Defining New Metrics in Microwave Ablation of Pulmonary Tumors: Ablation Work and Ablation Resistance Score.

Purpose: To investigate pulmonary microwave ablation metrics including ablation work, ablation resistance score, and involution.

Materials And Methods: Retrospective review was performed of 98 pulmonary tumor ablations using the NeuWave Certus Microwave Ablation System (NeuWave Medical, Madison, Wisconsin) in 71 patients (32 men and 39 women; mean age, 64.7 y ± 11.5). Ablation work was defined as sum of (power) * (time) * (number of antennas) for all phases during an ablation procedure. Ablation zone was measured on CT at 3 time points: after procedure, 1-3 months (mean 47 d), and 3-12 months (mean 292 d). Ablation zones were scored based on location for pulmonary lobe (upper = 1, middle/lingula = 2, lower = 3) and region (peripheral = 1, parenchymal = 2, central = 3), and the 2 were summed for ablation resistance score.

Results: Ablation zone on CT at 1-3 months was significantly smaller in regions with higher ablation resistance score (P < .05). There was a significant correlation between ablation work and ablation zone measured on CT performed after procedure (P < .001), at 1-3 months (P < .001), and at 3-12 months (P < .05). Ablation zone significantly decreased from after procedure to 1-3 months (P < .001) and from 1-3 months to 3-12 months (P < .001), with change from after procedure to 1-3 months significantly greater (P < .01).

Conclusions: Pulmonary microwave ablation zone is significantly smaller in regions with higher ablation resistance score. Ablation work correlates to ablation zone with a nonlinear involution pattern in the first year and may be useful for planning before the procedure.