¹⁸F-Fluciclovine (¹⁸F-FACBC) PET imaging of recurrent brain tumors

The aim of our study was to investigate the efficacy of ¹⁸F-Fluciclovine brain PET imaging in recurrent gliomas, and to compare the utility of these images to that of contrast enhanced magnetic resonance imaging (MRI) and to [¹¹C-methyl]-L-methionine (¹¹C-Methionine) PET imaging. We also sought to gain insight into the factors affecting the uptake of ¹⁸F-FACBC in both tumors and normal brain, and specifically to evaluate how the uptake in these tissues varied over an extended period of time post injection.

Twenty-seven patients with recurrent or progressive primary brain tumor (based on clinical and MRI/CT data) were studied using dynamic ¹⁸F-Fluciclovine brain imaging for up to 4 h. Of these, 16 patients also had ¹¹C-Methionine brain scans. Visual findings, semi-quantitative analyses and pharmacokinetic modeling of a subset of the ¹⁸F-Fluciclovine images was conducted. The information derived from these analyses were compared to data from ¹¹C-Methionine and to contrast-enhanced MRI.

¹⁸F-Fluciclovine was positive for all 27 patients, whereas contrast MRI was indeterminate for three patients. Tumor ¹⁸F-Fluciclovine SUVmax ranged from 1.5 to 10.5 (average: 4.5 ± 2.3), while ¹¹C-Methionine's tumor SUVmax ranged from 2.2 to 10.2 (average: 5.0 ± 2.2). Image contrast was higher with ¹⁸F-Fluciclovine compared to ¹¹C-Methionine (p < 0.0001). This was due to ¹⁸F-Fluciclovine's lower background in normal brain tissue (0.5 ± 0.2 compared to 1.3 ± 0.4 for ¹¹C-Methionine). ¹⁸F-Fluciclovine uptake in both normal brain and tumors was well described by a simple one-compartment (three-parameter: V_b,k₁,k₂) model. Normal brain was found to approach transient equilibrium with a half-time that varied greatly, ranging from 1.5 to 8.3 h (mean 2.7 ± 2.3 h), and achieving a consistent final distribution volume averaging 1.4 ± 0.2 ml/cc. Tumors equilibrated more rapidly (t_1/2ranging from 4 to 148 min, average 57 ± 51 min), with an average distribution volume of 3.2 ± 1.1 ml/cc. A qualitative comparison showed that the rate of normal brain uptake of ¹¹C-Methionine was much faster than that of ¹⁸F-Fluciclovine.

Tumor uptake of ¹⁸F-Fluciclovine correlated well with the established brain tumor imaging agent ¹¹C-Methionine but provided significantly higher image contrast. ¹⁸F-Fluciclovine may be particularly useful when the contrast MRI is non-diagnostic. Based on the data gathered, we were unable to determine whether Fluciclovine uptake was due solely to recurrent tumor or if inflammation or other processes also contributed.