Numerical Modeling of the Tissue Freeze-Thaw Cycle during Cutaneous Cryosurgery using Liquid Nitrogen Spray

INTRODUCTION It is common in some cryosurgical procedures to rely on freeze-thaw cycle(s) to destroy undesirable tissues. Most research in cryosurgery focuses on the freezing process and much less attention has been paid to thawing or re-warming. However, as ice melts during thawing, the extracellular solution can become locally hypotonic, driving water into cells, resulting in cell expansion and ultimately, membrane rupture. Therefore, the thermal history of the target tissue during both the freezing and thawing processes is critical for cell viability. Cryosurgery using freezing to destroy undesired tissues is extensively used by dermatologists for removal of diverse benign and malignant cutaneous lesions [1]. Compared to alternative treatment modalities, cutaneous cryosurgery has many merits including: (i) a brief treatment period; (ii) bloodless treatment field; (iii) a good cosmetic result with minimal scarring; (iv) ease of performance; (v) high cure rate and (vi) low cost [2]. With these profound advantages, cutaneous cryosurgery has witnessed widespread acceptance in dermatological clinical practice for treatment of a wide range of skin lesions including actinic keratosis, warts, hypertrophic scars, keloids, mucoceles and solar lentigos [3]. To better understand and predict the thermal history during cryosurgery, we developed a two-dimensional numerical model to describe the complete freeze-thaw cycle during liquid nitrogen cutaneous cryosurgery. A stratified anatomical structure of human skin is considered in the model. The numerical simulation applies temperature-dependent thermal and physical properties for human skin tissue and considers the typical thermal boundary conditions for clinical practice. Parametric studies are performed to explore the influence of spray cooling, spray duration and surface heating. Results are discussed concentrating on iceball front propagation, lethal temperature isotherm evolution, tissue temperature variation and cooling rates. These results are expected to provide both quantitative and graphical support to cutaneous cryosurgery and suggest approaches to optimize current cryosurgical protocols. Several techniques have been developed for cutaneous cryosurgery, including swab, probe and spray methods. The swab method uses a cotton-tipped applicator saturated with LN2 and applies LN2 directly onto the target skin surface. Histological and thermal studies have demonstrated that freezing induced by swab technique cannot destroy cells located deeper than 1.5 mm from skin surface. This is inadequate for treatment of thick tumors [4]. Cryoprobe, a closed system with continuous circulation of LN2, was first developed by Dr. Irving Cooper in 1961 and has been widely applied in cryosurgeries of several anatomic areas including the liver and prostate. Spray method utilizes a canister to store LN2 and changeable nozzles to spray LN2 onto the skin surface. For the majority of cutaneous cryosurgery, the spray technique is preferred because of ease of use and the potential for