- Positron emission tomography (PET) scans use a radioactive sugar known as 18F-FDG to try to determine if a tumor is active or not. Active tumors generally take up more sugar than the surrounding tissue, but because normal brain tissue uses the same sugar as brain tumors, it is then difficult to tell if tumor tissue is taking up sugar or not. A different radioactive agent, 18F-FLT, is now being studied in some adults with different kinds of tumors. Researchers are interested in determining whether it is possible to use this agent as a marker of tumor activity in children.