Prediction of the critical thermal zone during pulmonary cryoablation on computed tomography from correlated experimental and clinical findings

Objective: During cryoablation, cells are destroyed at temperatures less than -20°C. The determining factors for local cancer control in pulmonary cryoablation were assessed using computed tomography (CT), isothermal curves, and histologic findings in pigs. Experimental findings were compared with clinical CT findings and were extrapolated to local cancer control outcomes. Methods: Cryoablation was performed with thermal monitoring, and the ablated areas were divided into 3 zones: less than -20°C, -20°C to 0°C, and greater than 0°C and were compared with histologic findings. CT findings with multiplanar reconstruction in 36 nodules were compared with the porcine histologic findings. The relationship between CT findings and 3-year local cancer control was evaluated in 98 nodules. Results: The 3 concentric thermal zones correlated with histologic findings as follows: less than -20°C zone, complete tissue destruction (zone D); -20°C to 0°C zone (which surrounded zone D), hemorrhage with air trapping and maintenance of alveolar structures (zone H); and greater than 0°C zone (outermost), edema with sustained alveolar structures (zone E). The CT findings in 36 nodules showed a central solid zone, a surrounding air-containing zone, and an outside solid zone, corresponding to zones D, H, and E, respectively. Local cancer control at 3 years in 80 nodules contained within the central solid zone was significantly greater compared with the 18 nodules that were not (82% vs 33%, P = .0002). Conclusions: Pulmonary cryoablation should be performed such that tumors are contained within the central solid zone on CT, which represents the less than -20°C zone.