Monte Carlo calculated TG-43 dosimetry parameters for the SeedLink 125Iodine brachytherapy system

The SeedLink brachytherapy system is comprised from an assembly of I-Plant 3500 interstitial brachytherapy seeds and bioresorbable spacers joined together by a 6-mm-long titanium sleeve centered over each seed. This device is designed to maintain specified spacing between seeds during treatment thereby decreasing implant preparation time and reducing radionuclide migration within the prostate and periprostatic region. Reliable clinical treatment and planning applications necessitate accurate dosimetric data for source evaluation, therefore the authors report the results of a Monte Carlo study designed to calculate the AAPM Task Group Report No. 43 dosimetric parameters for the SeedLink brachytherapy source and compare these values against previously published Monte Carlo study results of the I-Plant 3500 brachytherapy seed. For this investigation, a total of 1 x 10(9) source photon histories were processed for each set of in-water and in-air calculations using the MCNP4C2 Monte Carlo radiation transport code (RSICC). Statistically, the radial dose function, g(r), and the dose-rate constant, lambda, were identical to the values calculated previously for the Model 3500 with the dose-rate constant evaluated to be lambda = 1.000+/-0.026 cGyh(-1) U(-1). The titanium sleeve used in SeedLink to bind together Model 3500 seeds and spacers resulted in slightly greater dosimetric anisotropy as exhibited in the anisotropy function, F(r, theta), the anisotropy factor, phi(an) (r), and the anisotropy constant, phi(an), which was calculated to be phi(an) = 0.91 +/- 0.01, or roughly 2% lower than the value calculated previously for the Model 3500. These results indicate that the radiological characteristics of the SeedLink dosimetry system are comparable to those obtained for previously characterized single seeds such as the Implant Sciences Model 3500 I-Plant seed.