Cryo-Forum

THERMAL GRADIENTS AND CRYOSURGICAL PROBE PERFORMANCE

February 1997
John Bauhst, H. Ma, C-M Zhang, A. Gage
SUNY at Binghamton and Buffalo and Cryomedical Sciences, Inc., Rockeville, MD USA
Successful cryosurgery requires knowledge of cryosurgical probe performance. Our objective was to determine the freezing capability of cryosurgical probes and to evaluate this capability in terms of the factors in the freeze/thaw cycle known to produce a destructive response.

Method - Cryosurgical probes, 3.4 mm and 8 mm in diameter, were placed in agelatin bath and cooled to tip temperatures of -195 C, -175 C and -100 C by cryosurgical apparatus (CMS AccuProbe® System).
Thermocouples, placed on probe freezing surface and at distances of 4, 8, 12, 16, and 20 mm from the surface, measured temperature changes during the growth of the iceball. Mutiple runs of 20 minutes at each of the settings provided data.

Results - The 8 mm probe achieved a tip temperature (PT) of -195C in 3 minutes and produced a 7cm iceball in 20 minutes. The 8mm probe used with a PT of -100C produced a 5.5cm iceball in 20 minutes. The -40C isotherm was 5mm inside of the 0C isotherm with PT of -195C, 7mm inside of the 0C isotherm with a PT of -175C, and 13mm inside o the 0C isotherm with a PT at -100C. The 3.4mm probe with PT at -175C produced a 5cm iceball in 20mins. Used at -100C, the resultant iceball was 4cm in 20 minutes. The -40C isotherm with PT at -175C was 10mm from 0C isotherm. With PT at -100C, the -40C isotherm was only 4mm from the probe, 16mm inside of the 0C isotherm.

Conclusions - The colder the probe temperature, the faster the rate of freezing and the larger the frozen volume. The colder the probe and the faster the freezing rate, the closer the -40C isotherm is to the 0C isotherm. Probe temperatures of -175C and colder are needed to create lethal temperatures more than 2cm from the probe, as is required for many cancers.


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