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 technique is the best for differentiating recurrence of brain tumour from radiation therapy induced necrosis? 
Select and explain. 
A. MRI, B. Contrast enhanced MRI, C. PET scan, D. CT scan
C: PET scan
Answer: C: PET scan 
Explanation: C i.e., PET scan MRI or CT scan are often unable to distinguish radiation necrosis from recurrent tumour PET or SPECT scans may demonstrate that glucose metabolism is increased in tumour tissue but decreased in radiation necrosis, used to distinguish recurrence from radiation necrosisQ.