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(Continued from page 80)
transitional zone of 150-200 um, which corresponded to approximately 30 cell diameters. Vasogenic edema forms in the specimens on the second post operative day. Rubinsky (26) confirmed these findings. Other (4, 14) pathology accounts describe neuronal karyorrhexis, blood vessel congestion and white blood cell infiltration 11 hours after cryosurgery. By 5 weeks, phagocytes, leaving puckered glial scars, resorb the white and grey matter. Quigley et al, postulates that the zone surrounding the ice ball (greater than 150-200 um), suffers cooling injury without histological signs at 2 days but thereafter, the injury becomes apparent as the lesion grows into the white matter. This is believed to be secondary to the white matter's decreased blood supply when compared to the grey matter. In 1987 Moser (19) demonstrated a zone of blood brain barrier breakdown extending no more than 1 mm beyond the maximal diameter of the ice ball as revealed on CT Scan.
Other experiments were conducted with freeze frame photography while the ice ball was generated in gelatin. Gelatin has a specific gravity approximating human brain and therefore provides some clues as to the ice balls formation. The gelatin of course lacks the blood vessels, which are vitally important to predicting the size and shape of the ice ball formation (9).
SIZE & SHAPE OF ICE BALL
The size of the ice ball is not static and its precise dimensions are unpredictable. Likewise the formation of the ice ball can not be precisely timed. Factors affecting cooling are the surface area of the tip, temperature and rate of flow of the freezing agent and the thickness and heat transfer of the noninsulated portion of the cannula. Factors affecting the shape of the lesion are the diameter of the probe, the length of the uninsulated tip, the heat transfer to the tissue, the probe temperature and the length of time of cold application. Based upon laboratory studies with gelatin and his own human subjects, Cooper (6) demonstrated that with a 2 mm cannula at a temperature of -40 C for 3 minutes there will be a lesion 6 mm in diameter, at -50 C an 8 mm lesion and at -100 C a 12 mm lesion. The expanding edge of the cryogenic lesion is in a reversible temperature range of permanent destruction for 30 seconds due to the sharp temperature gradient around the cannula tip. At 0 C the area of reversible inhibition is 3 mm around the tip. After the lesion is allowed to thaw the cannula can be removed without adherence of any tissue. Using modern probes of 3 and 4 mm and temperatures of -160 to -200 C ice balls of 2.5 to 4 cm can produced at 6 minutes (23). Since the exact size and shape of the ice ball is variable monitoring techniques should be employed.
ADVANTAGES AND DISADVANTAGES
Cryosurgery has multiple advantages over other techniques of tissue manipulation. The cryoprobe can deliver a precise, rapid, safe, (Continued on page 82)
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