Which of the following elements is obsolete in radiotherapy:

Radium 226 Abeloff's Clinical Oncology writes-"Radium was the most impoant implantation source used for more than 5 decades in the twentieth century. However, many propeies of radium make it undesirable as a radioactive source for therapy. During its decay, radium produces radon gas, which is colorless and odorless but highly radioactive. Moreover, radium has a very long half-life (1620 yrs.!!). These qualities make it a significant hazard in the case of contamination. Hence, more suitable isotopes have replaced radium in clinical applications. Cesium-137 is widely used for gynecologic brachytherapy implants. It has a lower-energy gamma ray than radium (i.e., it is less penetrating and easier to shield), and it has no gaseous daughter nuclei. Palladium-103, iridium-192, and iodine-125 are used for implantation in the body for brachytherapy treatments.............................................................................. Today, cobalt 60 is used primarily for palliative teletherapy or for stereotactic teletherapy to small malignant lesions in the brain and benign lesions such as acoustic neuromas or aeriovenous malformations."