[Methyl-11C]thymidine positron emission tomography in tumoral and non-tumoral cerebral lesions.

No ideal radiopharmaceutical exists for positron emission tomography (PET) that fulfills all clinical requirements for the study of brain tumors. The usefulness of a recently developed PET tracer, [methyl-11C]thymidine ([methyl-11C]TdR) is explored in brain tumors. Twenty patients with confirmed tumoral and non-tumoral brain lesions were investigated with [methyl-11C] TdR PET. The 11C activity was visually and quantitatively assessed. In two patients, dynamic scans were performed. The PET findings were compared to those of magnetic resonance imaging (MRI) or computed tomography (CT) of the brain and to the final diagnosis. Eight out of ten patients with confirmed tumoral lesions or tumor recurrence had increased 11C activity within the lesion. In ten non-tumoral lesions no increased 11C uptake was found. The dynamic PET studies showed that [methyl-11C] TdR first acts as a blood flow tracer, but that later on the uptake of 11C activity is due to labeled metabolites, crossing the blood-brain barrier. Increased tracer activity was only observed in tumoral and not in non-tumoral contrast-enhanced lesions on MRI or CT. [Methyl-11C] TdR is not a selective PET radiopharmaceutical for brain tumors, but can be used as a tracer for tumoral blood-brain barrier disruption.