F-DOPA PET identifies high tumor grade and anaplastic transformation in non-enhancing gliomas

Background and Aim. The absence of contrast enhancement on magnetic resonance imaging (MRI) characterizes WHO grade II Low-Grade Gliomas. However, up to 30% of non-enhancing lesions are histologically diagnosed as anaplastic gliomas (WHO grade III). Identification of this subset of high-grade non-enhancing gliomas, as well as early detection of malignant transformation is essential in treatment decision. We investigated the role of L-3,4-Dihydroxy-6 [18F]fluorophenylalanine (F-DOPA) PET/CT in tumor grading and in the evaluation of early progression in a population of non-enhancing gliomas. Materials and Methods. We retrospectively analyzed 22 patients with non-enhancing gliomas (17 grade II, 3 grade III, 2 gliomatosis cerebri). All patients had previously undergone at least one surgical procedure (either biopsy or surgical excision), 5/22 (23%) had received radiation therapy, 6/22 (27%) had received chemotherapy with Temozolomide (5/6 were in ongoing treatment at the time of PET). All patients underwent F-DOPA PET/CT in addition to conventional MRI surveillance. Static images of the brain were acquired on a Philips Gemini camera over 20 minutes, starting 15 minutes after the i.v. injection of 185 MBq F-DOPA. A low-dose CT scan was adopted for attenuation correction. SUVmax was recorded for all lesions and contralateral background. A tumor to contralateral background uptake ratio (T/B) >1.6 was used as the semiquantitative parameter identifying high grade or early progressive lesions. Results. All non-enhancing lesions showed a T/B >1.1, thus were considered as PET-positive. Median SUVmax was 2.26 ± 1.12. T/B values were suggestive of high-grade and low-grade gliomas in 12 and 10 patients, respectively. All but one patients with T/B <1.6 were indeed histologically confirmed as low-grade gliomas. Of the 12 patients with T/B >1.6, 2 were gliomatosis cerebri, 4 showed anaplastic transformation within 6 months, 1 was histologically confirmed as grade III non-enhancing glioma, 1 progressed after 18 months. The remaining 4 patients are progression-free after a median of 9 months. Conclusion. In patients with non-enhancing gliomas, the presence of high F-DOPA uptake is suggestive of high tumor grade (WHO grade III and gliomatosis) or predictive of an early anaplastic transformation. Therefore, F-DOPA semiquantitative analysis may be useful to identify a subgroup of patients requiring a short follow-up.