Impact of Energy and Access Methods on Extrahepatic Tumor Spreading and the Ablation Zone: An Ex vivo Experiment Using a Subcapsular Tumor Model

Abstract

Objective: To evaluate the impact of energy and access methods on extrahepatic tumor spreading and the ablation zone in an ex vivo subcapsular tumor mimic model with a risk of extrahepatic tumor spreading.

Materials and methods: Forty-two tumor-mimics were created in bovine liver blocks by injecting a mixture of iodine contrast material just below the liver capsule. Radiofrequency (RF) ablations were performed using an electrode placed parallel or perpendicular to hepatic surface through the tumor mimic with low- and high-power protocols (groups 1 and 2, respectively). Computed tomography (CT) scans were performed before and after ablation. The presence of contrast leak on the hepatic surface on CT, size of ablation zone, and timing of the first roll-off and popping sound were compared between the groups.

Results: With parallel access, one contrast leak in group 1 (1/10, 10%) and nine in group 2 (9/10, 90%) (p < 0.001) were identified on post-ablation CT. With perpendicular access, six contrast leaks were identified in each group (6/11, 54.5%). The first roll-off and popping sound were significantly delayed in group 1 irrespective of the access method (p = 0.002). No statistical difference in the size of the ablation zone of the liver specimen was observed between the two groups (p = 0.247).

Conclusion: Low-power RF ablation with parallel access is proposed to be effective and safe from extrahepatic tumor spreading in RF ablation of a solid hepatic tumor in the subcapsular location. Perpendicular placement of an electrode to the capsule is associated with a risk of extrahepatic tumor spreading regardless of the power applied.

Keywords: Experimental liver neoplasm; Neoplasm seeding; Radiofrequency ablation; Thermal ablation.

Fig. 1. US of subcapsular tumor mimic model.

US image reveals hyperechoic mass-like lesion (arrows) measuring approximately 1.6 cm in diameter just below liver capsule (arrowheads). US = ultrasound

Fig. 2. Photographs of experimental models and RF ablation system.

Sectioned bovine liver block with tumor mimic created in subcapsular portion is placed in water plate. (A) Electrode is inserted at lateral surface of liver block and advanced parallel to capsule (arrows). (B) Electrode is inserted at bottom of liver block opposite capsule and advanced through tumor mimic towards capsule (arrows). Lateral side of paper case is open for US guidance (arrowheads). RF = radiofrequency

Fig. 3. US of parallel and perpendicular access methods.

A. US image illustrating parallel access method. Electrode (arrow) is placed parallel to liver capsule (arrowheads) through tumor mimic (open arrows). B. US image illustrating perpendicular access method. Tip of electrode (arrow) extends towards liver capsule (arrowheads) through tumor mimic (open arrow).

Fig. 4. RF ablation with low-power protocol and parallel access.

A, B. Pre- and post-RF ablation CT images. Post-RF ablation CT image (B) reveals no visible contrast leak on upper hepatic surface, compared to that observed on pre-RF ablation CT image (A). C. Liver specimen corresponding to CT images. Analysis of specimen revealed no visible contrast leak on surface of liver. CT = computed tomography, RF = radiofrequency

Fig. 5. RF ablation with high-power protocol and parallel access.

A, B. Pre- and post-RF ablation CT images. Post-RF ablation CT image (B) reveals contrast leak on hepatic surface (arrow) compared to that observed on pre-RF ablation CT image (A). High-density lines (arrowheads) inferior to tumor mimic (open arrows) is track through which tumor mimic material was injected. C. Liver specimen corresponding to CT images. Specimen image illustrates track of contrast leak corresponding to tract on post-RF ablation CT (arrow).

Fig. 6. RF ablation with high-power protocol and perpendicular access.

A, B. Pre- and post-RF ablation CT images. Post-RF ablation CT image (B) reveals contrast leak on hepatic surface after RF ablation, compared to that observed on pre-RF ablation CT image (A) (arrow). C. Liver specimen corresponding to CT images. Analysis of specimen reveals contrast leak on hepatic surface (arrowheads). D. Another liver specimen with low-power protocol and perpendicular access. Analysis of specimen reveals no contrast leak on hepatic surface, and diameter of ablation zone just beneath capsule is larger than that of specimen shown in (C) (double-headed arrows on C, D) (Scale bar: 5 cm).