Which of the following elements is obsolete in radiotherapy
Radium needles that were once implanted into tumours as a cancer treatment are now obsolete and constitute a radioactive waste problem, as their half-life is approximately 1600 years.

           "Radioisotopes may be naturally occurring, such as radium - 226 (226 Ra)' which is now seldom used in radiotherapy".
Radium 226
Cobalt 60
Iridium 192
Cesium 137