A CT‐based Monte Carlo simulation tool for dosimetry planning and analysis

The Los Alamos code MCNP4A (M_onte C_arlo N_‐P_article version 4A) is currently used to simulate a variety of problems ranging from nuclear reactor analysis to boron neutron capture therapy. A graphical user interface has been developed that automatically sets up the MCNP4A geometry and radiation source requirements for a three‐dimensional Monte Carlo simulation using computed tomography data. The major drawback for this dosimetry system is the amount of time to obtain a statistically significant answer. A specialized patch file has been developed that optimizes photon particle transport and dose scoring within the standard MCNP4A lattice geometry. The transport modifications produce a performance increase (number of histories per minute) of approximately 4.7 based upon a 6 MV point source centered within a 30 × 30 × 30   cm 3lattice water phantom and 1 × 1 × 1   mm 3voxels. The dose scoring modifications produce a performance increase of approximately 470 based upon a tally section of greater than1 × 10 4lattice elements and a voxel size of 5   mm 3 . Homogeneous and heterogeneous benchmark calculations produce good agreement with measurements using a standard water phantom and a high‐ and low‐density heterogeneity phantom. The dose distribution from a typical mediastinum treatment planning setup is presented for qualitative analysis and comparison versus a conventional treatment planning system.