<prompt> Which of the following elements is obsolete in radiotherapyRadium 226Cobalt 60Iridium 192Cesium 137  <response>: 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. 