Dosimetric study of the new InterSource(125) 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 increase d these last few years. New seed models for I-125 have been recently introd uced. The aim of this study is to determine the dosimetric parameters as re commended by the AAPM in the TG43 formalism for a new iodine seed design: t he InterSource(125) (Furnished by IBt, Seneffe, Belgium). Measurements are made with LiF thermoluminescent dosimeters (size of 1 mm(3)) in solid water phantoms to obtain the dose constant, the radial dose function, and the an isotropy function. The TLDs were calibrated at 6 MV and an energy correctio n factor of 1.41 has been applied. The same dose parameters are also obtain ed 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 ca lculated 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 me asured TG43 functions for solid water are in excellent agreement. We have t hen calculated these functions in liquid water to obtain the dosimetric inf ormation for clinical applications as per TG43 recommendations. In WT1, the calculated dose rate constant is 0.98 +/- 1% and the measured value is 1.0 3 +/- 7%. The calculated value for water is 1.02 +/- 1%. In conclusion, the dosimetric functions for the new iodine seed InterSource(125) have been de termined. They are quite different from the data of the well-known model 67 11 from Amersham due to the absence of silver in the new seed. The characte ristics are very similar to those of model 6702. (C) 2001 American Associat ion of Physicists in Medicine.