[18F]FDG-PET-CT for early monitoring of tumor response: when and why.

(18)F-fluorodeoxyglucose positron emission tomography ([(18)F]FDG PET) has been recognized as a suitable tool in tumor response assessment of patients complaining with solid tumors who have undergone chemo- and radiotherapy. It offers the advantage of functional tissue characterization, which is independent from morphologic criteria allowing to differentiate disease relapse from therapy-induced fibrosis. At present, there is a growing body of evidence that PET semi-quantitative assessment of treatment-induced changes in tumor [(18)F]FDG avidity may predict early tumor response and patient outcome. Patient management might be changed. For instance, in non responder patients this novel diagnostic approach would hamper useless ''wait and watch'' attitude in implementing further options or identifying those needing additional therapeutic strategies. On the other hand, for those patients revealing promptly a favourable metabolic response a cost-sparing approach could be implemented avoiding expensive diagnostic procedures during the follow-up as well as the risk of over-treating. In any case, since even a partial metabolic response may be an indication for continuing therapy, the advantage of metabolic assessment over conventional procedures may be clinically relevant. Although a morphological assessment has been considered for long time the standard for detecting therapy response, limitations of conventional computed tomography-based evaluation in solid tumors are well-known. PET provides an independent means of assessing malignancy. However, no consensus has been achieved until now regarding the optimal timing in performing PET during or at completion of treatment.