¹⁸F-FET PET Uptake Characteristics in Patients with Newly Diagnosed and Untreated Brain Metastasis

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 [TBR_max/TBR_mean], biologic tumor volume [BTV], and time-activity curves with minimal time to peak [TTP_min]) 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 TBR_max ≥ 1.6 and were classified as ¹⁸F-FET-positive (median TBR_max, 2.53 [range, 1.64-9.47]; TBR_mean, 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 TTP_min 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.