SU-E-T-476: GPU-Based Monte Carlo Radiotherapy Dose Calculation Using Phase- Space Sources.

PURPOSE To design an efficient method for utilizing phase-space source models in the GPU-based Monte Carlo (MC) dose calculation engine gDPM. METHODS In GPU-based MC algorithms, particles are transported in parallel on different threads. Particles of different types and energies can require significantly different execution times. This can cause "thread divergence" and lower efficiency when source particles are read sequentially from a phase-space file. We have developed a strategy for utilizing phase- space files in a GPU compatible manner whereby the particles are grouped into phase-space-lets (PSLs) by type, energy, and location in the phase- space plane. This allows for dose calculations using only particles inside the field opening defined by the secondary collimators. For validation, the gDPM PSL implementation is compared with DOSXYZnrc using a BEAMnrc phase-space source model as input. RESULTS Two phase-spaces were generated using a BEAMnrc head model of a 6MV Varian Clinac 21EX, one above the upper jaws used to generate PSLs for gDPM and the other below the lower jaws used for DOSXYZnrc dose calculation. Profiles and depth dose curves for a variety of field sizes were generated in a water phantom. The agreement between gDPM and DOSXYZnrc is within 2% for all field sizes. For the 10 cm × 10 cm field, the calculation times of 650 million histories were 147 CPU hours and 54 GPU seconds for DOSXYZnrc and gDPM, respectively. In addition, we have tested the gDPM PSL implementation for dose calculation in a realistic 7-field IMRT tongue treatment plan. The calculation times were 59 CPU-hours and 66 GPU- seconds for DOSXYZnrc and gDPM for 485 million histories, respectively. Gamma pass rate for the two dose distributions was 99.54% for 3 mm/3% criteria within the 10% isodose. CONCLUSIONS Methods for the efficient use of phase-space sources for GPU-based MC dose calculations have been developed.