C1060 card in comparison with a 2.27GHz Intel
Xeon CPU. The computational time for calculating dose deposition coefficients
for a 9-field prostate IMRT plan with this new framework is less than 1 second.
This indicates that the GPU-based FSPB algorithm is well-suited for online
re-planning for adaptive radiotherapy.
