Metabolic imaging of amino acids by PET/CT: Comparison of 11C-methionine and 18F-FBPA

Objectives 18F-fluoro-boronophenylalanine (FBPA) is one of amino acid derivatives and has been used to select patients and to evaluate boron levels in the tumor for boron neutron capture therapy (BNCT). However, it is not easy to conduct FBPA-PET/CT study because synthesis of FBPA from 18F2 gas has some limitations of low radio-yielding, low specific activity and high cost. As an amino acid metabolic imaging, 11C-methionine (Met) PET/CT study has been applied in many hospitals. In this study, we demonstrated the biodistribution of FBPA and Met on whole-body PET/CT, and compared uptakes in brain tumors between FBPA and Met.

Methods Four patients with malignant tumor who were inspected both Met-PET/CT and FBPA-PET/CT within 2 weeks in our hospital were set as the target. Whole-body PET/CT studies of Met were started at 10 min post injection, and FBPA at 1h post injection. PET acquisition was carried out with 3min of imaging per bed position in 3D mode. Region of interest (ROI) were placed within tumor and normal tissue organ, and maximum of standardized uptake value (SUV) of Met and FBPA were calculated and compared.

Results The SUV of FBPA in tumor ranged 1.7 to 3.9 (average, 2.7±1.0), whereas that of Met ranged 2.1to 5.3 (average,3.4±1.4). SUV in the tumor showed acceptable correlations between Met and FBPA(r=0.96). SUV of Met in liver, pancreas and gastric wall showed 11.5±1.2, 21.9±4.8 and 13.7±1.7, respectively, whereas SUV of FBPA in liver, pancreas and gastric wall showed 1.8±0.3, 1.7±0.3 and 1.6±0.2, respectively. The difference of SUVs between Met and FBPA (p<0.001) in liver, pancreas and gastric wall may demonstrate the expression of amino acid transporter systems.

Conclusions The accumulation of FBPA in tumor was correlated with that of Met. The SUV in tumor of Met could be an acceptable index for selecting BNCT-applicable patient who has brain tumor.