Monitoring Renal Cryosurgery: Predictors of Tissue Necrosis in Swine

PURPOSE: To examine parameters for monitoring renal cryosurgery and correlate with histopathologic necrosis in a porcine model. Parameters include: 1) tissue temperature, 2) distance of tissue from the cryosurgical probe insertion site, and 3) inclusion of tissue by the visible iceball. MATERIALS AND METHODS: Following midline incision, 6 healthy kidneys from 3 domestic pigs were treated using a nitrogen-based cryosurgical system with 3.4 mm outer diameter cryoprobe. Temperatures were monitored at 6 sites in each kidney using prototype thermocouples. Gross and histologic analysis was performed on tissue harvested from each thermosensor site 24 hours postoperatively. RESULTS: All animals survived to complete the full protocol. Post-procedure bleeding was managed surgically. Histopathology revealed that complete ablation was uniformly produced at temperatures of -19.4C or lower in 13/13 tissue samples. Within 16 mm. of the probe insertion site, cells were uniformly ablated in 17/17 samples while degree of tissue death varied widely beyond this margin. Cell death was more likely found at points encompassed by the visible iceball (16/18, 89%) than those beyond it (2/18, 11%) [p <0.0001, Chi-square] while 2/18 (11%) of samples harvested from within the iceball margin were partially viable. CONCLUSIONS: Temperature monitoring using thermocouples during porcine renal cryosurgery demonstrated complete homogeneous necrosis of tissues reaching -19.4C or lower. Distance beyond 16 mm. from the cryoprobe and direct visualization of the iceball proved to be less reliable predictors of tissue necrosis. Management of bleeding post-thaw was necessary in every case.