PET imaging of recurrent brain tumors using ¹⁸F-FACBC

Objectives: We evaluated the efficacy of ¹⁸F-FACBC brain PET imaging in recurrent gliomas and compared the utility of these images to that of contrast enhanced anatomical (MRI or CT) imaging and to [¹¹C-methyl]-L-methionine (¹¹C-Methionine) PET imaging.

Methods: This prospective clinical study was performed after IRB approval (IRB#03-028). Twenty-seven patients with suspected recurrence or progression of a primary brain tumor (based on clinical and MRI/CT data) were studied. The initial pathological diagnoses included 10 astrocytoma, 12 glioblastoma, 4 oligodendroglioma and 1 oligoastrocytoma. All patients underwent from primary surgery followed by other local or systemic therapies. Dynamic PET brain imaging for up to 3 hours after injection of 370 MBq ¹⁸F-FACBC were performed within 30-days of the MRI/CT images. Of these, 16 patients also had ¹¹C-Methionine PET brain scans on the same days. Visual findings, semi-quantitative analyses and pharmacokinetic modeling of the ¹⁸F-FACBC images was conducted. The information derived from these analyses were compared to data from ¹¹C-Methionine PET and to contrast-enhanced anatomical imaging. To establish the standard of truth, histology results were used when a biopsy or a surgery was performed. For the other patients, clinical and imaging follow up was used.

Results: All patients were considered to have active tumor. ¹⁸F-FACBC PET was positive for all patients, whereas 3 patients were found to be indeterminate based on the MRI images. Furthermore, all tumors were clearly delineated with ¹⁸F-FACBC PET, whereas, tumor delineation in 6 patients on the ¹¹C-Methionine scans due to a comparable brain background. Tumor ¹⁸F-FACBC SUV max ranged from 1.5 to 10.4 (average 4.5 ± 2.3), while ¹¹C-Methionine’s tumor SUV max ranged from 2.2 to 10.2 SUVmax (average 5.0±2.2). In the 16 patients undergoing both ¹⁸F-FACBC and ¹¹C-Methionine scans, there was a close correlation in tumor uptake (r=0.92). However, image contrast was higher with ¹⁸F-FACBC compared to ¹¹C-Methionine due to ¹⁸F-FACBC’s lower background in normal brain tissue (SUV = 0.5 ± 0.2 compared to 1.3 ± 0.4 for ¹¹C-Methionine). A subset amounting to 5 patients had arterial blood sampling following the ¹⁸F-FACBC injection. In these patients the ¹⁸F-FACBC uptake in both normal brain and tumors was well described by a simple one-compartment (3-parameter: V_b, k₁, k₂) model. Normal brain was found to approach transient equilibrium with a half-time of 1.8 ± 0.7 hours and achieving a final distribution volume a little over unity (average 1.2 ± 0.3 ml/cc). Tumors equilibrated somewhat more rapidly with blood (t_1/2 = 45 ± 34 minutes), with an average distribution volume of 2.8 ± 1.1 ml/cc.

Conclusions: Compared to ¹¹C-Methionine PET, ¹⁸F-FACBC PET detected recurrent glioma with higher contrast and may be particularly useful when contrast MRI is non-diagnostic. Further investigations are needed to confirm the specificity of ¹⁸F-FACBC PET in brain tumor.