Exposure Contribution from Ti K X Rays Produced in the Titanium Capsule of the Clinical I-125 Seed

The iodine-125 radioactive nuclide produces three intrinsic photons, i.e., the 27.4-keV tellurium (Te) K alpha and 31.0-keV K beta x rays, and 35.5-keV gamma rays. In addition, a silver-wire-type seed is known to have silver(Ag) K alpha and K beta x rays as a result of interactions between silver atoms and the intrinsic photons. Since the radioactive iodine-125 is completely sealed with a 0.05-mm-thick titanium capsule, fluorescence x rays from titanium atoms are also expected. This study reports (1) abundance of Ti K x rays at 4.5 keV, (2) contribution to exposure from these photons amounting to approximately 19% of the total exposure at 1-cm source-to-detector distance (SDD), (3) exposure uncertainties of 23% at 1-cm SDD when Ti K x rays are excluded, and (4) negligible dose contribution to tissue from these photons.