Cooling Effects Occur in Hepatic Microwave Ablation At Low Vascular Flow Rates and in Close Proximity to Liver Vessels - Ex Vivo.

Background. The impact of vascular cooling effects in hepatic microwave ablation (MWA) is controversially discussed. The objective of this study was a systematic assessment of vascular cooling effects in hepatic MWA ex vivo. Methods. Microwave ablations were performed in fresh porcine liver ex vivo with a temperature-controlled MWA generator (902-928 MHz) and a non-cooled 14-G-antenna. Energy input was set to 9.0 kJ. Hepatic vessels were simulated by glass tubes. Three different vessel diameters (3.0, 5.0, 8.0 mm) and vessel to antenna distances (5, 10, 20 mm) were examined. Vessels were perfused with saline solution at nine different flow rates (0-500 mL/min). Vascular cooling effects were assessed at the largest cross-sectional ablation area. A quantitative and semi-quantitative/morphologic analysis was carried out. Results. 228 ablations were performed. Vascular cooling effects were observed at close (5 mm) and medium (10 mm) antenna to vessel distances (P < .05). Vascular cooling effects occurred around vessels with flow rates ≥1.0 mL/min (P < .05) and a vessel diameter ≥3 mm (P < .05). Higher flow rates did not result in more distinct cooling effects (P > .05). No cooling effects were measured at large (20 mm) antenna to vessel distances (P > .05). Conclusion. Vascular cooling effects occur in hepatic MWA and should be considered in treatment planning. The vascular cooling effect was mainly affected by antenna to vessel distance. Vessel diameter and vascular flow rate played a minor role in vascular cooling effects.

Periportal fields cause stronger cooling effects than veins in hepatic microwave ablation: an in vivo porcine study.

BACKGROUND: Vascular cooling effects are a well-known source for tumor recurrence in thermal in situ ablation techniques for hepatic malignancies. Microwave ablation (MWA) is an ablation technique to be considered in the treatment of malignant liver tumors. The impact of vascular cooling in MWA is still controversial. PURPOSE: To evaluate the influence of different intrahepatic vessel types, vessel sizes, and vessel-to-antenna-distances on MWA geometry in vivo. MATERIAL AND METHODS: Five MWAs (902-928 MHz) were performed with an energy input of 24.0 kJ in three porcine livers in vivo. MWA lesions were cut into 2-mm slices. The minimum and maximum radius of the ablation area was measured for each slice. Distances were measured from ablation center toward all adjacent hepatic vessels with a diameter of ≥1 mm and within a perimeter of 20 mm around the antenna. The respective vascular cooling effect relative to the maximum ablation radius was calculated. RESULTS: In total, 707 vessels (489 veins, 218 portal fields) were detected; 370 (76%) hepatic veins and 185 (85%) portal fields caused a cooling effect. Portal fields resulted in higher cooling effects (37%) than hepatic veins (26%, P < 0.01). No cooling effect could be observed in close proximity of vessels within the central ablation zone. CONCLUSION: Hepatic vessels influenced MWA zones and caused a distinct cooling effect. Portal fields resulted in more pronounced cooling effect than hepatic veins. No cooling effect was observed around vessels situated within the central white zone.

Microwave ablation zones are larger than they macroscopically appear - Reevaluation based on NADH vitality staining ex vivo.

BACKGROUND: Animal liver is established as an ex vivo model for studies on hepatic microwave ablation (MWA). Macroscopically visible color changes in the ablation zone are used to assess cell destruction and confirm successful ablation ex vivo. OBJECTIVE: Macroscopy and histology of MWA zones regarding cell viability in ex vivo porcine livers were compared in this study. METHODS: Six MWA were performed in porcine livers post mortem. A 14-G antenna and microwave generator (928 MHz; 9.0 kJ) were used. MWA were cut at the maximum cross section in vertical alignment to the antenna. NADH-diaphorase staining determined cell vitality. Macroscopic and microscopic ablation zones were statistically analyzed. RESULTS: Histology showed two distinct ablation zones: central white zone (WZH) with no cell viability and peripheral red zone (RZH) with partial cell viability. However, the macroscopically visible WZM was significantly smaller than the microscopic WZH with an area difference of 43.1% (p < 0.05) and a radius difference of 21.2% (1.6 mm; p < 0.05). Macroscopy and histology showed a very high correlation for the complete lesion area (WZH/M+RZH/M; r = 0.9; p = 0.001). CONCLUSIONS: The avital central zone is significantly larger as the macroscopically visible WZ which is commonly used to assess successful ablation in MWA ex vivo studies. Irreversible cell destruction can be underestimated in macroscopic evaluation.

Intermittent Pringle maneuver may be beneficial for radiofrequency ablations in situations with tumor-vessel proximity.

BACKGROUND: Radiofrequency ablation (RFA) represents a treatment option for non-resectable liver malignancies. Larger ablations can be achieved with a temporary hepatic inflow occlusion (Pringle maneuver - PM). However, a PM can induce dehydration and carbonization of the target tissue. The objective of this study was to evaluate the impact of an intermittent PM on the ablation size. METHODS: Twenty-five multipolar RFAs were performed in porcine livers ex vivo. A perfused glass tube was used to simulate a natural vessel. The following five test series (each n=5) were conducted: (1) continuous PM, (2-4) intermittent PM, and (5) no PM. Ablations were cut into half. Ablation area, minimal radius, and maximal radius were compared. RESULTS: No change in complete ablation size could be measured between the test series (p>0.05). A small rim of native liver tissue was observed around the glass tube in the test series without PM. A significant increase of ablation area could be measured on the margin of the ablations with an intermittent PM, starting without hepatic inflow occlusion (p<0.05). CONCLUSION: An intermittent PM did not lead to smaller ablations compared to a continuous or no PM ex vivo. Furthermore, an intermittent PM can increase the ablation area when initial hepatic inflow is succeeded by a PM.