18F-FLT PET/CT imaging is not competent for the pretreatment evaluation of metastatic gastric cancer: a comparison with 18F-FDG PET/CT imaging.

OBJECTIVE: The aim of this study was to evaluate the utility of 3'-deoxy-3'-F-fluorothymidine (F-FLT) PET/computed tomography (CT) imaging in the pretreatment evaluation of metastatic gastric cancer in comparison with F-fluorodeoxyglucose (F-FDG) PET/CT imaging.

METHODS: A total of 39 metastatic gastric cancer patients were enrolled in the study. Attenuation-corrected whole-body F-FLT and F-FDG PET/CT (low-dose CT) imaging was performed on two consecutive days before chemotherapy.

RESULTS: Accumulation of focal activity was visible in primary tumors on F-FLT PET/CT in 36/39 patients and on F-FDG PET/CT in 37/39 patients, with sensitivities of 92.3 and 94.9%, respectively. Further, three of the 36 FLT-avid primary tumors were almost undetected because they were covered by a high background hepatic uptake. Because of the high physiological uptake of F-FLT in the liver [median maximum standardized uptake value (SUVmax) 5.5, range 4.5-8.3] and the bone marrow (median SUVmax 14.8, range 10.8-22.0), the sensitivity of F-FLT PET/CT versus F-FDG PET/CT for detecting liver metastases and bone metastases was 30.0% (6/20) versus 100% (20/20) and 1/5 (20.0%) versus 5/5 (100%), respectively (P<0.05). Metabolically positive findings of lymph node, peritoneal, and ovarian metastases were similar between the two modalities: 96.8% (30/31) versus 93.5% (29/31), 89.5% (17/19) versus 94.7% (18/19), and 90.9% (10/11) versus 90.9% (10/11) of patients for F-FLT versus F-FDG, respectively (P>0.05).

CONCLUSION: F-FLT PET/CT imaging is not recommended for pretreatment assessment of metastatic gastric cancer as it is not competent enough to evaluate liver and bone metastases; moreover, the high background hepatic uptake may cover the gastric primary tumors located adjacent to the liver.