Use of 11C-methionine positron emission tomography in basal germinoma: assessment of treatment response and residual tumor

Prognosis of germinoma arising in the basal ganglia or thalamus is worse compared to that in the pineal or suprasellar region. One of the reasons for poor prognosis is the difficulty in evaluating the efficacy of treatment by conventional neuroimaging tools. The usefulness of C-11-methionine (MET) positron emission tomography (PET) in monitoring the biological nature of brain tumors has been proved in glioma patients. Herein, we describe MET-PET findings in three cases of germinomas in the basal ganglia or thalamus and discuss the use of MET-PET in the assessment of treatment response and residual tumor for the next treatment strategy. The patients showed transient increase of MET uptake in the lesions after the initial treatment. Although we did not perform histological verification, MET- PET findings suggested that active tumor cells were still alive in the lesions after the initial treatment. MET uptake gradually decreased during the course of intensive therapy in these patients. MET-PET also revealed germinoma invasion in the brain before discernible signal abnormality or mass lesion in conventional magnetic resonance images in two patients. Further studies including histological verification and long-term follow-up might validate the use of MET-PET in monitoring the treatment efficacy and evaluation of active residual tumor after the treatment. Until we understand what MET uptake truly represents, treatment strategy based on MET uptake must be carefully designed to prevent overtreatment and resultant complications.