EANM procedure guidelines for brain tumour imaging using labelled amino acid analogues

Abstract

Increased amino acid transport in brain tumour cells results from overexpression of the transporter systems and is related to alterations in the tumour vasculature and tumour cell proliferation. Radiolabelled amino acids offer significant improvements in the diagnostic evaluation of cerebral tumours in comparison with conventional anatomical imaging. They also display contrast far superior to that obtained with FDG because of the low uptake of amino acids in normal brain tissue, and they might be more tumour specific as their uptake is less influenced by inflammation [1]. The most frequently used radiolabelled amino acid is [methyl-11C]-L-methionine (MET), used in conjunction with PET. In an effort to overcome the disadvantages of its short half-life and complex metabolism, and despite a changed amino acid structure, several fluoro- and iodo-amino acids have been developed. These agents include 3-[123I]iodo-α-methyl-L-tyrosine (IMT) for SPECT and O-(2-[18F]fluoroethyl)-L-tyrosine (FET) for PET, which are transported by the same specific amino acid transport system L as MET, but are not incorporated into proteins [2]. Their rapid accumulation into brain tumours is independent of blood-brain barrier disruption. Among the 18F-labelled amino acids, FET has been selected as representative of this category owing to ease of synthesis, high in vivo stability, and fast brain and tumour uptake kinetics [3]. Other natural or artificial amino acids have been labelled to measure tumour metabolism; they are beyond the scope of these guidelines. Despite differences in blood clearance, uptake kinetics and relation to protein synthesis, MET, IMT and FET show similar results in the diagnostic evaluation of cerebral tumours, supporting their parallel review in these guidelines.