Secondary cancer in the liver. Comparison of axial computed tomography (CT, top) and positron emission tomography (PET, bottom) scans of a patient with kidney cancer that has spread to the liver. The secondary cancer in the liver is not seen on the CT scan, but has been detected by the PET scan as the circular pink area. The primary cancer is of the right kidney, and it has spread (metastasised) to the liver. The PET scan has been obtained with 2-18F-fluoro-2-deoxy-D-glucose (FDG), a radioactive tracer that can be used to identify cancer sites.