Cryosurgery: Analysis and Experimentation of Cryoprobes in Phase Changing Media

This article presents a retrospective of work performed at the Technion, Israel Institute of Technology, over the last 3-odd decades. Results of analytical and numerical studies are presented briefly as well as in vitro and in vivo experimental data and their comparison to the derived results. Studies include the analysis of both the direct (Stefan) and the inverse-Stefan phase-change heat transfer problems in a tissue-simulating medium (gel) by the application of both surface and insertion cryoprobes. The effects of blood perfusion and metabolic heat generation rates on the advancement of the freezing front are discussed. The simultaneous operation of needle cryoprobes in a number of different configurations and the effects of a thermally significant blood vessel in the vicinity of the cryoprobe are also presented. Typical results demonstrate that metabolic rate in the yet nonfrozen tissue, will have only minor effects on the advancement of the frozen front. Capillary blood perfusion, on the other hand, does affect the course of change of the temperature distribution, hindering, as it is increased, the advancement of the frozen front. The volumes enclosed by the “lethal” isotherm (assumed as $−40°C$⁠), achieve most of their final size in the first few minutes of operation, thus obviating the need for prolonged applications. Volumes occupied by this lethal isotherm were shown to be rather small. Thus, after 10 min of operation, these volumes will occupy only about 6% (single probe), 6–11% (two probes, varying distances apart), and 6–15% (three probes, different placement configurations), relative to the total frozen volume. For cryosurgery to become the treatment-of-choice, much more work will be required to cover the following issues: (1) A clear cut understanding and definition of the tissue-specific thermal conditions that are required to ensure the complete destruction of a tissue undergoing a controlled cryosurgical process. (2) Comprehensive analyses of the complete freeze/thaw cycle(s) and it effects on the final outcome. (3) Improved technical means to control the temperature variations of the cryoprobe to achieve the desired thermal conditions required for tissue destruction. (4) Improvement in the pretreatment design process to include optimal placement schemes of multiprobes and their separate and specific operation. (5) Understanding the effects of thermally significant blood vessels, and other related thermal perturbations, which are situated adjacent to, or even within, the tissue volume to be treated.