PET Scan in Cancer

What malignancies can PET and PET/CT be used for? 

PET is a proved modality in the evaluation of various malignancies, including esophageal, gastric, pancreatic, and colon cancers, as well as GI stromal tumors (GISTs), carcinoid tumors, and lymphoma. In addition, PET/CT has demonstrated utility in the assessment of inflammatory bowel disease. 

Combined with CT, PET/CT exemplifies the value of combined metabolic and anatomic imaging. The most common scenarios in which PET and PET/CT have been useful are staging and surveillance of malignancies and evaluation of postoperative sites. 

Although routine staging of colon cancer is not recommended, 18 F-FDG PET/CT has demonstrated significant benefit in assessing recurrence and restaging. Studies with 18 F-FDG have been useful in patients with increasing carcinoembryonic antigen levels without anatomic abnormalities and in the evaluation of liver metastases, which are often underestimated with other radiologic modalities. 

Detection of primary and metastatic pancreatic cancer using 18 F-FDG PET/CT has been proven as well. Unfortunately, there are some lesions that are not 18 F-FDG avid and the sensitivity of this test has been limited in detecting cystic pancreatic malignancies, mucinous tumors, and low cellular density lesions. In addition, pancreatitis and inflammatory pseudotumors can be falsely positive for malignancy and demonstrate FDG avidity. 

GISTs typically have a rounded, exophytic appearance with well-defined borders on CT imaging. 18 F-FDG PET imaging demonstrates intense activity in malignant GISTs, with lower metabolic activity in non-malignant GISTs. 18 F-FDG PET can also serve in predicting response to therapy. 

One malignancy that has a low sensitivity on 18 F-FDG PET is hepatocellular carcinoma. Typically, poorly differentiated HCC will be 18 F-FDG avid. Well-differentiated HCC can have higher levels of glucose-6-phosphatase, which will dephosphorylate the phosphorylated 18 F-FDG and permit it to leach out of the cell. Alternative PET radiotracers such as 11 C-choline and 11 C-acetate have been shown to have better avidity for well-differentiated HCC but are not as widely available as 18 F-FDG.


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