Clinical evidence on PET-CT for radiation therapy planning in gastro-intestinal tumors

A large number of histological and anatomically distinct malignancies originate from the gastro-intestinal (GI) tract. Radiotherapy (RT) plays an increasing role in the multimodal treatment of most of these malignancies. The proximity of different organs at risk such as the kidneys, the spinal cord and the small bowel and the potential toxicity associated with combined treatment modalities make accurate target volume delineation imperative. The ability of positron emission tomography (PET) imaging to visualize a so-called 'biological target volume' (BTV) may be helpful in this respect. Currently the most widely used tracer for diagnosis, staging, restaging and response assessment is [18F]Fluoro-deoxyglucose (FDG). Promising preliminary results in esophageal, pancreatic and anorectal cancers and colorectal liver metastasis suggest that FDG-PET might provide us with additional information useful in target volume delineation. Poor image resolution and a low sensitivity for lymph node detection currently obstructs its widespread implementation. Moreover, validation in large prospective trials and the pathological validation of the correct tumor volume is still lacking. In hepatocellular carcinoma (HCC) and gastric adenocarcinoma there is currently little evidence for the use of FDG-PET in target delineation. However more extensive research is warranted before the true value of FDG-PET in these sites can be assessed. Also other tracers are constantly being developed and investigated. Up to now however none of these tracers has found its way into the daily practice of target volume delineation.