O-(2-[18F]Fluoroethyl)-L-tyrosine (FET): a tracer for differentiation of tumour from inflammation in murine lymph nodes
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High uptake of [18F]fluoro-2-deoxy-D-glucose (FDG) by inflammatory cells is a frequent cause of false positive results in lymph node (LN) staging by positron emission tomography. Previous studies suggest that radiolabelled amino acids may be more specific markers for viable tumour tissue than FDG. The aim of this study was to investigate quantitatively the uptake of FDG, [3H]methyl-L-methionine (MET) and O-2-([18F]fluoroethyl)-L-tyrosine (FET) in tumour-infiltrated and immunologically stimulated LNs. Popliteal LNs of Balb/c and DBA/2 mice were stimulated by injection into the right posterior foot pad of mice of either streptozotocin (STZ), causing chronic lymphadenitis, or concanavalin A (Con A), resulting in acute lymphadenitis. Tumour-infiltrated popliteal LNs were induced by inoculation of 2×105 lacZ-tagged T cell mouse lymphoma cells into the right posterior foot pad of syngeneic mice. Twenty-one days post inoculation of tumour cells or at various time points after STZ or Con A injection, mice were simultaneously injected intravenously with MET and FDG or MET and FET. After 30 min, mice were sacrificed and tracer uptake was determined in popliteal LNs. Contralateral LNs and LNs of untreated mice served as controls. Histopathological and immunohistochemical analysis demonstrated typical signs of chronic inflammation (non-specific sinus hyperplasia with macrophages) in STZ-treated animals and acute inflammatory changes (accumulation of neutrophilic granulocytes, vascular dilation, follicular hyperplasia) in Con A-treated animals. X-Gal staining confirmed the presence of tumour cells in the LNs of the injected side of tumour-inoculated mice. In the chronic lymphadenitis model, FDG uptake increased 3.0±0.1 fold [from 2.7±0.2 to 8.2±1.2 percent of injected dose per gram tissue (%ID/g)] and MET uptake 2.0±0.01 fold (from 4.5±0.6 to 9.2±1.1 %ID/g). In the acute lymphadenitis model, FDG uptake increased 3.9±0.3 fold (from 2.7±0.2 to 10.6±2.4 %ID/g) and MET uptake 1.9±0.1 fold (from 4.5±0.6 to 8.5±1.4 %ID/g). In contrast, FET uptake in both lymphadenitis models (1.0±0.03 and 1.2±0.04 fold) was not significantly different from that in controls (from 4.2±0.3 to 4.7±0.7 and to 5.1±0.4 %ID/g, respectively). Uptake of all three tracers in tumour-infiltrated LNs was significantly higher than that in control LNs. FDG uptake increased 2.8±0.15 fold (from 2.7±0.2 to 7.6±1.3%ID/g), MET uptake 1.7±0.11 fold (from 4.5±0.6 to 7.5±1.3 %ID/g) and FET uptake 2.4±0.15 fold (from 4.2±0.3 to 10.0±1.8 %ID/g). MET and FDG uptake was similar or higher in inflammatory than in tumour-infiltrated LNs (P=0.01 and P<0.01, respectively). In contrast, uptake of FET showed no overlap between tumour-infiltrated and inflammatory LNs (P<0.00001). In conclusion, tumour-infiltrated and inflammatory LNs could not be differentiated by means of FDG and MET uptake. FET, in contrast, proved to be a specific tracer for differentiating between tumour-infiltrated and inflammatory LNs in the murine models studied.
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