Role of F-18 FDG PET or PET/CT in the evaluation of gastric cancer

Full text: PET detects only less than 50% of early gastric cancer. Therefore, mass screening programs using endoscopy or various radiological tests are recommended for all adults over the age of 40 for early detection and early treatment of gastric cancer. The sensitivity of FDG PET in detecting AGCs appears to be quite variable from 48% to 98% mainly depending on histologic subtypes and depth of invasion of gastric cancers included. Of histologic subtypes, many suggested that signet ring cell and mucinous subtypes show low degree of FDG uptake because of an extracellular or intracellular mucin component. Some proposed microscopic tumor growth according to Lauren's classification as a factor showing a positive correlation with the degree of FDG uptake. Others reported the effect of macroscopic tumor type and the grade of differentiation on FDG uptake with inconsistent conclusions between studies. EGC seems to be the only finding in the prediction of low grade FDG uptake without controversy. The most important step after being diagnosed with gastric cancer is accurate staging, which mainly evaluates tumor resectability to avoid unnecessary surgery. Important factors that affect tumor resectability are whether the tumor can be separated from adjacent organs or important blood vessels, the extent of lymph node metastasis, presence of peritoneal metastasis, or distant organ metastasis. To evaluate the extent of local tumor invasion, anatomical imaging that has superior spatial resolution is essential. There are a few studies on prognostic significance of FDG uptake with inconsistent results between them. In spite of lower sensitivity for lymph node staging, the specificity of CT and PET are very high, and the specificity for PET tends to be higher than that for CT. Limited data published so far show that PET seems less useful in the detection of lung and bone metastasis. In the evaluation of pleural or peritoneal metastasis, PET seems very specific but insensitive as well. When FDG uptake of the primary tumor is low, metastasis is also known to show low FDG uptake reducing its detection. There are only a few data available in the evaluation of recurrence detection and treatment response using FDG PET. (author)