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sharply delineated, avascular tumor bed and a reproducible method of tissue ablation with an extremely low risk of hemorrhage and minimal brain reaction. Cryosurgery does not require the resection of large volumes of normal surrounding tissue, but instead can be preformed through a burr hole under local anesthesia. Cryosurgery can be applied to multiple areas in the brain because of its small area of application and can be used to retreat areas. Cryosurgery may also have an immunological benefit by possibly sensitizing the patient to tumor antigens as verified by reports of tumor regression in adjacent areas left untreated after cryosurgery (20). With a certain degree of accuracy the extent of the lesion can be determined in advance. The procedure can be performed under stereotactic guidance, minimizing brain retraction and edema, can be performed in an awake patient allowing physiologic and clinical monitoring and can be utilized in the physiologic cooling method to determine if the permanent lesion is appropriate and without side effects (23). It is possible to produce a test lesion in a preselected location in the conscious cooperative patient before inducing a permanent lesion, and if needed instantly adjust the location, for a different affect or because of a side effects. (5) There is some associated edema from the cryolesion but this is smaller than other stereotactic procedures of destruction such as electrocoagulation. The edema emerges during the first few days and if not taken into account and treated appropriately with dehydrating agents, will lead to a lesion of a different proportion than expected (30). Cryosurgery can be performed on deep seated lesions or in areas of eloquent brain. It can assist in obtaining frozen biopsies during open procedures, with less chance of malignant cell spillage. Cryosurgery has additional operational benefits as well, a thermographic recording can be made during the procedure, allowing the surgeon to follow the exact course of events and its use can allow the patient to ambulate earlier and reduce hospitalization time by 50% when compared to open operations. During the 1960's and 1970's, when cryosurgery was at its peak, over 5000 operations for movement disorders were performed; there was a modest 1.2% mortality rate and a 0.8% chance of hemorrhage. (27).
The limitation of the current cryogenic probe is the maximum size that one lesion can be generated, 4.0 cm diameter. The exact size and shape of the cryogenic lesion, or ice ball, can only be predicted based upon previous observations. Since its precise boundary may be uncertain, an exact means of monitoring the freezing process is needed. The tissue temperature gradient can vary at the ice ball edge by 10 C/mm to 30 C/mm. When cryosurgery is attempted in proximity to large patent blood vessel, irregular shapes may occur especially at the periphery because of the heat dissipation from moving blood. Therefore, vascular tissue especially at the periphery of a lesion should be monitored closely with a temperature probe or impedance monitor.
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