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 is used to differentiate tumour recurrence and rediation necrosis? 
Select and explain. 
A. PET scan, B. MRI, C. 3D CT, D. USG
A: PET scan
Answer: A: PET scan 
Explanation: Ans. is'a'i.e., PET scanPET may be used in therapeutic monitoring after chemotherapy or radiotherapy, with a classic indication being assessment for recurrent tumor versus radiation necrosis.Conventional imaging can not differentiate recurrent tumor from radiation necrosis.