Positron emission tomography (PET) is a new diagnostic method recently widely used in oncology. It is applied to distinguish benign and malign tumors, to diagnose relaps or posttherapeutic changes. PET is also complementary method to the standard used methods such as CT or NMR. Now it is used even as the predict factor of the treatment response and it can choose patients with worse prognosis. PET can help define a target volume in the radiotherapeutic planning. Daily using of PET in the oncologic praxis can change treatment strategy and it can improve therapeutic results. In our review we would like to summarize new informations about using PET in the treatment planning.