Which one of the following radioisotope is not used as permanent implant in brachytherapy?

Cesium 137 is used as a temporary implant and not a permanent implant. It is used almost exclusively for low dose rate brachytherapy. In low dose rate brachytherapy, temporary implant which is the source of low energy radiation is kept adjacent to the tumor. Temporary implants are commonly used in head and neck cancer. It is used as an interstitial implant in which sources are loaded through catheters implanted into the tumor volume. Most commonly used temporary radioisotope is Ir 192. Ref: Brachytherapy: Applications and Techniques By Phillip M. Devlin, Page 52; Radiation in Medicine: A Need for Regulatory Reform By U.S. Nuclear Regulatory Commission, Page 71