Dosimetric intercomparison of permanent Ho-166 seed's implants and HDR Ir-192 brachytherapy in breast cancer

Abstract

AimTo provide a comparative dosimetric analysis of permanent implants of Ho166-seeds and temporary HDR Ir192-brachytherapy through computational simulation. BackgroundBrachytherapy with Ir192-HDR or LDR based on temporary wires or permanent radioactive seed implants can be used as dose reinforcement for breast radiation therapy. Permanent breast implants have not been a practical clinical routine; although, I125 and Pd103-seeds have already been reported. Biodegradable Ho166-ceramic-seeds have been addressed recently. Material and methodsSimulations of implants of nine Ho166-seeds and equivalent with HDR Ir192-brachytherapy were elaborated in MCNP5, shaped in a computational multivoxel simulator which reproduced a female thorax phantom. Spatial dose rate distributions and dose–volume histograms were generated. Protocol's analysis involving exposure time, seed's activities and dose were performed. ResultsPermanent Ho166-seed implants presented a maximum dose rate per unit of contained activity (MDR) of 1.1601μGyh−1Bq−1; and, a normalized MDR in standard points (8mm, equidistant to 03-seeds – SP1, 10mm – SP2) of 1.0% (SP1) and 0.5% (SP2), respectively. Ir192-brachytherapy presented MDR of 4.3945×10−3μGyh−1Bq−1; and, 30% (SP1), and 20% (SP2). Therefore, seed's implant activities of 333MBq (Ho166) and 259GBq (Ir192) produced prescribed doses of 58Gy (SP1; 5d) and 56Gy (SP1, 5 fractions, 6min), respectively. ConclusionsBreast Ho166-implants of 37–111MBq are attractive due to the high dose rate near 6–10mm from seeds, equivalent to Ir192-brachytherapy of 259GBq (3 fractions, 6min) providing similar dose in standard points at a week; however, with spatial dose distribution better confined. The seed positioning can be adjusted for controlling the breast tumor, in stages I and II, in flat and deep tumors, without any breast volumetric limitation.