New strategies for the placement of cryoprobes in malignant tumors of the liver for reducing the probability of recurrences after hepatic cryosurgery.

Abstract

We examined modifications in the conventional techniques of cryosurgery for reducing the relatively large number of recurrences after hepatic cryosurgery. Since recurrences are likely to develop in tumor regions which are not frozen to lethal temperatures, the temperature distribution in frozen livers was studied by thermocouple techniques and by calculating the spatial and temporal alterations in the thermal fields for detecting, if present, nonlethally frozen sectors in a frozen tumor tissue. Misplacement of the cryoprobes and the presence of blood vessels in the vicinity of the target tissue were found to be the main sources for nonoptimal freezing of the tumor. Furthermore an overestimation of the size of the lethally frozen tumor regions is possible because of the slow fusion and long-lasting shape alterations in the lethally frozen regions growing together from several cryoprobes. The placement of cryoprobe was optimized by aiming for positions of the cryoprobes by which the tumor is totally enclosed by the calculated -50 degrees C isotherm. The effect of blood vessels could be compensated by thermally shielding the target tissue with an additional cryoprobe between tumor and vessel. In irregularly shaped tumors the placement of cryoprobes was improved by calculating the temperature distribution for cryoprobes in positions thought to be optimal and examining whether this selected placement of cryoprobes cooled all regions of the tumor to lethal temperatures. This procedure was performed before actual cryosurgery on a digitized image of the tumor displayed on the PC screen, and, if required, positions of the cryoprobes were altered until calculations showed that all of the target tissue was frozen to lethal temperatures. For small tumors the placement of the cryoprobes outside the target was found to be advantageous. Under such conditions the tumor was so rapidly cooled to lethal temperatures by synergistic effects between the cryoprobes that excessive cryodamage in normal liver tissue was avoided.