Modelling the cryogenic treatment of warts and recommendations for changes to current clinical practice

Cryotherapy is a commonly used and recommended treatment for warts and there is wide variation in technique between clinicians applying the same mode of cryotherapy. To address this uncertainty in clinical practice a mathematical model is used to demonstrate the relationship between freezing time and tissue freezing zone for two commonly used cryogens (liquid nitrogen spray and nitrous oxide probes). It is found that nitrous oxide probes need substantially longer application time to obtain the same necrosis region as liquid nitrogen sprays. Overspray from liquid nitrogen spray is shown to lead to increased damage to healthy tissue surrounding a wart. Using a barrier of petrolatum (petroleum jelly) on the healthy tissue is demonstrated to be an effective means for limiting this damage. Changes to clinical practice to take account of these findings are recommended and need clinical trials to verify their applicability. References G. Aguilar, W. Verkruysse, B. Majaron, L. O. Svaasand, E. J. Lavernia, and J. S. Nelson. Measurement of heat flux and heat transfer coefficient during continuous cryogen spray cooling for laser dermatologic surgery. J. on Selected Topics in Quantum Elect., 7:1013--1021, 2001. doi:10.1109/2944.983307. Anonymous. Clinical effectiveness group: National guidelines for the management of anogenital warts. http://www.bashh.org/documents/86/86.pdf. M. Connolly, K. Bazmi, M. O'Connell, J. F. Lyons, and J F. Bourke. Cryotherapy of viral warts: a sustained 10-s freeze is more effective than the traditional method. Brit. J. Derm., 145:554--557, 2001. doi:10.1046/j.1365-2133.2001.04449.x. FlexPDE. http://www.pdesolutions.com. H. McClean and S. Shann. A cross-sectional survey of treatment choices for anogenital warts. Int. J. STD and AIDS, 16:212--216, 2005. doi:10.1258/0956462053420130. G. N. Mercer and H. S. Sidhu. A heat transfer model describing burns to the skin from automotive airbags. In Andrew Stacey, Bill Blyth, John Shepherd, and A. J. Roberts, editors, Proceedings of the 7th Biennial Engineering Mathematics and Applications Conference, EMAC-2005, volume 47 of ANZIAM J., pages C339--C354, October 2006. \protect http://anziamj.austms.org.au/V47EMAC2005/Mercer [October 3, 2006]. G. N. Mercer and H. S. Sidhu. Mathematical modelling of the effect of fire exposure on a new type of protective clothing. In Geoffry N. Mercer and A. J. Roberts, editors, Proceedings of the 8th Biennial Engineering Mathematics and Applications Conference, EMAC-2007, volume 49 of ANZIAM J., pages C289--C305, January 2008. http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/346 [January 8, 2008]. J. Sherrard and L. Riddell. Comparison of the effectiveness of commonly used clinic-based treatments for external genital warts. Int. J. STD and AIDS, 18:365--368, 2007. doi:10.1258/095646207781024711. J.C. Sterling, S. Handfield-Jones, and P. M. Hudson. Guidelines for the management of cutaneous warts. Brit. J. Derm., 144:4--11, 2001. doi:10.1046/j.1365-2133.2001.04066.x. D. A. Torvi and J. D. Dale. A finite element model of skin subjected to a flash fire. J. Biomechanical Eng., 116:250--255, 1994