Background: Metastases of unknown origin (MUO) have diverse clinical 
presentations and, unfortunately, poor outcomes in most cases. Identifying the 
initial tumor location is still a considerable issue in many MUO cases. Our aim 
in this research was to assess the ability of F18-Fluorodeoxyglucose Positron 
Emission Tomography/Computed Tomography (18F-FDG PET/CT) to detect the 
site of the primary tumor before starting active treatment. 
Materials and methods: The study included 60 patients referred to our unit as 
cases of metastatic lesions with unknown primary (34 male, 26 female). 
PET/CT was done for all patients with its standard protocol.
Results: PET/CT could suggest primary lesion in 44 cases (73.3%) with an 
estimated sensitivity of 97%, specificity 57%, positive predictive value 80.9%, 
and negative predictive value 91%.
Conclusion: PET/CT is a very valuable tool in cases of MUO and is 
recommended as a diagnostic test for identification of the primary in cancer 
patients whose primary is unknown.