ABSTRACT
Radiotherapy (RT) is a standard approach in the multimodal treatment of various gastrointestinal (GI) tumors; these tumors include esophageal cancer, stomach cancer, rectal cancer and anal cancer. Additionally, RT is preferred as an alternative to surgery in patients with liver cancer, biliary tract cancer and pancreatic cancer. Positron emission tomography (PET), often combined with computed tomography (CT), is important in diagnosing, assessing treatment response, and restaging a variety of GI tumors. However, the additional value of PET also in terms of adaptive RT, especially in the treatment planning and dose definition process, is still unclear. When performed during RT, PET aims to evaluate changes in functional tumor volume due to treatment, thus helping to reduce the radiation target volume. Additionally, by including PET images in RT planning, tumor identification can be more accurate and can help determine metabolic tumor volume. This review addresses the additional value of PET in adaptive RT protocols and its contribution to target volume adaptation for individualized treatment strategies in esophagus, stomach, pancreas, liver, biliary tract, rectum and anal neoplasias.