[Positron emission tomography in CUP syndrome]

In approximately 2% of all cancers, no primary tumor can be detected and cancer of unknown primary (CUP) syndrome, a diagnosis 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 a novel, 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.