Simple SummaryGastrointestinal cancers represent a major burden in oncology worldwide. As radiotherapy is a cornerstone of the treatment strategies, accurate treatment planning is necessary. Imaging modalities, such as positron emission tomography with computed tomography (PET/CT) and magnetic resonance (MRI) could improve target volume determination. This review summarizes the role of PET/CT and MRI in radiotherapy treatment planning for anal, esophageal, rectal and pancreatic cancer.AbstractPositron emission tomography with computed tomography (PET/CT) and magnetic resonance imaging (MRI) could improve accuracy in target volume determination for gastrointestinal cancers. A systematic search of the PubMed database was performed, focusing on studies published within the last 20 years. Articles were considered eligible for the review if they included patients with anal canal, esophageal, rectal or pancreatic cancer, as well as PET/CT or MRI for radiotherapy treatment planning, and if they reported interobserver variability or changes in treatment planning volume due to different imaging modalities or correlation between the imaging modality and histopathologic specimen. The search of the literature retrieved 1396 articles. We retrieved six articles from an additional search of the reference lists of related articles. Forty-one studies were included in the final review. PET/CT seems indispensable for target volume determination of pathological lymph nodes in esophageal and anal canal cancer. MRI seems appropriate for the delineation of primary tumors in the pelvis as rectal and anal canal cancer. Delineation of the target volumes for radiotherapy of pancreatic cancer remains challenging, and additional studies are needed.