Dose characteristics of the 125i seeds for prostate brachytherapy

In Japan, two ¹²⁵I brachytherapy sources, the Amersham Health model 6711 and the SourceTech Medical model STM1251, areused for the treatment of prostate cancer. Because of the differences of the external geometry and the internal material between two sources, the dose characteristics are also different. We calculated the photon energy spectra, the radial dose function, and the anisotropy function of two sources using Monte Carlo (MC) simulation. The photon energy spectra of the 6711 included characteristic X-rays from the internal rod (Ag), but that of the STM¹²⁵1 didn't include characteristic X-rays from the internal rod (Au). The discrepancies of the radial dose function between two ¹²⁵I sources ranged from -3.2 to 3.0 %. The discrepancy of the anisotropy function between two ¹²⁵I sources was -24.6 % (5°), and ranged from -4.0 to 5.3 % (> 10°). It seems that these discrepancies were derived from existence or nonexistence of characteristic X-rays from the internal rod and the differences of the geometry of Ti capsule.