*2.4. Nucleoside Transporter-Targeting Probes*

Radiolabeled or fluorescent nucleobase analogs are currently used to diagnose solid tumors, including cancers of the bladder, breast, lung, ovary, and pancreas. Regarding diagnosis of TC specifically, only [18F]fluorothymidine ([18F]FLT) has been tested to date. [ 18F]FLT, which can be taken up by equilibrative nucleoside transporter 1 (ENT1), is a marker of cell proliferation [78]. In one study, 20 DTCs were assessed with [18F]FLT and [ 18F]FDG on PET/CT. While 69% of the metastatic lesions were identified by focal increases in [18F]-FLT uptake, a lower result than the 92% identified by [18F]FDG PET/CT. It is also demonstrated that [18F]FDG has the advantage in terms of specificity and accuracy over [18F]FLT in finding local lymph node malignancy and distant metastases [79]. So far, [ 18F]FLT PET/CT has not progressed very far in diagnosing TCs.
