Silver fluorescent x-ray yield and its influence on the dose rate constant for nine low-energy brachytherapy source models.

The physical characteristics of the photons emitted by a low-energy brachytherapy source are strongly dependent on the source's construction. Aside from absorption and scattering caused by the internal structures and the source encapsulation, the photoelectric interactions occurred in certain type of source-construction materials can generate additional energetic characteristic x rays with energies different from those emitted by the bare radionuclide. As a result, the same radionuclide encapsulated in different source models can result in dose rate constants and other dosimetric parameters that are strikingly different from each other. The aim of this work was to perform a systematic study on the yield of silver fluorescent x rays produced in nine 125I sources that are known to contain silver and its impact on the dose-rate constant. Using a high-resolution germanium spectrometer, the relative 125I spectra emitted by the nine sources on its bisector were measured and found to be similar to each other (the maximum variation in the 125I-Kbeta relative intensity was less than 4%). On the other hand, the measured silver fluorescent x-ray spectra exhibited much greater variations from model to model; the maximum change in the measured Ag-Kalpha relative intensity was over 95%. This larger variation in the measured silver fluorescent x-ray yield was caused by (1) the different amount of silver that was directly exposed to the 125I radionuclide in different source models and (2) the stronger influence of the source's internal geometry on the silver fluorescent x rays. Because the addition of silver fluorescent x rays can significantly alter the photon characteristics emitted by the radioactive sources, a precise knowledge on the silver fluorescent x-ray yield is needed in theoretical calculations of the sources' intrinsic dosimetric properties. This study concludes that the differences in silver fluorescent yield are the primary causes of the variable dose rate constant observed among these source models.