Evaluation of primary brain tumors with FLT-PET: usefulness and limitations

The purpose of this report was to investigate the potential of positron emission tomography using F-18 fluorodeoxythymidine (FLT-PET) in evaluating primary brain tumors.

FLT-PET was performed in 25 patients with primary brain tumors. FLT uptake in the lesion was semiquantitatively evaluated by measuring the maximal standardized uptake value (SUVmax) and the tumor-to-normal tissue ratio (TNR). SUVmax and TNR were compared with the histologic grade and the expression of the proliferation marker (Ki-67).

FLT uptake in normal brain parenchyma was very low, resulting in the visualization of brain tumors with high contrast. Both SUVmax and TNR significantly correlated with the malignant grade of brain gliomas, in which high SUVmax/TNR was obtained for high-grade gliomas. Patients with primary lymphoma also showed SUVmax/TNR equivalent to glioblastoma. There was a positive correlation between SUVmax/TNR and the Ki-67 index. In contrast, spuriously high SUVmax and TNR were obtained in 3 of 6 patients with suspected recurrent tumors (2 patients with recurrent grade 2 glioma and one patient with postoperative granuloma), all of which showed lesion enhancement on MRI after Gd administration.

FLT-PET can be used to evaluate the malignant grade and proliferation activity of primary brain tumors, especially malignant brain tumors. However, the presence of benign lesions showing blood-brain barrier disruption cannot be distinguished from malignant tumors and needs to be carefully evaluated.