  Purpose: Analysis of [18F]-Fluorodeoxyglucose (FDG) kinetics in cancer has
been most often limited to the evaluation of the average uptake over relatively
large volumes. Nevertheless, tumor lesion almost contains inflammatory
infiltrates whose cells are characterized by a significant radioactivity
washout due to the hydrolysis of FDG-6P catalyzed by glucose-6P phosphatase.
The present study aimed to verify whether voxel-wise compartmental analysis of
dynamic imaging can identify tumor regions characterized by tracer washout.
Materials & Methods: The study included 11 patients with lung cancer submitted
to PET/CT imaging for staging purposes. Tumor was defined by drawing a volume
of interest loosely surrounding the lesion and considering all inside voxels
with standardized uptake value (SUV) >40% of the maximum. After 20 minutes
dynamic imaging centered on the heart, eight whole body scans were repeated.
Six parametric maps were progressively generated by computing six regression
lines that considered all eight frames, the last seven ones, and so on, up to
the last three. Results: Progressively delaying the starting point of
regression line computation identified a progressive increase in the prevalence
of voxels with a negative slope. Conclusions: The voxel-wise parametric maps
provided by compartmental analysis permits to identify a measurable volume
characterized by radioactivity washout. The spatial localization of this
pattern is compatible with the recognized preferential site of inflammatory
infiltrates populating the tumor stroma and might improve the power of FDG
imaging in monitoring the effectiveness of treatments aimed to empower the host
immune response against the cancer.
