Principles of Cryosurgery

Tissue destruction from cryosurgery is typically described in two phases: freezing and thawing. A liquid nitrogen or liquid argon system is used during the freeze part of the treatment cycle in modern cryosurgery, whereas the thaw portion is usually carried out with helium gas. The freezing phase is performed

Fig. 1. Iceball rapidly with the generation of an iceball with a core temperature of -196°C (Fig. 1). Freezing creates cell death by several mechanisms. Subfreezing conditions cause formation of intracellular ice crystals, which creates a hyperosmolar environment. This environment in turn causes cell dehydration and shrinkage, enzyme denaturation, and dysfunction of the cytoskeleton and membrane. Additional damage may be produced by microvascular thrombosis and cellular anoxia. Rapid freezing followed by gradual thawing, known as a double freeze-thaw cycle, is used to maximize these effects [13].

The process of cryoablation is dependent on the temperature of the cryoprobe, the area of contact of the freezing surface, and the thermal conductivity of the tissue [14]. Differences between tissue resistances to freezing are also dependent on the distance from large vessels. This phenomenon is known as the heat sink effect. Theoretically, this heat sink or thermal conduction can affect the rate of cooling and the size of the cryoablation and prolong the time needed to reach any given temperature. Renal artery clamping has been attempted to increase the ablation effect but has not proved valuable.

The most important parameter affecting tissue necrosis within the cryolesion is temperature. An experiment in a porcine model using thermocouples indicated that a temperature of -19.4°C must be achieved to ensure completely homogeneous necrosis [15]. It also demonstrated uniform ablation of tissue at a distance of 16 mm or less from the cryoprobe, whereas all samples taken from a distance >21 mm from the cryoprobe were viable. The area of necrosis produced by the cryoprobe is visually approximated as the iceball, but temperatures higher than -20°C exist at its periphery. Campbell and associates [16] achieved a temperature of -20°C at 3.1mm behind the leading edge of the iceball in all 10 animals tested, suggesting extension of the iceball at least this distance beyond the edge of the tumor being treated to ensure adequate cooling.

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