| Cancer of the prostate can be treated in different ways. One of them, brachytherapy, is an internal irradiation method consisting of the placement of radioactive sources, called seeds, into the tumor.; This work deals with the dosimetry of the 198Au interstitial brachytherapy source. In order to facilitate its clinical use and to obtain the data to be employed in the latest treatment planning systems, new quantities and a potential calibration standard are studied. These quantities, based on dose rates, were recommended in 1995 by the American Association of Physicists in Medicine Task Group 43, AAPM TG-43, and have not previously been obtained for 198Au. They are measured in a solid water phantom using thermoluminescent detectors, and calculated using the Monte Carlo N-Particle code, MCNP, and simple analytic models. In the last part of this work, the “198Au equivalent” activity of 137Cs and 192Ir surrogate seeds is calculated since the National Institute of Standards and Technology, NIST, does not provide a standard for the short half-life 198Au source that would allow checking the activity of the seeds before use on patients. This calculation is done by simulating the response of the Sun Nuclear ionization chamber, model 1008, with MCNP 4C.; The air kerma strength, Sk, per unit apparent activity is found equal to 2.0627 (MCNP) and 2.0889 U mCi−1 (measured). Sk per unit activity is 1.8050 U mCi−1 (MCNP). The dose rate constant per unit apparent activity, Λ/Aapp, is equal to 2.3099 (MCNP) and 2.2878 cGy h−1 mCi −1 (measured). This same quantity per unit air kerma strength is 1.1198 (MCNP) and 1.0952 cGy h−1 U−1 (measured). The values of the radial dose function, g(r), the anisotropy function, F(r,&thgr;), the anisotropy factor, &phis;an(r), and the anisotropy constant are also given. Finally, the “198Au equivalent” activity for the 192Ir surrogate seed is equal to 1.9549 times the real activity of the 192Ir seed, and that for the 137Cs surrogate seed is 1.4895 times its real activity. |
