Positron emission tomography in CUP syndrome

Abstract

In approximately 2% of all cancers, no primary tumor can be detected and cancer of unknown primary (CUP) syndrome, adiagnosis of exclusion, is made. In CUP syndrome, computed tomography (CT) and/or magnetic resonance imaging (MRI) do not lead to the detection of primary tumors. In the advanced diagnostic workup of CUP syndrome, 18F‑fluordeoxyglucose positron emission tomography/computed tomography (18F‑FDG PET/CT) can be used. In addition, 68Ga-fibroblast activation protein inhibitor (FAPI) PET/CT as anovel, experimental imaging technique may be considered. 18F‑FDG PET/CT is clinically established for the detection of primary tumors of cervical CUP syndrome. High detection rates have also been reported for 18F‑FDG-PET/CT in extracervical CUP syndrome. 68Ga-FAPI PET/CT has not yet been clinically established, but remarkably high detection rates have been shown for 18F‑FDG-negative cervical CUP syndrome due to its low background activity. The benefit of 18F‑FDG PET in CUP syndrome has been documented in several meta-analyses. To date, the evidence for the use of 68Ga-FAPI PET/CT in CUP syndrome is still rudimentary. 18F‑FDG PET should be applied regularly in cervical CUP syndrome and can be individually considered in extracervical CUP syndrome.