High-Uptake Areas on ¹⁸F-FRP170 PET Image Necessarily Include Proliferating Areas in Glioblastoma: A Superimposed Image Study Combining ¹⁸F-FRP170 PET with ¹¹C-methionine PET

Abstract

Areas of uptake on positron emission tomography with 1-(2-[ 18 F]fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole (FRP170 PET), a hypoxic cell radiotracer, can include regions retaining highly proliferative activity despite tissue hypoxia. The aim of this study was to clarify whether FRP170 image can detect densely populated hypoxic areas without proliferating potential in glioblastoma. We performed FRP170 PET scan and L-methyl- 11 C-methionine (MET) PET scan in eight patients with non-treated glioblastoma. Standardized uptake values (SUVs) within tumor and apparent normal brain were measured on each FRP170 and MET image for all patients. To visualize actively proliferative areas on MET image we initially extracted pixels showing a ratio of SUV in tumor to SUV in normal brain (T/N) > 1.6. For FRP170 image, we changed the thresholds between minimum SUV and maximum SUV in tumor. Pixels showing SUV above each threshold were extracted and superimposed on previously extracted pixels from MET image. We estimated whether pixels extracted with MET and FRP170 were visually separated on superimposed image for each pa-tient. When no threshold was established, uptake areas on MET image and FRP170 image overlapped widely on superimposed image in all patients. The higher threshold for FRP170 image diminished FRP170-extracted pixels, and shrunk overlapped areas on superimposed image. However, pixels extracted from FRP170 images could not be completely separated from pixels extracted from MET images in all patients, even with threshold raised to almost maximum SUV. The current findings suggest that uptake areas on FRP170 PET scan necessarily include proliferating areas in glioblastoma.