Investigation of interseed attenuation and tissue composition effects in 125I seed implant prostate brachytherapy

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 D-90 by 4.2 Gy (2.9%), prostate V-100 by 0.5 cm(3) (1.4%), urethra D-10 by 8.6 Gy (3.5%), rectal D-2cm(3) 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 D-90 reduced from 185 to 149.8 Gy (-19%). For patients with calcifications, larger tissue effects were observed, prostate D-90 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 <0.5% for all DVH parameters, with up to 4% difference for patients with calcifications. CONCLUSIONS: Advanced brachytherapy dose calculation methods that take account of ISA and tissue effects show that clinical I-125 implant dose is different from TG-43U1 (AAPM Task Group No. 43 Report Update 1) calculations, reducing DVH parameter values particularly for patients with calcifications. Peripheral dose and areas of the implant with relatively poorer coverage are particularly affected. However, dose reductions are small compared with other postimplant dose uncertainties. (C) 2014 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.