Hepatic cryosurgery precision: evaluation of ultrasonography, thermometry, and impedancemetry in a pig model

One of the main problems of the use of liver cryosurgery is to be sure that a defined hepatic volume has been completely destroyed. We undertook an experimental pig study to determine histopathological evolution of cryolesions, to evaluate the value of intraoperative sonography, thermometry, and impedancemetry to monitor necrosis and to evaluate clinical and biological repercussions of hepatic cryosurgery. Forty-eight cryolesions were obtained by freezing each liver lobe of 12 experimental pigs during a 5-min contact with a flat cryoprobe cooled with liquid nitrogen. Cryolesions and the surrounding liver were monitored during cryosurgery by six thermocouple electrodes, five impedance electrodes, and intraoperative sonography. Animals were sacrificed immediately, 6 hr and between day 1 and day 32 after the procedure. Cryolesions were excised, and a full size pathological study was carried out. No morbidity or mortality was observed. At the end of the freezing time, cryolesions were hemispheric in shape, and their radius measured by sonography was 17.7 +/- 1.2 mm (mean +/- SD). Microscopic study showed sequential tissue alterations with edema, ischemic necrosis, tissue slough, and granulation. Cryolesions were sharply delineated from the normal liver tissue. The radius of necrosis at days 2 and 3 was 17 +/- 0.3 mm (mean +/- SD). It showed good correlation with the cryolesion size measured by intraoperative sonography. The temperature threshold to obtain complete normal liver necrosis was -15 degrees C. We found impedancemetry too difficult to use and not precise enough to monitor cryonecrosis. We conclude that intraoperative sonography and thermometry are useful means to monitor the extent of cryonecrosis during liver cryosurgery.