Brachytherapy is the internal radiation therapy procedure for treating tumors. All are used in brachytherapy, EXCEPT:

Clinically used isotopes in brachytherapy include iridium 192 (192 Ir), iodine 125 (125 I), cobalt 60 (60 Co), cesium 137 (137 Cs), gold 198 (198 Au), and palladium 103 (103 Pd). Iodine 131 is not used in brachytherapy. Ref: Nuclear Development Beneficial Uses and Production of Isotopes 2004 Update ... By OECD, Nuclear Energy Agency, 2005, Page 23; Brachytherapy: Applications and Techniques By Phillip M. Devlin, M.D., 2007, Page 52.