Dosimetric characterization of a newly designed encapsulated interstitial brachytherapy source of iodine-125-model...

A newly designed encapsulated 125I source has been introduced (Model LS-1 BrachySeed manufactured by DRAXIMAGE Inc.) for interstitial brachytherapy. In this source 125I radionuclide is contained in two ceramic beads positioned at each end of a titanium capsule. The source contains a rod of Pt-Ir, which serves as a radiographic marker for source localization in the patient. Principle photon emissions are 27.4 and 31.0 keV X-rays and a 35.5 keV gamma-ray. The 22.2 and 25.5 keV silver X-rays produced by fluorescence of the silver dopant in the ceramic bead radioisotope carriers, are also emitted. In this work, the dosimetric characteristics of the 125I source were measured with micro LiF TLD chips and dosimetry parameters were characterized based upon the American Association of Physicists in Medicine, Task Group, No. 43 formalism. The corrected 1999 National Institute of Standards and Technology standard for low energy interstitial brachytherapy sources was used to specify the air kerma strength of the sources used in this study. The dose rate constant of the sources was determined to be 1.02+/-0.07 cGy h(-1) U(-1). The radial dose function was measured and was found to be similar to that of the silver-based model 6711 125I source. However, the anisotropy function of the Model LS-1 BrachySeed source is considerably better than that of model 6711 125I source, especially on the points along and close to the longitudinal axis of the source. The BrachySeed model LS-1 provides more isotropic angular dose distribution in tissue than model 6711 125I source. The anisotropy constant for the model LS-1 source was determined to be 1.006, which is considerably better than the value of 0.93 for the model 6711 source.