Microwave liver ablation: Influence of hepatic vein size on heat-sink effect in a porcine model

PURPOSE: To determine influence of hepatic vein size on perfusion-mediated attenuation in adjacent microwave thermal ablation. MATERIALS AND METHODS: With approval of the institutional animal research committee, seven Yorkshire pigs underwent percutaneous (n = 2) or open (n = 5) microwave liver ablation under general anesthesia. In each, multiple ultrasound-guided, nonoverlapping thermal lesions were created within 1 cm of hepatic veins in a 5-10-minute ablation at 45 W. After euthanasia, the liver was harvested and sectioned at 0.5-cm intervals and the degree of perivascular coagulation attenuation was graded on histopathologic analysis. Correlation between venous size (small, <= 3 mm; medium, 3-6 mm; and large, > 6 mm) and attenuation grade was performed with use of the Spearman rank test. RESULTS: In 63 of 103 sections (61%)-29 of 37 (78%) small, 27 of 48 (56%) medium, and seven of 18 (39%) large veins-the thermal injury extended to the vein wall around the entire circumference of the coagulation front without distortion of the ablation margin. In 40 of 103 sections (38.9%), varying degrees of concave distortion of perivenous ablation margins were noted, with significant correlation between vein size and heat-sink extent (P <.01). However, thermal injury extended to the vascular wall in all sections without complete circumferential sparing of liver tissue. Around two thrombosed veins, thermal lesions encased the vessels, producing paradoxically convex ablation margins. CONCLUSIONS: Although the heat-sink effect was significantly dependent on hepatic vein size, the majority of pathologic section's exhibited no or minimal effect. Further study is required to assess clinical implications.