In patients with brain
metastasis, PET using labeled
amino acids has gained clinical importance, mainly regarding the differentiation of viable
tumor tissue from treatment-related effects. However, there is still limited knowledge concerning the uptake characteristics in patients with newly diagnosed and untreated
brain metastases. Hence, we evaluated the uptake characteristics in these patients using dynamic O-(2-18F-fluoroethyl)-l-tyrosine (18F-FET) PET. Methods: Patients with newly diagnosed
brain metastases without prior local
therapy and 18F-FET PET scanning were retrospectively identified in 2 centers. Static and dynamic PET parameters (maximal/mean
tumor-to-brain-ratio [TBRmax/TBRmean], biologic
tumor volume [BTV], and time-activity curves with minimal time to peak [TTPmin]) were evaluated and correlated with MRI parameters (maximal lesion diameter, volume of contrast enhancement) and originating primary
tumor. Results: Forty-five
brain metastases in 30 patients were included. Forty of 45
metastases (89%) had a TBRmax ≥ 1.6 and were classified as 18F-FET-positive (median TBRmax, 2.53 [range, 1.64-9.47]; TBRmean, 1.86 [range, 1.63-5.48]; and BTV, 3.59 mL [range, 0.04-23.98 mL], respectively). In 39 of 45
brain metastases eligible for dynamic analysis, a wide range of TTPmin was observed (median, 22.5 min; range, 4.5-47.5 min). All 18F-FET-negative
metastases had a diameter of ≤ 1.0 cm, whereas
metastases with a > 1.0 cm diameter all showed pathologic 18F-FET uptake, which did not correlate with lesion size. The highest variability of uptake intensity was observed within the group of
melanoma metastases. Conclusion: Untreated
metastases predominantly show increased 18F-FET uptake, and only a third of
metastases < 1.0 cm were 18F-FET-negative, most likely because of scanner resolution and partial-volume effects. In
metastases > 1.0 cm, 18F-FET uptake intensity was highly variable and independent of
tumor size (even intraindividually). 18F-FET PET might provide additional information beyond the
tumor extent by reflecting molecular features of a
metastasis and might be a useful tool for future clinical applications, for example, response assessment.