Purpose: To use Monte Carlo (MC) simulation and sector analysis to assess interseed attenuation and scatter (ISA) and tissue effects in permanent seed implant prostate brachytherapy and to compare methods for modeling tissue.Methods And Materials: CT-based postimplant plan simulations for 40 patients were evaluated using dose-volume histogram (DVH) parameters and sector analysis. Simulations in water (to evaluate ISA alone) and tissue assigned from contours or CT data, with and without calcifications, were compared. Results: For patients without calcifications, mean combined ISA and tissue effect reduced prostate D90 by 4.2 Gy (2.9%), prostate V100 by 0.5 cm(3) (1.4%), urethra D10 by 8.6 Gy (3.5%), rectal [Formula: see text] by 11.6 Gy (10.5%), and the 100% isodose volume by 4.7 cm(3). Larger differences were observed comparing planned dose to postimplant, mean prostate D90 reduced from 185 to 149.8 Gy (-19%). For patients with calcifications, larger tissue effects were observed, prostate D90 reduced by up to 7.4%. Sector analysis showed dose reductions were larger in anterior and base sectors of the prostate. For patients without calcifications, contour- and CT-based tissue model results agreed within