ABSTRACT
Radiotherapy (RT) is used alone or in combination with chemotherapy in the treatment of many cancers in oncology. Fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography (PET/CT), which is widely used in evaluating the response to RT, is a guide in the treatment process and course of the patient and has an important role in determining survival and prognosis. In case of response to treatment, the planned target volume and RT dose can be reduced, and treatment intensity can be increased in resistant tumors. The most important problem in evaluating the response to RT with PET/CT is to distinguish between residual tumor and radiation-induced inflammation and changes. In evaluating response to treatment, the metabolic and anatomical response can be evaluated together or a metabolic evaluation can be performed. Although visual and/or quantitative criteria and PET parameters are used in the metabolic evaluation, there are not standard quantitative criteria for evaluating response to RT. In this review, the place and importance of PET/CT, which has an important role in radiation oncology practice, in evaluating the response to RT will be discussed.