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.