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
[{'content': 'Which of the following elements is obsolete in radiotherapy: \nSelect and explain. \nA. Radium 226, B. Cobalt 60, C. Iridium 192, D. Cesium 137', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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."', 'role': 'assistant'}]