Experimental and theoretical dosimetry of a new polymer encapsulated iodine-125 source--SmartSeed: dosimetric impact of fluorescence x rays

The detailed study of a new permanent iodine-125 brachytherapy source, SmartSeed, is presented in this article. It is the first iodine seed made with biocompatible polymer and is manufactured by the IBt-Bebig group.

Three dosimetric studies have been performed: The first one used thermoluminescent detectors in a solid water phantom with NIST (National Institute of Standards and Technology, USA) calibrated seeds, and two separate studies were of Monte Carlo photon transport calculations (MCNP5 code). The TG-43U1 protocol was applied to derive dosimetric parameters for clinical applications.

The radial dose function g(r) was determined at different distances ranging from 0.5 to 10 cm; and the anisotropy function F(r, theta) at angles ranging from 0 degrees to 350 degrees in 10 degrees increments. Monte Carlo calculations were performed in liquid water to obtain values for lambda, g(r), F(r, theta), and phi(an)(r) as recommended by the TG-43U1 protocol for use in treatment planning system software. SmartSeed's biocompatible polymer capsule permits fluorescence x rays (3, 5, and 12 keV), generated by lead glass marker, to be present in the emission spectrum, influencing the dose rate constant. The impact on near field dosimetry in water from these x rays was also investigated and reported. The capsule also attenuates iodine-125 energies much less than typical titanium encased sources, resulting in a highly isotropic source.

SmartSeed has a dose rate constant of 0.895 +/- 7.3% cGy h(-1) U(-1), a radial dose function nearly identical to the IBt-Bebig model I25.S06 seed, and a highly isotropic dose distribution. Fluorescence x rays account for the relatively low value of lambda, yet their variable contribution to dosimetry arising from seed dimensional uncertainties is estimated to be < 0.2%.