2541 - Evaluation of 18F-DOPA PET Imaging Features and Impact on Radiation Target Volumes for Patients with Oligodendroglioma

A. Kessel¹, D. H. Pafundi², D. R. Johnson³, J. N. Sarkaria¹, P. D. Brown¹, A. Mahajan¹, U. Sener⁴, T. Burns⁵, J. H. Uhm⁴, N. N. Laack II¹, D. H. Brinkmann¹, and W. Breen¹; ¹Department of Radiation Oncology, Mayo Clinic, Rochester, MN, ²Department of Radiation Oncology, Mayo Clinic, Jacksonville, FL, ³Mayo Clinic, Division of Nuclear Medicine, Rochester, MN, ⁴Mayo Clinic, Department of Neurology, Rochester, MN, ⁵Department of Neurosurgery, Mayo Clinic, Rochester, MN

Purpose/Objective(s): Radiation therapy (RT) target volumes for oligodendroglioma (oligo) are typically defined using MRI T2 sequences, as these tumors are often non-enhancing. Because of the lack of specificity of T2 hyperintensity, this practice may result in target volumes larger or smaller than optimal for a patient. The amino acid radiotracer 18F-DOPA does not require breakdown of the blood-brain barrier for transport, and therefore demonstrates high uptake in glioma even beyond T1 contrast enhancement but low uptake in normal tissue. While 18F-DOPA PET has been prospectively utilized for glioblastoma, data on its utility in oligo is lacking. We hypothesized that 18F-DOPA PET would result in changes in RT target volumes for most patients with oligodendroglioma. Materials/

Methods: This is a single institution analysis of patients with WHO 2016 oligo who underwent 18F-DOPA PET prior to RT on two prospective clinical trials. MRI T2 and T1 contrast enhancement (when applicable) were compared with 18F-DOPA PET defined tumor volumes. Tumor maximum SUV (SUVmax) and tumor mean SUV (SUVmean) were calculated. Patients were followed for radiographic disease progression.
Results: Ten patients with IDH-mutant, 1p/19q co-deleted oligo (N=8 newly diagnosed/2 recurrent; n=8 WHO Grade II, n=2 Grade III) were enrolled between November 2010 and June 2020. Median age was 46 (range 30-61). Five patients were female. Median T/N ratio to define tumor volume on PET was 2.6 (2.1-3.3). Median MRI T2-defined tumor volume was 66.6 cc (range 5.6-217.3), and 18F-DOPA volume was 19.83 cc (range 5.6-121.2). WHO Grade 2 oligo median 18F-DOPA SUVmax and SUVmean were 4.2 (range 2.2-8.6) and 2.1 (range 1.7-3.3), respectively. In all 10 patients, 18F-DOPA imaging identified additional tumor volume outside the T2 volume (median 2.23 cc, range 0.1-6.6 cc). Two of the ten patients had contrast enhancing tumor. Median tumor volume outside of enhancement was 126.9cc (56.8-197.0) by T2 and 64.7cc (28.6-100.7) by 18F-DOPA. At last follow-up, 3 of 7 patients with long-term follow-up had experienced progression. Median progression-free survival was 80.7 months (range 65.4 – not reached). Compared to patients without progression, patients with progression had a median SUVmax of 6.2 vs 4.2, SUVmean of 2.7 vs 2.1, and total volume of 37.6cc vs 25.4cc.
Conclusion: 18F-DOPA PET imaging in patients with low-grade oligo consistently demonstrated high tumor SUVmax and SUVmean with tumor identified both within and outside of the standard T2-hyperintensity defined tumor volume in all patients. 18F-DOPA PET has the potential to improve target delineation for patients with oligodendroglioma.