Response of LiF-TLD micro-rods around 125I radioactive seed

The EGSnrc Monte Carlo system has been used to calculate the energy response of LiF-TLDs of different sizes around I-125 permanent brachytherapy sources. The source model includes the effects of an encapsulation, self-absorption within the source and in the welded ends of the encapsulation. The LiF-TLD material has cylindrical geometry (micro-rod) with diameters ranging from I mm to 5 mm and a length of 6 mm. The energy response factor (relative to Co-60 gamma-rays) for a LiF-TLD calibrated in Co-60 gamma-rays and then irradiated by an 1251 permanent brachytherapy source varies between 1.32 +/- 0.2% (1 SD) and 1.406 +/- 0.2% (1 SD) for 5 mm and 1 mm diameter micro-rods, respectively. The energy response factor depends on the radius and the polar angle (r, theta) of the measurement point. For a LiF-TLD of diameter 1 mm calibrated at 1 cm on the transverse axis (r = 1.0 cm, theta = 90degrees) of the I-125 source in water, the energy response factor decreases by a maximum of 3.5% within the 6 cm x 6 cm x 6 cm calculation region. For the 5 mm diameter LiF-TLD, the energy response factor decreases by a maximum of 5% in the same region. An examination of the photon energy spectra showed that the photon spectrum does not change significantly in water within the 3D calculation region (6 cm x 6 cm x 6 cm). The mass energy absorption coefficient ratio of water to LiF-TLDs does not vary by more than 0.5% in this calculation grid. The results, however, show that there is a change in the photon spectrum with distance from the source and with polar angle for LiF-TLDs. This difference in the energy spectrum gives rise to a difference in the mass energy absorption coefficient ratio of water to LiF (calculated by taking into account the difference in photon fluence in water and LiF) and that calculated assuming that the photon spectrum in water and in the LiF-TLD is identical.