Indolent dorsal midbrain tumor: new findings based on positron emission tomography

Intrinsic tumors arising in the dorsal midbrain cause obstructive hydrocephalus and have an indolent clinical course. Positron emission tomography (PET) with fluorine-18-labeled fluorodeoxyglucose (FDG) and l- [methyl-(11)C]methionine (MET) was used to evaluate the biological behaviors of dorsal midbrain tumors.

The authors report on 4 patients (3 males and 1 female) with dorsal midbrain tumors who presented with obstructive hydrocephalus. A diagnosis was made with MR imaging in each patient. To manage the hydrocephalus, endoscopic third ventriculostomy was performed in all cases. The patients did not undergo any other surgical procedures except endoscopic biopsy procedure, chemotherapy, or radiation therapy. The patients in 3 cases underwent FDG- and MET-PET within 6 months of CSF-diverting procedures, and the patient in 1 case underwent PET 10 years after the procedure.

After the CSF-diverting procedure, clinical symptoms resolved or improved in all patients. Gliosis or glial proliferation was diagnosed in 1 patient, and possible low-grade glioma in 2 patients. Although all tumors appeared hyperintense on T2-weighted MR images, their appearance on T1-weighted images was variable (iso- and/or hypointense), and partial lesion enhancement was observed on images from 2 patients. On the other hand, the PET features of these lesions were almost identical, and the scans did not show a high uptake of FDG and MET compared with the cortical uptake in a normal brain. The mean tumor tissue/normal tissue ratio of FDG uptake was 0.65, and that of MET was 0.99.

Positron emission tomography findings suggested that the indolent dorsal midbrain lesion had nontumorous characteristics, thus supporting a good prognosis. Positron emission tomography studies may be more informative and predictive of the biological behavior of dorsal midbrain tumors than a biopsy procedure.