This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neoplasia, Subject name: Radiology
Which of the following techniques in 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: Ans. is 'c' ie PET scan (Ref: Harrison, 16/e, p2460)."MRI or CT scans are often unable to distinguish radiation necrosis from recurrent tumor, but PET or SPECT scans may demonstrate that glucose metabolism is increased in tumor tissue but decreased in radiation necrosis." - Harrison.