False-Positive FDG Positron Emission TomographyUptake in Nonmalignant Chest Abnormalities

Positron emission tomography (PET) with FDG monitors the enhanced glycolytic activity and increased expression of glucose transporters associated with tumor cells. FDG accumulates in tumors via the same transporters used by glucose. Similarly, once in the cell, the radiotracer is phosphorylated to FDG-6PO4. At this point the handling of FDG differs from glucose. Because of the lack of an oxygen at the 2-position in FDG, it cannot proceed further in glycolysis or glycogen synthesis and becomes a “trapped tracer.” This characteristic of FDG has led to its widespread use for whole-body imaging of patients with cancer. Also, because of the relatively long physical half-life of fluorine-18, it can be distributed to imaging facilities without on-site cyclotrons. In general, the clinical evaluation of glucose metabolism of FDG is based on qualitative inspection or semiquantitative analysis of region-of-interest values or lesion radioactivity normalized to the injected dose and body weight (standard uptake value).

Qualitative inspection focuses on the identification of abnormal regions of increased uptake greater than the background blood pool (as gauged by mediastinal uptake). Semiquantitative analysis has been cited and implemented with variable results because of its reliance on steady-state conditions for glucose uptake and metabolism. Conditions such as blood glucose and insulin levels and the number of transporters on a specific tumor cell(which is not generally known) have a large impact on diseased and nondiseased soft-tissue uptake and should be taken into consideration.