Accelerated dose calculation engine for interstitial brachytherapy

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. Material and Methods: 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 ¹⁰³Pd (or ¹²⁵I) 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 ¹⁰³Pd and ¹²⁵I prostate dose distributions with 2% average statistical uncertainty in 23 seconds using a single Intel Core2 processor. More than 30 minutes 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. MCPI is used to quantify the effect of calcification on prostate DVH: 5% calcified volume decreases D₁₀₀ by 58%. Conclusion: The use of multiprocessor parallel calculation can further increase the speed of MCPI and makes quasi-instantaneous dose calculations for prostate implant planning a reality. MCPI can be easily extended to handle other interstitial brachytherapy modalities (e.g. wire sources).