Quantitative analysis of lung microwave ablation zone volume and shape

We quantified volume and shape variability in lung microwave ablation (LMWA) zones, comparing them with expected ablation zones, exploring the correlation with tissue contraction.

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.

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.

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.

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.

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.