Treatment-related uptake of O-(2-[18F]fluoroethyl)-L-tyrosine and L-[methyl-3H]-methionine after tumor resection in rat glioma models.

Purpose: Assessment of residual tumor after resection of cerebral gliomas can be difficult with MRI and may be improved by amino acid PET. The aim of this experimental study was to investigate uptake of (2-[¹⁸F]fluoroethyl)-L-tyrosine (¹⁸F-FET) and L-[methyl-³H]-methionine (³H-MET) in residual tumor after surgery and possible false positive uptake in treatment-related changes. Methods: F98 or GS-9L rat gliomas were implanted into the brain of 64 rats. Tumors were resected after one week of tumor growth and sham surgery was performed in additional 10 animals. At different time points after surgery (1, 2, 3, 7, 14-16 days), rats underwent ex vivo dual tracer autoradiography using ¹⁸F-FET and ³H-MET. Histological slices were evaluated by immunostainings for cell density and astrogliosis. Tracer uptake was determined by regions of interest and quantified by lesion-to-brain ratios (L/B). Four animals underwent PET to examine time activity curves of ¹⁸F-FET uptake. Results: Treatment-related uptake with a mean L/B of 2.0 ± 0.3 for ¹⁸F-FET and a mean L/B of 1.7 ± 0.2 for ³H-MET was noted at the rim of the resection cavity in the first week after surgery which decreased significantly by 14-16 days (P < 0.01). Treatment-related tracer uptake was significantly higher for ¹⁸F-FET than for ³H-MET (P < 0.001). Reactive astrogliosis was noted in the vicinity of the resection cavity from the second day after surgery. Time-activity curves of ¹⁸F-FET uptake in those areas revealed constantly increasing uptake. Conclusion: Surgery may induce significant treatment-related ¹⁸F-FET and ³H-MET uptake in the vicinity of the resection cavity in the first week after surgery presumably caused by reactive astrogliosis. Assessment of residual tumor after surgery by amino acid PET seems to be more reliable after an interval of 14 days.