THEORY OF CRYOSURGICAL-DAMAGE TO TISSUES
The potentialities of cryosurgery in the field of urology had been examined already in the 60ís-70ís, but the number of complications was such as to suggest abandoning this procedure. Last-generation cryosurgical apparata now allow a controlled destruction of the tissues in situ, thus preventing major and more frequent complications.
The latest achievement of this field is the ultrasound and thermo-guided percutaneous transperineal cryoablation of the prostate. Its theoretical premises may be resumed in the physiopathological behaviour of tissues to be damaged by low temperatures. This event can be distinguished in 3 phases: hypothermia, freezing and thawing.
Four different factors are responsible for the entity of the damage caused:minimum temperature, swiftness of the phenomenon, duration, number of repetitions.
Hypothermia occurs at about -10/15°C. Table 3 shows the phenomena which take place at these temperatures.
There will be irreversible damage to cells and, therefore, tissues only when ice will form in the cytoplasm. For this reason the efficacy of the freezing phase depends on its swiftness. If the freezing is slow, as the temperature decreases the ice formed in the interstitiae withdraws water causing an extracellular hyperosmotic phase therefore subtracting water from inside the cells . The loss of liquids brings to intracellular hypersmosis which hinders freezing and consequently protects, in some way, the tissue from damage. Intracellular freezing is the key-moment in this phase: when it occurs, it starts to extend to all the tissue, in a "domino effect" aided by all the intercellular bridges.
At the end of this freezing process, thawing starts. This also causes damage to the target-organ. The critical moment is when the crystalline structure of the ice is modified in a process known as re-crystallisation (-40° -20°C), with osmotic processes opposite to the ones described above taking place. This phase, also, is highly destructive.
The effects of these events last weeks with tabification intermediated by the anoxia derived from the circle destruction, by flogosis and oedema phenomena and by a possible cryoimmunitary action.