Whole-body tumor imaging: O-[¹¹C]methyl-l-tyrosine/positron emission tomography

This chapter describes the development of O-[¹¹C]methyl-L-tyrosine (OMT) and its first clinical application to whole-body imaging as well as brain tumor imaging. Radiolabeled amino acids such as L-[methyl-¹¹C]methionine (MET) are taken up into normal brain tissue at much lower levels, which enables imaging of the majority of low- and high-grade brain tumors. MET is applied for diagnosing tumors in the chest region as well as the brain, but not in the abdominal region where the natural amino acids are taken up at high levels in organs such as the pancreas and liver. On the other hand, the artificial amino acid O-[¹⁸F]fluoroethyl-L-tyrosine is accumulated at very low levels in the human pancreas and liver but excreted from the kidneys to the bladder through the urinary tract. This finding could be characteristically common to artificial amino acid tracers that reflect amino acid transport. Multiple tracers can be administered to the individuals on the same day for a differential diagnosis with OMT-PET followed by FDG-PET. The use of ¹¹C-labeled tracers usually results in decreased radiation absorbed doses compared with the use of ¹⁸F-tracers that have longer half-lives. Positron-labeled amino acids have been identified as a suitable imaging probe to evaluate the pathophysiology of brain tumors using PET. Among the various amino acid tracers, MET is used in most clinical studies of brain tumors because of its simple and efficient labeling procedure.