cesium-137 (137 Cs), a radioactive material with a half-life of 30.2 years that is used in radiotherapy as a sealed source of gamma rays intended for application to various malignancies that are treated by brachytherapy.Cesium has replaced radium for such applications.See also brachytherapy. radioactive material with a half-life of 30.2 years that is used in radiotherapy as a sealed source of gamma rays intended for application to various malignancies that are treated by brachytherapy. Cesium has replaced radium for such applications. See also brachytherapy.