Source of Support: None, Conflict of Interest: None
125I seeds employed for permanent and temporary interstitial implants exhibit significant radiation
fluence anisotropy due to self absorption in the marker and oblique filtration through the encapsulation jacket. A silver wire 1251 seed (Model 6711), introduced in 1983 by 3M Company, failed to improve the photon distribution anisotropy. In addition to the known 1251 photon spectrum, the new seed emitted two silver characteristic x-rays, lowering the mean photon energy from 28.4 to 27.4 keV. A double wall uniform thickness encapsulated 125I source, laser welded at one end, has been developed for clinical use. The source uses a carbon coated thin tungsten filaments for enhanced radiographic visualization. Measurements made by Nal and intrinsic Ge detectors indicate that the 2300 series 125I source emits a pure 125I spectrum. The 6ngular dependence of individual photon peaks and total photon spectrum as well as the corresponding anisotropy factors were measured. The 4x averaged anisotropy factor for the total radiation fluence is 0.92 compared to 0.87 for model 6711 seed. The dose distribution around the new 125I source in water is very isotropic.