Liquid nitrogen with a boiling point of -196°C is the coldest practical cryogenic agent. When placed in contact with a surface with a higher temperature, the liquid nitrogen will boil, extracting latent heat from its surroundings. In addition, if maintained in a pressurized environment, a gas, upon being allowed to expand, will lead to a drop in temperature. This is known as the Joule-Thomson effect. Most early applications involved the open application of liquid nitrogen into or onto the surface of a lesion. This involved pouring the liquid nitrogen into the tumor cavity, following intralesional excision or spraying along the margins with a probe (Fig. 1). Drawbacks of this approach include the formation of a thin layer of vapor, which may actually insulate the tissues, and an unpredictable delivery of the cryogen, resulting in temperature variability.
The initial closed systems also employed liquid nitrogen, which was continually circulated through the tip of the probes. However, liquid nitrogen is limited to probes of a diameter greater than 3 mm. Using pressurized argon gas (boiling point of -185.7°C), it is possible to create probes with a diameter as small as 1.4 mm, making directed treatment of small tumors possible (29).
FIGURE 1 Use of liquid nitrogen in an open system intraoperatively. Source: Photo courtesy of James Wittig, MD.
Systems are now available which utilize argon gas as a coolant and helium to aid in the thawing process. This makes it possible to reach temperatures as low as -100°C within a few seconds. An active thawing process is induced through the delivery of helium gas through the probe. With the current systems, it is possible to deploy up to eight probes at once.
What results at the tip of the probe is an "ice ball," which has a predictable geometry based on the length and diameter of the noninsulated tip of the probe. This ice ball can be visualized by various imaging technique including ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) (Fig. 2). Ultrasound, though very practical for certain applications, does not permit visualization deep into the most superficial portion of the ice ball. Most centers do not have the capability to employ MRI during the treatment process. This leaves CT imaging as the most practical and widely employed modality for this purpose.
It is important to remember that a temperature of 0°C should be assumed at the edge of the ice ball. Data published by the equipment manufacturers on the size and geometry of the different thermal zones surrounding the tip of the probe make it possible to predict the area of the ice ball at approximately -20°C or lower (Fig. 3). For complete necrosis of the tumor, it is important to extend the margins of the ice ball an appropriate distance beyond the tumor margins in order to ensure complete cell death. Although this is often the case with tumors in solid organs such as the liver and kidney, where the destruction of a small cuff of surrounding normal tissue will not result in undue morbidity to the patient, this is not the case in many musculoskeletal applications for several reasons. Much of the early data has been accumulated
FIGURE 2 Axial computed tomographic image demonstrating the rounded low attenuation ice ball surrounding the cryoprobes.
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