This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
Which of the following elements is obsolete in radiotherapy 
Select and explain. 
A. Radium 226, B. Cobalt 60, C. Iridium 192, D. Cesium 137
A: Radium 226
Answer: A: Radium 226 
Explanation: 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".