Most common radio-isotopes used as a source for external beam radiotherapy in the treatment of cancer:

Cobalt 60 is commonly used as a source for external beam radiotherapy in the treatment of cancer patients Gamma rays are used in some forms of radiotherapy. Elements such as radium, cobalt 60 and cesium can naturally emit gammarays. A gamma ray unit consist of a quantity of cobalt encased in thick lead shielding. A small apeure in the shielding is opened during treatment to release gamma ray beam. Ref: Encyclopedia of Family Health By David B. Jacoby, 3rd Edition, Page 1773; Principles and Practice of Gynecologic Oncology, 4th Edition, Page 414; Nuclear Safeguards, Security and Nonproliferation: Achieving Security with ... By James Doyle, Page 455.