The PET/CT images show several examples of physiologic / non-neoplastic FDG findings.
Maximum intensity projection (MIP) image
Symmetric hilar/mediastinal lymph node hypermetabolism, a classic pattern for granulomatous disease such as sarcoidosis
Linear FDG uptake in the colon, which in absence of focal uptake or soft tissue mass is usually physiologic (and often more prominent in patients taking Metformin – as in this patient)
Physiologic excretion of FDG in the renal collecting systems, left distal ureter, and bladder
The retroperitoneal mass has homogeneous low-moderate FDG uptake similar in intensity to the liver.
MRI demonstrates T2 hyperintense lesions in the medial right hepatic dome (top row) and adjacent to the gallbladder fossa (bottom row). These lesions have arterial enhancement and decreased enhancement on delayed images (although not washout to less intense than background liver).
The liver and retroperitoneal mass were biopsied, showing hepatocellular carcinoma with retroperitoneal nodal metastasis.
If you review the PET/CT images, you’ll find no definite abnormal FDG uptake in the liver. Hepatocellular carcinoma is well known to have variable FDG uptake and may be occult on PET/CT.
In reviewing the mechanism of FDG uptake into tumors, HCC cells are reported have a high level of glucose-6-phosphatase activity (K4 in the graphic below) accounting for their lower FDG uptake. Wahl RL, ed. Principles and Practice of PET and PET/CT, 2nd ed. Philadelphia: Lippincott Williams and Wilkins, 2009.
However the situation may not be so simple, HCC cases with increased FDG uptake have been attributed to higher hexokinase (K3) activity (Torizuka T. JNM 1995;36:1811-1817) and Glut (K1) expression (Paudyal B. Int J Oncol 2008;33:1047-54).
