A radioactive substance being studied in the diagnosis of cancer. Fluorothymidine F 18 is injected into the blood and builds up in cells that are dividing, including cancer cells. The radiation that it gives off as it decays (breaks down) helps make clear pictures of tumors during positron emission tomography (PET) scans. It is a type of radiopharmaceutical. Also called 18F-FLT and 3’-deoxy-3’-(18F) fluorothymidine.