Fast CT/SPECT derived 3D Monte Carlo dose computations for 131I internal emitter therapy

The DPM (Dose Planning Method) Monte Carlo electron and photon transport program, designed for fast computation of radiation dose in external electron beam radiotherapy, has been adapted to the determination of dose in patient-specific I-131 internal emitter therapy. Because both its photon and electron transport mechanics algorithms have been optimized for computation in 3D voxelized geometries (in particular, those from CT scans), DPM is perfectly suited for patient-specific dose calculations in internal emitter therapy. The necessary inputs are a patient CT image, a registered SPECT image, and any number of registered masks defining regions of interest. DPM has been benchmarked for this application by comparing computed absorption fractions and self-absorption fractions for a variety of organs using a Zubal phantom with results from other Monte Carlo programs and reference results from the Medical Internal Radionuclide Dose (MIRD) Committee standards, and the sourcing and photon tracking algorithms of DPM have been further benchmarked by comparison to experimental data. This paper presents a description of the program, the results of the benchmark studies, and some sample computations using patient data from radioimmunotherapy studies.