Numerical modeling of cryospray for treatment of skin tumor

Abstract

Cryospray as a mode of cryosurgery has come up as a novel and revolutionary clinical technique to ablate undesired tissue, malignant and benign, ranging from skin lesions and boils to keratosis and skin tumors. Cryosurgery is effective and fast way to treat such skin abnormalities. A numerical model for the same is hence highly desirable that can prefigure the various thermal and physical characteristics changes in the targeted tissue. The numerical solutions are derived using a two-dimensional model of the skin having three layers namely epidermis, dermis and subcutaneous fat, these layers are all considered to be homogenous and the thermal and physical properties of these layers are incorporated into the model to generate results. The Pennes equation for bio heat transfer in polar coordinates is taken and different constraints for the blood perfusion and the metabolic heat generated in each layer are considered. The equation is solved using typical boundary conditions like Dirichlet and Robin’s conditions for clinical use. We use implicit method for discretization of unsteady term and central difference scheme for discretizing diffusion term which give us second order accurate result in time and space. The model not only takes into account the freezing process but also puts into perspective the thawing process which is equally effective in the destruction and damage of the target tissue. The slow thawing increases the water content causing the low osmotic pressure on the outside and hence large uptake of water by cells leading to their rupture. The damage to the tissue is calibrated in terms of the propagation of the frozen area during the spray time and then during the thawing process.
We also study the temperature changes during the freeze-thaw cycle at various depths within the skin tissue for further understanding and establishment of the process. The results for the propagation of the frozen zone are expected to provide substantial numerical and graphical information about cryospray and hence give the basis for further improvements in the current cryosurgical procedures.