What is your differential at this point?
The slight hypervascularity and hypoenhancement in the portal venous phase are very suspicious for a mass with neovascularity such as HCC... but without risk factors and with the nonoptimized study it is difficult to come down hard on the diagnosis. The appearance is not classic for FNH, but hepatic adenoma or an atypical metastasis could be considered. This mass was definitely arising off the liver, but in a more ambiguous situation, a mass arising from some adjacent structure could also be considered.
What is your follow up suggestion?
Although well-marginated, this mass cannot be written off as benign due to its enhancement characteristics. Biopsy would be a reasonable next step in this situation since MRI findings would be unlikely to definiteively distinguish between a well-differentiated HCC and adenoma. Biopsy may still be necessary. MRI could be considered if there is a possibility that the imaging study might obviate biopsy.
How common is HCC in the absence of classic risk factors (cirrhosis or chronic viral hepatitis)?
Much less common, but the more common in North America and Western Europe than elsewhere in the world. Exact % incidence of HCC in a "normal" liver is not well-defined, partly because even without classic risk factors "normal" livers may have less well-correlated risk factors (such as steatosis).
Which is better for characterizing a liver lesion... the early arterial phase or the late arterial phase?
The late arterial phase is the better phase for evaluating liver lesions. If caught too early, a lesion may not have had enough time to demonstrate its hypervascular nature. Irregular neovascularity may be seen around an HCC, but the mass is much harder to detect than on the late arterial phase, at which time all the neovascular capillaries fill with contrast.
There is a ~5 cm round, well-circumscribed mass extending exophytically off the inferior aspect of segment 5. It demonstrates mild arterial phase hypervascularity and mild hypoattenuation on the portal venous phase. Its enhancement is slightly heterogeneous. No evidence of a central scar.
The mass abuts the hepatic flexure of the colon, the gallbladder, and the ventral abdominal wall, without evidence of invasion.
There is no other lesion in the liver. The liver is otherwise normal appearing with no evidence of cirrhosis or portal hypertension. No portal vein thrombosis.