Needle Implantation Cryoprobes: Biophysical and Thermal Characteristics

There has been increased recent interest in hepatic cryosurgery for primary and metastatic tumors in the liver. This has been realized through technological advances in intraoperative ultrasound monitoring and cryoprobe technology. Purpose-designed needle implantable cryoprobes have been developed to freeze deep-seated tumors in the hepatic parenchyma. However, the biophysical and thermal characteristics of these implantable cryoprobes have not been studied. This article discusses the biophysical and thermal characteristics of recently developed high-efficiency, implantable needle cryoprobes that can be used laparoscopically. The cryolesion was formed along the whole length of the cryoprobe implanted into hepatic tissue. It was cylindrical in shape and extended 8 mm beyond the tip of the cryoprobe after 20 minutes of freezing. During this period of freezing, the volume of the cryolesion increased in a near constant manner, but the diameter increased in a logarithmic fashion. In addition, single-cycle freezing produced a larger cryolesion than a dual freeze cycle interrupted by a 5-minute thaw. Further, increasing the length of cryoprobe implantation increased the volume of the cryolesion, although the diameter was smaller. The rate of cooling at the tip of the cryoprobe was also faster with partial implantation of the cryoprobe than with complete implantation. The cooling rate varied vertically along the length of the cryoprobe, as well as horizontally from the cryoprobe.