Cryotherapy, also called cryosurgery, cryoablation or targeted cryoablation therapy, is a minimum invasive treatment that uses extreme cold temperatures to freeze and destroy damage tissue, like tumors or cancer cells. During cryotherapy, a refrigerant as liquid nitrogen or argon gas is forced to flow inside a probe. This probe is similar to a needle and it is called cryoprobe. Once the refrigerant is inside this cryoprobe the temperature decreases below zero Celsius in a given time, creating an intense cold that contacts the diseased tissue. Physicians use image guidance techniques to monitor the cryoprobe, such as ultrasound, computed tomography (CT) or magnetic resonance imaging (MRI). To destroy diseased tissue located outside the body, liquid nitrogen is applied directly to the infected zone with a cotton swab or spray. For tumors located below the skin surface and depth in the body, the medical image guidance to insert one or more cryoprobes is used. Living tissue, whether healthy or sick, cannot tolerate extremely low temperatures. For this reason cryotherapy involves a series of steps leading to cell death. Tumors are repeatedly frozen and thaw, typically two freeze – thaw cycles are used. Once the cells have been destroyed, white blood cells of the immune system remove dead tissue.

The present work is a 3D simulation. The skin is modeled with a regular geometry, divided into three layers: epidermis, thinner and superficial part of the skin; dermis, 40 times thicker than the epidermis (also this layer has a thermoregulatory function because of the blood flow, which also contributes to vasoconstriction and vasodilatation of the skin), and finally the last layer is the hypodermis or subcutaneous fat layer (which mainly stores fat). For a transient analysis of this three layers of the skin, the bio-heat transfer equation of Pennes is used, since it contains terms that involve energy released during metabolism, blood perfusion, body core temperature and certain physical properties such as density, specific heat, thermal conductivity, latent heat of phase change and heat capacity ratio. The malignant tumor, melanoma, is modeled as an irregular symmetric geometry. Three different melanoma Clark levels are analyzed, Clark II, III and IV. Each level is analyzed with three size variations. These levels are chosen because most people who are diagnosed with melanoma have Clark II level. Clark V level was not considered because when melanoma reaches subcutaneous cellular tissue the metastasis process begins. In this work a thermal recovery analysis of the skin during certain periods of time in the freezing-thaw cycles is carried out. Each of this time periods vary according to the type of refrigerant, liquid nitrogen or argon gas. The analysis contemplates the phase change suffered by the skin.

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