A MONTE-CARLO PROGRAM CONVERTING ACTIVITY DISTRIBUTIONS TO ABSORBED DOSE DISTRIBUTIONS IN A RADIONUCLIDE TREATMENT PLANNING SYSTEM

In systemic radiation therapy, the absorbed dose distribution must be calculated from the individual activity distribution, A computer code has been developed for the conversion of an arbitrary activity distrib ution to a 3-D absorbed dose distribution. The activity distribution c an be described either analytically or as a voxel based distribution, which comes from a SPECT acquisition, Decay points are sampled accordi ng to the activity map, and particles (photons and electrons) from the decay are followed through the tissue until they either escape the pa tient or drop below a cut off energy, To verify the calculated results , the mathematically defined MIRD phantom and unity density spheres ha ve been included in the code, Also other published dosimetry data were used for verification, Absorbed fractions and S-values were calculate d. A comparison with simulated data from the code with MIRD data shows good agreement, The S values are within 10-20% of published MIRD S va lues for most organs, Absorbed fractions for photons and electrons ire spheres (masses between 1 g and 200 kg) are within 10-15% of those pu blished, Radial absorbed dose distributions in a necrotic tumor show g ood agreement with published data, The application of the code in a ra dionuclide therapy dose planning system, based on quantitative SPECT, is discussed.