Intraoperative Complications
Inadequate Choice of Cryosurgical Equipment and Technique
Definition
Choice of a cryosurgical equipment and/or technique that do not allow sufficiently fast freezing of the tumoral mass and may result in:
incomplete tumor destruction with local recurrence as a late postoperative complication
excessive prolongation of the procedure for horses treated under general anesthesia
Risk factors
Thick tumors
Tumors with a large base
Pathogenesis
Maximal tissue destruction is obtained by fast freezing and slow thawing. To ensure destruction of all tumoral cells, the obtained tissue temperature should be low enough. The larger and thicker the tumoral mass is, the longer it takes to obtain sufficiently low temperatures in the entire lesion to ensure destruction. Although initial experimental data suggested that a tissue temperature of –20°C for ≥1 minute is adequate for tissue destruction [17], other studies have shown that lower temperatures are often required to destroy tumor cells [18]. Biological susceptibility to cryoablation varies according to the tumor type and moreover, not all cells within the tumor will be subjected to the same cooling and thawing temperatures and rates [19]. When a cold probe is applied to the tissue, a steep temperature gradient develops in the tissue. Whereas the tissue close to the probe freezes very fast to very low temperatures, the tissue in the peripheral lesion cools slowly and to a higher temperature, with a less destructive effect. Therefore, the application of lower temperatures is advised. Recent studies in human medicine suggest that exposure of cancer cells to low temperatures of –30°C to –40°C for ≥1 minute using a double freeze‐thaw protocol yields complete cell destruction [12, 18].
Although cryosurgery of superficial skin lesions can be performed under local analgesia in the standing sedated animal [13, 14], some locations (e.g. ventral abdomen, inguinal region, ocular tumors, etc.), or tumor or patient characteristics require general anaesthesia. When the cryoablation is seriously prolonged, there is a higher risk of anesthesia‐related intra‐ and postoperative complications (hypoxia, myositis, nerve paralysis, etc.).
Diagnosis and monitoring
Exact determination of the correct speed of freezing and thawing is not possible and the surgeon should rely on achieving an “as fast as possible” freezing and a “slow spontaneous” thawing. Tissue temperatures of –30 to –40°C should be reached to ensure tissue destruction and this should be monitored using thermocouple needles (see Prevention below).
Prevention
The choice of cryogen and cryosurgical equipment should be adequate for the size of lesion to ensure that the tumor can be frozen fast and sufficiently deep:
Liquid nitrogen is the most powerfull cryogen (–196°C) and is most commonly used in cryosurgery in horses.
Hand‐held instruments for gas‐cooling with N2O are also only suited to treat smaller lesions, even though the size of the tip can be adapted to the size of the lesion to be frozen.
Copper probes or cotton tips immersed in liquid nitrogen quickly warm up in contact with living tissue and do not allow deep freezing [13]. Their use is restricted to small and superficial lesions.
Liquid nitrogen circulation probes should be large enough to cover the entire lesion to assure a more or less equal temperature distribution. In case the tumor base exceeds the size of the largest probe, cryosurgery should be performed at multiple locations with an overlap of a few millimeters. A contact gel assures good contact between the probe and lesion and the probe should stick to the lesions once the freezing starts [13]. Air bubbles between probe and tissue must be avoided as these have an insulating effect.
Spraying liquid nitrogen directly onto the tissue results in much faster freezing compared to the use of probes held in contact with the tissue [13].
Very fast freezing of larger lesions can also be obtained by pouring the liquid nitrogen directly onto the lesion. This requires the use of a cup to confine the liquid nitrogen to the lesion (see below).
Besides a correct choice of cryogen and cryosurgical equipment, several measures can be taken to promote fast freezing and limit the duration of the procedure:
Lesions that exceed the level of the surrounding tissues should be debulked. This decreases the tissue volume to be frozen and helps achieving lower temperatures at a high freezing speed [13, 14].
For lesions located close to vessels or in a well vascularized area, the efficacy of freezing can be enhanced by the placement of a tourniquet (distal limbs) or a Chalazion clamp (eyelid) to temporarily slow down or stop the circulation in the treated area [8, 16].
In cases of multiple tumors, time can be gained by freezing a second tumor during the slow thaw phase of a first tumor.
Similarly, when a large tumor base needs to be frozen in consecutive areas, time can be gained by freezing a second zone during the slow thaw phase of a first zone.
The slow thawing phase is the most time‐consuming. Nevertheless, it is not advised to speed up that phase of the cycle, for example by heating the probe or using a hair dryer. Indeed, the process of recrystallization resulting in direct cellular damage mainly occurs during the slow thawing phase which is essential for cryosurgical success.
Finally, temperature should be monitored during the freezing process:
Use thermocouple needles inside the lesion to make sure –30 to –40°C is reached for ≥1 minute. In larger lesions, multiple thermocouple needles should be placed and it is more important to position them at the periphery of the lesion compared to the center. Placement of the needle close to a blood vessel (heat source) can also influence the temperature measured [18].
When thermocouple needles are not available or cannot be placed safely, the tissue temperature should be estimated as accurately as possible by:
Visual inspection of the formed ice‐ball at the level of the cornea [5].
Inspection and palpation of the formed ice‐ball at the level of the skin [14]. Be aware that the outer edge of the palpable ice‐ball only reaches a tissue temperature of 0°C which is inadequate for cell destruction [20].
Ultrasonographic monitoring of the ice‐ball. This is used more commonly in human medicine (e.g. for cryotherapy of the prostate and other internal organs). Frozen tissue has a hypoechoic appearance and the boundary between frozen and unfrozen tissue shows as a white hyperechoic rim (HER). At the border between the HER and the hypoechoic frozen tissue, tissue temperature is approximately –15°C. At the outer border of the HER, tissue temperature is approximately 0°C [18].
Treatment
