Positron Emission Tomography (PET) is a nuclear medicine imaging technique using radiopharmaceuticals labelled with short-lived radionuclides for the in-vivo study of physiological, biochemical and pharmacological processes in human health and disease.

Alterations in tumour biology precede structural changes. Tumours show accelerated metabolism, including increases in glycolysis, nucleic acid and protein synthesis. Fluorodeoxyglucose (FDG), a glucose analogue, can differentiate benign from malignant tissue on the basis of increases in glucose metabolism. FDG is the workhorse of PET imaging in oncological disease.

FDG PET has clinical utility in gastrointestinal tumours in initial diagnosis, staging, restaging and evaluating therapy response. FDG PET has the ability to detected occult malignancies in the gastrointestinal tract. Focal colonic metabolic lesions have an incidence of over 40% of premalignant or malignant neoplasms at colonoscopy. FDG PET has proven utility in staging of oesophageal, gastric, pancreatic and colonic malignancies. PET alters management in at least 30% of patients being considered for surgery for limited metastatic disease. FDG PET is indicated in patients being evaluated for resection of pulmonary or liver metastases to stage the extent of disease.

FDG PET has value in therapeutic monitoring of patients treated with chemo- or radiotherapy. PET can detect a response to therapy earlier than conventional imaging methods. PET is useful in evaluating response to neoadjuvant chemotherapy in oesophageal and rectal malignancies. PET has prognostic significance following chemotherapy. Change in FDG uptake after therapy correlates with histopathological response and survival. 

PET has an established and expanding role in evaluating gastrointestinal tumours. The emergence of cytostatic therapeutic agents presents a challenge for conventional imaging methods, as tumours may not show morphological changes. PET has the ability to evaluate the effectiveness of cytostatic drugs early in treatment.