This is my take on it.
As already indicated the PET scan looks for metabolic anomalies while the CT scan looks at the morphology (structure). The PET scan also may indicate spread more clearly. Both of these are used for planning IMRT treatments and are performed together (generally ?). You need to know which areas to zap and which to protect, for this a 3D representation is needed which shows where the cancer is or is suspected to be after surgery for instance.


Markus