Cyclophosphamide unmasks an antimetastatic effect of local tumor cryoablation.
= 12) were sacrificed after induction of liver tumors at 4 and 6 weeks. The animals in the treatment group (n= 16) underwent cryosurgery 2 weeks after tumor induction and were allowed to recover before sacrifice 2 and 4 weeks later. Pulmonary metastases were identified in 9 of 12 (75%) control animals at postmortem examination (2 and 6 weeks after tumor inoculation) and in 6 of 16 (38%) rats in the treatment group at sacrifice 2 and 4 weeks after cryosurgery (p= 0.11, Fisher's test for unpaired analysis). Peritoneal deposits were observed in 5 of 12 (42%) control animals at postmortem examination and in 8 of 16 (50%) of the treated animals at the time of cryosurgery (p= 0.95, Fisher's test for unpaired analysis). Two other study animals developed deposits after cryosurgery. The prevalence of peritoneal deposits in the study group was not altered by cryosurgery (p= 0.5, McNemar's test for paired analysis). The importance of the final subzero temperature at the edge of the iceball during tumor ablation by cryosurgery was confirmed by the histologic findings. Complete ablation with no residual viable tumor was obtained only when the subzero temperature had reached −38°C or below. The results of this study do not support the suggestion that hepatic cryosurgery enhances tumor dissemination. The findings also confirm that a subzero temperature at the edge of the iceball of −38°C or lower is necessary to ensure complete ablation of tumor.