Abstract No. 494 Microwave ablation zone observations in a large series with recommendations for adjustments

Abstract

Microwave ablation (MWA) is and established technology that has recently been applied to interventional oncology. Ablations rely on a certain power (watts) for a certain time (minutes/seconds). Charts provided by manufacturers are produced using ex-vivo bovine liver at 20°C. Results in living patients do not conform with the charts. This retrospective analysis of 121 ablation zones evaluates differences and suggests how to adjust parameters in clinical practice. 73 patients underwent MWA of 136 liver lesions in 85 procedures. Lesions measured mean 2.7 × 2.3 × 2.4 cm (SDev 1.4 × 1.2 × 1.4 cm), HCC n = 91, Cholangio n = 6, metastasis n = 39. Each lesion was treated with a single ablation. Manufacturer anticipated ablation zone and actual ablation zone measured within 24 hours on venous phase CECT images were compared. Power/time values on manufacturer charts compared to operator selected (standard (S) vs non-standard (NS) ablations); HCC vs other lesion pathology sought to assess the effect of a presumed fibrotic liver parenchyma. Overall mean anticipated ablation zone dimensions were 4.8 × 3.7 × 3.7 cm (SDev 1.1 × 0.8 × 0.8 cm) compared to actual 4.1 × 3.0 × 3.2 cm (SDev 1.2 × 0.9 × 1.1 cm) (n = 121). Differences in individual dimensions were highly significant ttest p<0.0005. Zone volumes using volume of an oval, were 37.5 ± 25 cm3 vs 24.6 ± 24.1cm3 (p<0.0001) mean 68% of predicted (range, 8.5-217%). In 107/136 lesions the zone was mean 50.4 ± 23.1% smaller, in 29/136 mean 133.6 ± 27.3% larger. In S ablations, zones were mean 64.8 ± 41.8% predicted versus 78.7 ± 40.5% predicted in NS ablations. HCC ablation zones were mean 63.5 ± 40.1% predicted versus 67.2 ± 46.1% in non-cirrhotic livers. In approximately ¾ ablations the actual ablation zone is significantly smaller than suggested based on manufacturer charts. Individualizing power/time values may improve this by inadvertent underestimation of anticipated ablation zones. Cirrhosis may exacerbate discrepancies. Larger zones did not increase complication rates. Planning of larger ablation zones is recommended to improve outcomes.