Quantitative analysis of lung microwave ablation zone volume and shape

OBJECTIVE: We quantified volume and shape variability in lung microwave ablation (LMWA) zones, comparing them with expected ablation zones, exploring the correlation with tissue contraction. MATERIALS AND METHODS: After Institutional Review Board approval, we retrospectively included patients who underwent LMWA between January 2015 and January 2019. Exclusion criteria were ablations with multiple burns/probes, overlapping ablation zones, or indistinguishable background lung parenchyma. Ablation zones were oriented along an applicator-centric coordinate system. We used Pyradiomics to generate volumes and the Euler characteristic transform for three-dimensional shape space analysis. Wilcoxon paired signed-rank tests compared the expected versus the actual ablation zone. Tissue contraction was quantified using paired anatomical landmarks before and after computed tomography scans. RESULTS: We included 111 ablations in 72 patients (31 male, 41 female; median age 59). Median ablation power was 65 watts (range 20‒65), median ablation time 5 min (range 1‒10). Total energy correlated with volume and width (p = 0.007, p = 0.003, respectively). Ablation volume did not differ from vendor predictions (p = 0.452), whereas length (p < 0.001) and maximum width (p < 0.001) were greater than predicted. Ablation shapes were more elongated (p < 0.001), less spherical (p < 0.001), asymmetric (wider in back than in front, p = 0.007), and diverged from expected ellipsoids. There was no correlation between tissue contraction and volume, power, or time. CONCLUSION: The provided vendor model offers a reasonable estimate of mean ablation volume, but wide variability in volume and shape necessitates a more bespoke method of ablation zone prediction. Tissue contraction did not correlate with ablation zone variability. RELEVANCE STATEMENT: High variability in actual ablation volume, shape, length, width, and position along the needle and divergence from ellipsoids necessitates the development of improved ablation zone prediction models. KEY POINTS: While median ablation volumes do not differ from vendor predictions, ablation length (p < 0.001) and maximum width (p < 0.001) are greater than vendor predictions, and volumes are variable. Ablation shapes diverge from ellipsoids, are more elongated (p < 0.001), less spherical (p < 0.001), and asymmetric (p = 0.007). Tissue contraction does not account for the volume variability of ablation zones.