99mTc- methionine: a potential radiopharmaceutical for brain tumor SPECT

In the recent years SPECT, PET and production of short-lived isotopes through Cyclotron, metabolic imaging gained an important arena over CT and MRI. The higher C 11 methionine uptake in low-grade glioma and reduced uptake in high-grade glioma and comparison of their uptake variability with non-tumoral lesion was reported. Using MRI/CT the differentiation of non-tumorous lesion from gliomas is still a difficult task. The evaluation of tumor staging, grading and therapy in various tumors is still a challenging priority. The functional endothelial uptake of C 11 methionine and/or blood brain barrier changes demonstrated the usefulness of C 11 methionine for evaluation and follow-up of low-grade glioma. The cost effectiveness and nonavailability of Cyclotron in underdeveloped and developing countries is a very basic problem and therefore it is very much essential to have reactor produced isotopic labeled compounds. With this aim studies were undertaken to develop Tc-99m labeled Methionine for its possible use in brain tumor imaging. In our studies we labeled Methionine by chelation and transchelation. The ascorbic acid and stannous chloride were used as reducing agents. The labeling efficiencies as calculated were in order of 99% and 97% for chelation and transchelation methods respectively. This technology can be utilized further to for (a) grading low-grade glioma from high-grade malignancy, (b) differentiating glioma with non-tumoral lesions. Our pilot studies conducted using SPECT in patients indicated the accumulation of 99mTc-Methionine in brain tumors. Region of interest (ROI) was drawn on the slice with greatest tumor activity and average pixel count was obtained. To obtain the background activity a similar ROI was drawn on the opposite lobe and the uptake was obtained. The tumor uptake was correlated with tumor grade using methionine retention index