Microwave ablation in a hepatic porcine model: correlation of CT and histopathologic findings

Abstract

Background: Thermal ablative techniques have gained increasing popularity in recent years as safe and effective options for patients with unresectable solid malignancies. Microwave ablation has emerged as a relatively new technique with the promise of larger and faster burns without some of the limitations of radiofrequency ablation (RFA). Here we study a new microwave ablation device in a living porcine model using gross, histologic, and radiographic analysis.

Materials and methods: The size and shape of ablated lesions were assessed using six pigs in a non-survival study. Liver tissue was ablated using 2, 4, and 8 min burns, in both peripheral and central locations, with and without vascular inflow occlusion. To characterize the post-ablation appearance, three additional pigs underwent several 4 min ablations each followed by serial computed tomography (CT) imaging at 7, 14, and 28 days postoperatively.

Results: The 2 and 4 min ablations resulted in lesions that were similar in size, 33.5 cm(3) and 37.5 cm(3), respectively. Ablations lasting 8 min produced lesions that were significantly larger, 92.0 cm(3) on average. Proximity to hepatic vasculature and inflow occlusion did not significantly change lesion size or shape. In follow-up studies, CT imaging showed a gradual reduction in lesion volume over 28 days to 25-50% of the original volume.

Discussion: Microwave ablation with a novel device results in consistently sized and shaped lesions. Importantly, we did not observe any significant heat-sink effect using this device, a major difference from RFA techniques. This system offers a viable alternative for creating fast, large ablation volumes for treatment in liver cancer.

Keywords: ablation; cancer; hepatocellular; liver; metastatic; microwave; porcine; thermal.

Figure 1.

Gross section of 100 W, 4 min ablation from freshly harvested pig liver. Microwave ablation applicator probe is shown in situ for reference (each line on the probe represents 1 cm). A large portal venous branch is shown near the probe tip (black arrow). No heat sink effect is apparent. Histology from the transition zone (white box) is shown in Figure 2.

Figure 2.

Histology of transition zone at margin of ablated lesion (inset from Figure 1). At the top of the image is necrotic tissue; normal hepatic parenchyma is at the bottom. The transition zone in the center of the image reveals infiltration of erythrocytes, leukocytes, and disruption of the normal hepatic architecture. H&E, ×20.

Figure 3.

CT imaging demonstrating contraction of lesion over time. The same lesion (white arrows) in a single pig imaged on postoperative days 7, 14, and 28 is shown.

Figure 4.

Survival study CT data showing changes in lesion volume over time. Average volume of ablated lesions on postoperative days 7, 14, and 28 based on CT imaging calculations. Horizontal lines indicate the averages (n=8) for each postoperative day with value labeled next to each line.