Findings: Post contrast axial CT images through the liver demonstrate no abnormality. Specifically, there is no evidence for a metastatic focus.
Following intravenous administration of 20.8 mCI of Fluorine 18 Florodeoxyglucose, a PET scan of the torso was acquired after a one hour delay. Images demonstrate a 1.5 cm. focus of intense radiopharmaceutical uptake within the medial segment of the left lobe of the liver.
Diagnosis: Metastatic disease to the liver.
Discussion: Positron Emission Tomography (EPT) with Fluorine -18 Florodeoxyglucose (FDG) can detect metastatic foci because of increased metabolic rate. Following intravenous injection of the radiopharmaceutical, one hour delayed images are obtained in three orthogonal planes for viewing purposes. Any abnormality with an increased metabolic rate will show increased activity. This includes both neoplasms as well as infections. However, areas of necrosis will demonstrate decreased activity.
A recent study compared staging of colorectal carcinomas with PET and CT scans. PET demonstrated a positive predictive value of 90% and a negative predictive value of 100%. In addition, FDG PET depicted liver metastasis in 7 out of 8 patients compared to CT which depicted live metastasis in 3 out of 8 patients. However, it should be noted that evaluation for lymph node metastasis was somewhat lower with a sensitivity of 29%.
The half-life of Fluorine -18 FDG is 110 minutes. FDG is similar to glucose and is phosphorylated by hexokinase to FDG-6-phosphate. This compound is then tracked in tissue and accumulates at a rate proportional to its metabolic activity.
References:
Abdel-Nabi A, Doerr RJ, Lamonica DM, et al. Staging of Primary Colorectal
Carcinomas with Fluorine -18 Florodeoxyglucose Whole/Body PET: Correlation
with Histopathologic and CT Findings. Radiology, 1998;206:755-760.
Shields AF, Graham MM, Spence AM. The Role of PET Imaging in Clinical
Oncology. Nuclear Medicine Annual, New York, NY: Raven Press, Ltd;
1995:129-168.
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