Percutaneous radiofrequency ablation for hepatic tumors: factors affecting baseline impedance

PURPOSEWe aimed to evaluate factors that affect baseline impedance of percutaneous radiofrequency ablation.METHODSIn this retrospective study, we analyzed 51 patients with 55 hepatic tumors from November 2015 until April 2018. We measured the baseline impedance nine times with three adjustable tip sizes (2 cm, 2.5 cm, 3 cm) and three different pad locations (two pads attached on the thigh, four on the thigh, two on the back). The first roll-off time was measured with two grounding pads attached on the back. Body mass index, cirrhotic or non-cirrhotic liver parenchyma, previous procedure, tumor location, artificial ascites, active tip size, and the pad location were evaluated as potential factors affecting baseline impedance using the Mann–Whitney U test, t-test and analysis of variance test.RESULTSComplete radiofrequency ablation was achieved in 51 patients. Body mass index (p = 0.897), cirrhotic or non-cirrhotic liver parenchyma (p = 0.767), previous procedure (p = 0.957), tumor location (p = 0.906), and artificial ascites (p = 0.882) did not significantly affect baseline impedance. Grounding pads located on the back showed the lowest baseline impedance (p < 0.001). Increase in active tip size showed gradual decrease in baseline impedance (p = 0.016).CONCLUSIONThe factors affecting baseline impedance were the pad location and the tip size. Positioning pads on the back lowers the baseline impedance and can shorten the first roll-off time, ultimately resulting in reduced total ablation time.

Hepatocellular carcinoma (HCC) continues to be the most prevalent primary liver malignancy and is the third leading cause of cancer-related deaths worldwide (1–3). According to the 2018 European Association for the Study of the Liver clinical practice guidelines (4), ablation is the standard of care for patients with very early stage HCC (Barcelona clinic liver cancer staging system, BCLC-0), and can be adopted as first-line therapy even in surgical patients. Additionally, thermal ablation in single tumors sized 2 to 3 cm is an alternative to surgical resection based on tumor location and patient condition (4). Baseline impedance of radiofrequency ablation (RFA) is measured with a single monopolar electrode before ablation automatically (5). The impedance-controlled mode is generated with an initial set value, which can be adjusted by means of a radiofrequency (RF) output control (5). The first roll-off occurs when the impedance is increased 60% over the initial value (5). Hence, low baseline impedance suggests a short first roll-off time, which is expected to eventually reduce the entire ablation time (6). The aim of this study was to evaluate factors that significantly affect baseline impedance during RFA.