title: ARCHERRT - a GPU-based and photon-electron coupled Monte Carlo dose computing engine for radiation therapy: software development and application to helical tomotherapy.
abstract: OBJECTIVE
  Using the graphical processing units (GPU) hardware technology, an extremely fast Monte Carlo (MC) code ARCHERRT is developed for radiation dose calculations in radiation therapy. This paper describes the detailed software development and testing for three clinical TomoTherapy® cases: the prostate, lung, and head & neck.
  
  
  METHODS
  To obtain clinically relevant dose distributions, phase space files (PSFs) created from optimized radiation therapy treatment plan fluence maps were used as the input to ARCHERRT. Patient-specific phantoms were constructed from patient CT images. Batch simulations were employed to facilitate the time-consuming task of loading large PSFs, and to improve the estimation of statistical uncertainty. Furthermore, two different Woodcock tracking algorithms were implemented and their relative performance was compared. The dose curves of an Elekta accelerator PSF incident on a homogeneous water phantom were benchmarked against DOSXYZnrc. For each of the treatment cases, dose volume histograms and isodose maps were produced from ARCHERRT and the general-purpose code, GEANT4. The gamma index analysis was performed to evaluate the similarity of voxel doses obtained from these two codes. The hardware accelerators used in this study are one NVIDIA K20 GPU, one NVIDIA K40 GPU, and six NVIDIA M2090 GPUs. In addition, to make a fairer comparison of the CPU and GPU performance, a multithreaded CPU code was developed using OpenMP and tested on an Intel E5-2620 CPU.
  
  
  RESULTS
  For the water phantom, the depth dose curve and dose profiles from ARCHERRT agree well with DOSXYZnrc. For clinical cases, results from ARCHERRT are compared with those from GEANT4 and good agreement is observed. Gamma index test is performed for voxels whose dose is greater than 10% of maximum dose. For 2%/2mm criteria, the passing rates for the prostate, lung case, and head & neck cases are 99.7%, 98.5%, and 97.2%, respectively. Due to specific architecture of GPU, modified Woodcock tracking algorithm performed inferior to the original one. ARCHERRT achieves a fast speed for PSF-based dose calculations. With a single M2090 card, the simulations cost about 60, 50, 80 s for three cases, respectively, with the 1% statistical error in the PTV. Using the latest K40 card, the simulations are 1.7-1.8 times faster. More impressively, six M2090 cards could finish the simulations in 8.9-13.4 s. For comparison, the same simulations on Intel E5-2620 (12 hyperthreading) cost about 500-800 s.
  
  
  CONCLUSIONS
  ARCHERRT was developed successfully to perform fast and accurate MC dose calculation for radiotherapy using PSFs and patient CT phantoms.
authors.affiliation: Nuclear Engineering Program, Rensselaer Polytechnic Institute, Troy, New York 12180.
authors.forename: Lin
authors.initials: L
authors.lastname: Su
authors.affiliation: Medical Physics, University of Wisconsin, Madison, Wisconsin 53706.
authors.forename: Youming
authors.initials: Y
authors.lastname: Yang
authors.affiliation: Medical Physics, University of Wisconsin, Madison, Wisconsin 53706.
authors.forename: Bryan
authors.initials: B
authors.lastname: Bednarz
authors.affiliation: Molecular Imaging, Radiotherapy and Oncology, Université catholique de Louvain, Brussels, Belgium 1348.
authors.forename: Edmond
authors.initials: E
authors.lastname: Sterpin
authors.affiliation: Nuclear Engineering Program, Rensselaer Polytechnic Institute, Troy, New York 12180.
authors.forename: Xining
authors.initials: X
authors.lastname: Du
authors.affiliation: Nuclear Engineering Program, Rensselaer Polytechnic Institute, Troy, New York 12180.
authors.forename: Tianyu
authors.initials: T
authors.lastname: Liu
authors.affiliation: Nuclear Engineering Program, Rensselaer Polytechnic Institute, Troy, New York 12180.
authors.forename: Wei
authors.initials: W
authors.lastname: Ji
authors.affiliation: Nuclear Engineering Program, Rensselaer Polytechnic Institute, Troy, New York 12180.
authors.forename: X George
authors.initials: XG
authors.lastname: Xu
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D003201:Computers; D004583:Electrons; D005720:Gamma Rays; D006258:Head and Neck Neoplasms / Q000532:radiotherapy; D006801:Humans; D008175:Lung Neoplasms / Q000532:radiotherapy; D008297:Male; D008954:Models, Biological; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012984:Software; D013997:Time Factors; D014057:Tomography, X-Ray Computed