Assessment of ablative margin by MRI with ferucarbotran in radiofrequency ablation for liver cancer: comparison with enhanced CT

Objectives

Our aim was to determine whether ablated liver parenchyma surrounding a tumour can be assessed by MRI with ferucarbotran administered prior to radiofrequency ablation (RFA) compared with enhanced CT.

Methods

55 hepatocellular carcinomas (HCCs) in 42 patients and 5 metastatic liver cancers in 3 patients were treated by RFA after ferucarbotran administration. We then performed T₂* weighted MRI after 1 week and enhanced CT after 1 month. T₂* weighted MRI demonstrated the ablated parenchyma as a low-intensity rim around the high intensity of the ablated tumour in these cases. The assessment was allocated to one of three grades: margin (+), high-intensity area with continuous low-intensity rim; margin zero, high-intensity area with discontinuous low-intensity rim; and margin (−), high-intensity area extending beyond the low-intensity rim.

Results

Margin (+), margin zero and margin (−) were found in 17, 35 and 5 nodules, respectively. All 17 nodules with margin (+) and 13 of those with margin zero were assessed as having sufficient abalative margins on CT. The remaining 22 nodules with margin zero had insufficient margins on CT. The overall agreement between MRI and CT for the diagnosis of the ablative margin was moderate (κ=0.507, p<0.001). No local recurrence was found in 15 HCC nodules with margin (+), whereas local recurrence was found in 4 (11.8%) out of 34 HCC nodules with margin zero.

Conclusion

Administration of ferucarbotran before RFA enables the ablative margin to be visualised as a low-intensity rim, and also enables the evaluation of the ablative margin to be made earlier and more easily than with enhanced CT.Radiofrequency ablation (RFA) has become a widely used treatment for hepatocellular carcinoma (HCC) [1], with some studies reporting significant long-term survival results [2,3]. One of the most difficult and troublesome issues in RFA is the lack of a reliable method for confirming that complete necrosis has been achieved in the treated lesion. CT and MRI are commonly used to evaluate the therapeutic response in the ablated tumours. The imaging hallmark of successful treatment is a lack of enhancement in the index tumour on CT or MRI [4,5]. However, previous pathological examination has demonstrated the presence of microsatellite nodules around the original tumour [6,7]. Therefore, it is necessary to ablate liver parenchyma surrounding the original tumour, as well as the tumour itself, and the ablation zone of the surrounding normal tissue needs to be recognised. In fact, several studies [8-10] have reported that the local recurrence rate in nodules with sufficient ablative margin is lower than that in those without sufficient ablative margin. The ablative margin is conventionally assessed by comparing enhanced CT images before and after RFA for HCC tumours.Mori et al [11] reported a new method of evaluating the ablative margin using ferucarbotran (Resovist; Bayer Schering Pharma, Berlin, Germany), and demonstrated that the ablative margin is easily assessed by MRI. Ferucarbotran is a clinically approved superparamagnetic iron oxide (SPIO) that is liver specific on MRI. It is composed of SPIO microparticles (γ-Fe₂O₃) coated with carboxydextran. After intravenous administration, ferucarbotran is phagocytosed by Kupffer cells and equally distributed throughout the entire liver [12]. Kupffer cells are much more dominant in hepatic parenchyma than in cancer tissue. Therefore, the signal intensity from cancer in T₂* weighted sequences becomes relatively high compared with that from hepatic parenchyma. Ferucarbotran in ablated hepatic parenchyma would remain after ablation, showing low intensity around high-intensity cancer on post-ablational MR images.The aim of this study was to determine the usefulness of ablative margin assessment by enhanced MRI using ferucarbotran administered before RFA in patients with liver cancer in comparison with post-ablation enhanced CT images after 1 month.