Pulmonary cryoablation zones: More aggressive ablation is warranted in vivo

OBJECTIVE. The objective of our study was to determine the effective cryoablation zone when treating pulmonary tumors in vivo and to create pulmonary-specific ablation maps to guide clinical procedure planning. MATERIALS AND METHODS. Ablation volume was measured retrospectively in human patients after pulmonary tumor cryoablation with a triple-freeze protocol. Single-probe ablations were performed with 17-, 14-, and 13-gauge cryoprobes; multiple-probe ablations were performed with two or three 17-gauge probes. Statistical comparisons of ablation volumes to manufacturer reference values were calculated using the Wilcoxon rank-sum test. Comparisons of ablation sizes by the number of probes were evaluated by the Kruskal-Wallis test. RESULTS. Mean volume of in vivo lung ablation with a single 17-gauge cryoprobe measured 3.0 cm ³ , which is a statistically significant difference compared with the in vitro -20°C isotherm volume of 22.6 cm ³ (p < 0.01). Mean ablation volume of larger 13- and 14-gauge cryoprobes were 4.3 and 1.8 cm ³ , respectively, both of which are smaller than the in vitro -20°C isotherm volume. Mean cryoablation zone was not significantly affected by distance to the pleura (p = 0.54) or distance to a vessel (p = 0.55). Ablation volume was significantly increased (p < 0.01) with the use of multiple cryoprobes, at a rate of a 10.8-cm ³ increase per additional probe. The increased ablation zone size was more attributable to increased short-axis width (9.6-mm increase per probe) compared with long-axis length (5.6-mm increase per probe). CONCLUSION. The in vivo effective pulmonary cryoablation zone is significantly smaller than the manufacturer-published in vitro isotherm. Larger ablation margins in lung are best achieved by using multiple cryoprobes.