Thermoluminescent dosimetry of the Symmetra TM I-125 model I25.S06 interstitial brachytherapy seed

As the efficacy of brachytherapy prostate treatment is becoming realized, n ew models of I-125 seeds are being introduced. In this article we present t hermoluminescent dosimetry (TLD) in a solid water phantom for a new design of I-125 seed (UroMed/Bebig Symmetra (TM), Model 125.S06). TLD Cubes, LiF T LD-100, from Bicron (Solon, OH) with dimension 1 X 1 X 1 mm(3) were irradia ted at various distances from the seed at angles ranging from 0 degrees to 90 degrees in 10 degrees increments. The TLD detectors were calibrated by i rradiation in a Co-60 teletherapy beam. Monte Carlo simulation was used to account for TLD energy dependence and the deviation of solid water composit ion (as determined by chemical analysis of a sample) from liquid water. Dos e rates per unit air kerma strength were determined based on calibrations t raceable to the 1999 NIST standard (corrected for NIST measurement errors m ade in 1999) for the Symmetra (TM) seed. Dose data is presented in TG-43 fo rmat as a function of distance and angle. Values for Lambda, F(r, theta), g (r), and the anisotropy constant are obtained for use in radiation treatmen t planning (RTP) software. The dose rate constant was determined to be 1.03 3 +/-6.4% cGy h(-1) U-1, which is comparable to model 6702 and higher than model 6711. We find the relative dose distributions of the Symmetra (TM) se ed are similar to model 6702, and less anisotropic than model 6711. After a ccounting for deviation of measured solid water composition from the manufa cturer's specification, good agreement between TLD results and Monte-Carlo- aided values was found. (C) 2001 American Association of Physicists in Medi cine.