Which of the following elements is obsolete in radiotherapy: (A) Radium 226, (B) Cobalt 60, (C) Iridium 192, (D) Cesium 137
Answer is A. 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".