MCPI, an accelerated Monte Carlo dose-calculation engine for real-time prostate implant dosimetry

Purpose: To present a new accelerated Monte Carlo code (MCPI: Monte Carlo for Prostate Implant) intended for use as a dose calculation engine for planning clinical prostate implants. MCPI simulates physically a set of radioactive seeds with arbitrary positions and orientations, merged in a 3D CT‐based heterogeneous phantom representing the prostate and surrounding tissue. Method and Materials: MCPI uses a phase space data source‐model to account for seed self‐absorption and seed anisotropy. A “hybrid geometry” model (full 3D seed geometry merged in a 3D mesh of voxels) is developed for rigorous treatment of the interseed attenuation effect. MCPI is based upon the GEPTS general‐purpose Monte Carlo code. Compton scattering, coherent scattering, and photoelectric effect (with emission of fluorescence X‐rays) are modeled in detail, using the XCOM/EPDL97/NIST95 cross‐section data. MCPI is benchmarked against the MCNP5 code for the case of an idealized prostate implant, consisting of 83 [formula omitted] (or [formula omitted]) seeds. Results: MCNP5 and MCPI are in excellent agreement. The average difference between the dose distributions from the two codes is less than 0.5% for both seed models. For a 2×2×2‐mm3 voxel mesh, MCPI calculates the [formula omitted] and [formula omitted] prostate dose distributions with 2% average statistical uncertainty in 2.1 to 2.2 minutes using a single Pentium 4 PC. More than 3 hours calculation time is required for MCNP5 to achieve the same statistical precision. MCPI is about 90 and 700 times faster than MCNP5 for 2 and 1‐mm3 voxels, respectively. Conclusion: The use of multiprocessor parallel calculation can further increase the speed of MCPI and makes sub‐minute dose calculations for prostate implant planning a reality. © 2005, American Association of Physicists in Medicine. All rights reserved.