Dosimetric study of the new InterSource125 iodine seed

The use of low energy photon emitters for brachytherapy applications, as in the treatment of the prostate or of eye tumors, has significantly increased these last few years. New seed models for 125I have been recently introduced. The aim of this study is to determine the dosimetric parameters as recommended by the AAPM in the TG43 formalism for a new iodine seed design: the InterSource125 (Furnished by IBt, Seneffe, Belgium). Measurements are made with LiF thermoluminescent dosimeters (size of 1 mm3) in solid water phantoms to obtain the dose constant, the radial dose function, and the anisotropy function. The TLDs were calibrated at 6 MV and an energy correction factor of 1.41 has been applied. The same dose parameters are also obtained by Monte Carlo calculations (MCNP4B) in solid water and in liquid water. The radial function was measured at 1, 1.5, 2, 3, 4, 5, 6, and 7 cm and calculated between 0.3 and 7 cm. The anisotropy functions were measured at 2, 3, and 5 cm and calculated between 0.3 and 7 cm. The calculated and the measured TG43 functions for solid water are in excellent agreement. We have then calculated these functions in liquid water to obtain the dosimetric information for clinical applications as per TG43 recommendations. In WT1, the calculated dose rate constant is 0.98±1% and the measured value is 1.03±7%. The calculated value for water is 1.02±1%. In conclusion, the dosimetric functions for the new iodine seed InterSource125 have been determined. They are quite different from the data of the well-known model 6711 from Amersham due to the absence of silver in the new seed.