The role of PET in monitoring therapy

Positron emission tomography (PET) is being increasingly used for the evaluation of patients with known or suspected cancer at all phases of the management process from diagnosis, through staging to follow-up after treatment. The role of PET in therapeutic monitoring is expanding rapidly due to its ability to provide earlier and more robust identification of non-responders than provided by conventional non-invasive imaging approaches. PET can thereby potentially provide important benefits to the individual patient by allowing an earlier change to alternative treatments that may be more efficacious or by avoiding the unnecessary toxicity related to ineffective therapy. As therapies become ever more expensive, this could also produce cost savings because of earlier termination of ineffective treatment. Conversely, PET may demonstrate important biological effects despite a lack of apparent morphological response and therefore prevent premature withdrawal of effective therapies. Globally, the vast majority of therapeutic monitoring studies use the glucose analogue, fluorine-18 fluorodeoxyglucose (FDG) but new tracers such as fluorine-18 fluorothymidine (FLT) also offer promise for this application. In this review, the potential benefits and limitations of FDG PET are discussed along with suggestions regarding the most practical methodologies for response evaluation using this modality.