Lipid metabolism in hepatocelullar carcinoma as one of diagnostic goals by functional imaging

Accurate detection of HCC foci in the liver and at the whole-body level has a significant impact on patient management. Functional whole-body imaging by PET (fused with CT or MRI) with spatial resolution compatible with the detection of lesions <2 cm in size has been proposed to overcome some limitations of morphologic imaging. 18F-fluorodeoxyglucose, the reference PET tracer in oncology, has limitations in the functional imaging of liver tumours. In particular the detection rate of intra-hepatic well-differentiated hepatocellular carcinoma (HCC) is low and incompatible with an effective staging of those patients. To overcome this lack of sensitivity, choline PET racers have been used by several teams: 11C-choline or its analogues 18F-fluorocholine (FCH) and 18F – fluoroethylcholine (FEC). They showed sensitivity compatible with an accurate staging of well-differentiated HCC and also of intermediate or poorly differentiated HCC. Actually dual tracer PET with FDG and a lipid tracer has the best performance, as some lesions in a given patients may be of a different aggressiveness and take-up only one tracer. This can even be the case in one large liver nodule, with a different metabolic marker in different portions of the nodule. Some evidence has also been brought that the detection of distant metastasis benefit from a dual tracer approach. The selection of patients for liver transplantation or HCC tumour resection also benefits from dual tracer PET for an optimal pre-therapeutic staging and potentially for prediction of recurrence. In this aim, visualisation of HCC tumours with FDG is pejorative, whereas visualisation with a lipid tracer was indicative of a better prognosis in pilot studies. Among non-HCC liver malignancies in adults, only cholangiocarcinoma has been reported to take up lipid tracers in small series; FCH uptake has been reported in a child with recurrent hepatoblastoma. Concerning benign liver tumours, adenoma is rarely visible on choline PET, whereas focal nodular hyperplasia (FNH) is visible as a hot focus in the vast majority of cases. To characterise a liver nodule as HCC, this uptake by FNH may be seen as a source of false-positive results. According to one team, FCH could be a good tracer to differentiate in difficult cases between FNH and hepatocellular adenoma which has the potential to degenerate.

Key words: choline, PET, FCH, liver tumour, hepatocellular carcinoma, focal nodular hyperplasia, hepatoblastoma.