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Background: Post-radiotherapy nasopharyngeal changes represent a diagnostic dilemma. Early detection of persistent or recurrent disease may be translated to better cure rate if salvage therapy is implemented. Neither clinical exam nor current radiological anatomical studies (CT/MRI) can differentiate between benign post therapy changes and recurrence. PET scan is a functional study capable of identifying viable tumors as areas of increased radiotracer uptake., Methods: Fifty-five patients underwent 18-FDG PET scans post radiation therapy for nasopharyngeal carcinoma at King Faisal Specialist Hospital and Research Centre. We compared the 18-FDG PET scan with the clinical, radiological and pathological findings., Results: Clinical examination and CT of the head and neck showed post-treatment abnormality in the nasopharynx in 40 patients. Among these, 28 patients had asymmetry in the CT scan. Three out of the 28 patients had positive PET scan. Out of the 12 patients with positive primary disease in the CT scan, 3 had negative PET scan which was also confirmed by biopsy in 2 patients. Eleven patients had positive PET scan in the primary site; this was pathologically confirmed to be recurrent disease in 5 patients. In 2 patients repeat PET scan was converted to negative. The remaining 4 patients did not have biopsy due to the presence of concurrent distant disease. None of the patients with negative PET scan in the neck exhibit recurrence or persistent neck disease to the day of reporting the study. PET scan showed persistent higher sensitivity, specificity, positive and negative predictive values at both the primary site and the neck region than the CT did., Conclusion: PET scan is a useful tool in differentiating between post radiotherapy fibrosis and recurrent nasopharyngeal cancer., Key Words: Nasopharyngeal cancer - FDG/PET - Post therapy changes.