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': 'All of the following are used in Interstitial brachytherapy except: \nSelect and explain. \nA. CO6deg, B. 4192, C. Au198, D. C5137', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. CO6deg', 'role': 'assistant'}]