1 - Molecular Imaging in Early Therapy Monitoring

Abstract

This chapter presents the roles of 2-[18F]fluoro-2-deoxy-D-glucose (F-18-FDG) and positron emission tomography (PET) in cancer imaging. Tumor uptake of FDG is usually defined using the standard uptake value (SUV) as a quantitative parameter. Under standardized circumstances, a change in SUV of >20% can be considered significant. Several factors influence the SUV, and, therefore, it is indispensable that SUV measurements follow a strict protocol. Plasma glucose levels have a significant influence on SUV since both FDG and glucose compete for glucose transport. The physical properties of the PET scanner (geometric resolution, noise level, and sensitivity), the definition of SU, and reconstruction algorithms are important factors influencing SUV measurements. Palliative systemic therapy is performed in patients with stage IV non-small cell lung cancer (NSCLC). This therapy is often associated with high toxicity and life-threatening side effects. Thus, an early interruption of unnecessary therapies is an important issue. Absolute measurements of tumor glucose metabolism can be derived from a single PET study prior to, during, or after therapy. However, these single measurements are of no or little value for prediction of response or survival. To some extent, the lack of predictive power resides in the considerable variability of glucose metabolism in tumors of the upper digestive tract. This variability overshadows the course of the PET signal in responding tumors compared with nonresponding tumors. For this reason, measurement of relative changes between baseline and follow-up is preferred to single measurements of absolute tumor FDG uptake. Pretherapeutic values of tumor FDG uptake tend to be higher in responding tumors, whereas those after therapy tend to be lower compared with nonresponding tumors.