['To\xa0perform a detailed analysis of microsphere distribution in biopsy material from a patient treated with (90)Y-labeled resin spheres and characterize microsphere distribution in the hepatic artery tree, and to construct a novel dichotomous bifurcation model for microsphere deposits and evaluate its accuracy in simulating the observed microsphere deposits.', 'Our virtual model consisted of arteries that successively branched into 2 new generations of arteries at 20 nodes. The artery diameter exponentially decreased from the lowest generation to the highest generation. Three variable parameters were optimized to obtain concordance between simulations and measure microsphere distributions: an artery coefficient of variation (ACV) for the diameter of all artery generations and the microsphere flow distribution at the nodes; a hepatic tree distribution volume (HDV) for the artery tree; and an artery diameter reduction (ADR) parameter. The model was tested against previously measured activity concentrations in 84 biopsies from the liver of 1 patient. In 16 of 84 biopsies, the microsphere distribution regarding cluster size and localization in the artery tree was determined via light microscopy of 30-μm sections (mean concentration, 14 microspheres/mg; distributions divided into 3 groups with mean microsphere concentrations of 4.6, 14, and 28 microspheres/mg).', 'Single spheres and small clusters were observed in terminal arterioles, whereas large clusters, up to 450 microspheres, were observed in larger arterioles. For 14 microspheres/mg, the optimized parameter values were ACV=0.35, HDV\xa0=\xa050\xa0cm(3), and ADR=6\xa0μm. For 4.6 microspheres/mg, ACV and ADR decreased to 0.26 and 0\xa0μm, respectively, whereas HDV increased to 130\xa0cm(3). The\xa0opposite trend was observed for 28 microspheres/mg: ACV\xa0=\xa00.49, HDV\xa0=\xa020\xa0cm(3), and ADR = 8\xa0μm.'] Does simulation Model of Microsphere Distribution for Selective Internal Radiation Therapy agree With Observations?