Methionine Uptake by Tumor Tissue: A Microautoradiographic Comparison with FDG

L-methyl-11C-methionine (11C-Met) and 2-deoxy-2-18F-fluoro-D-glucose (18F-FDG) are used for tumor diagnosis and treatment evaluation by PET. In order to examine the role of these tracers in cancer imaging, intratumoral properties of 14C-Met were studied and compared to those of 18F-FDG. The distribution of 14C-Met in various cellular elements of two different mouse malignant tumor tissues, MH134 and FM3A, was analyzed serially using microautoradiography within a period of 120 min after injection of the tracer. Carbon-14-Met and 18F-FDG showed different distributions in tumor tissue. Carbon-14-Met uptake by the tumor was mostly by viable cancer cells. The uptake by macrophages and other cellular components was low. The uptake was higher in the highly proliferative tumor but did not reflect protein synthesis. The rapid and slow growing tumors demonstrated that 14C-Met uptake ratio was lower than that of 18F-FDG, reflecting de novo DNA synthesis ratio. Carbon-14-Met uptake represents the presence of viable cancer cells. Carbon-11-Met may be suitable for treatment evaluation of individual tumors but not growth rates of different tumors. Fluorine-18-FDG reflects tumor-host immune system reaction and is an excellent tool for pretreatment evaluation of tumors and determination of tumor proliferative activity.