Objective: This study was performed to evaluate risk factors predictive of local tumor control after microwave ablation of primary and secondary lung malignancies up to 3 cm in maximal diameter.
Materials and methods: The single-antenna microwave ablation treatment of 91 index tumors in 57 patients was studied retrospectively. Time to local tumor progression was monitored on CT scans over the follow-up period. Estimation of overall time to local tumor progression was performed with the Cox regression model. Factors hypothesized to correlate with ablation response included tumor diameter, tumor shape (round or oval versus irregular), clear versus ill-defined tumor margin, adjacency to the pleura, adjacency to bronchi, presence of vessels at least 3 mm in diameter a maximum of 5 mm from the index tumor, energy applied to the index tumor, and the occurrence of cavernous formations after ablation. A logistic regression model was used to correlate the data.
Results: Thirty of 91 (33.0%) index tumors, found in 21 of 57 (36.8%) patients, underwent local progression. The mean time to local tumor progression was 8.3 ± 5.5 months (range 2.1-25.2 months), and the estimated median time to local tumor progression was 22.6 ± 12.4 months. The risk factors that correlated significantly with local tumor progression were a maximal diameter greater than 15.5 mm (p < 0.01), irregular shape of the index tumor (p < 0.01), pleural contact (p = 0.02), and less than 26.7 J/mm(3) applied to the index tumor (p < 0.001). After regression analysis, shape of the index tumor (p = 0.03) and energy deployed per unit volume of the index tumor (p = 0.001) were found to be independent risk factors. Conversely, tumor margin definition (p = 0.06) and proximity of cavernous formations (p = 0.19), juxtatumoral vessels (p = 0.08), and bronchi (p = 0.89) did not affect tumor progression after ablation.
Conclusion: The independent predictive factors for local tumor progression in primary and secondary lung neoplasms up to 3 cm in diameter observed in this study were irregular shape of the index tumor and energy application of less than 26.7 J/mm(3) to the index tumor.