A model for the time-dependent thermal distribution within an iceball surrounding a cryoprobe

The optimal cooling parameters to maximize cell necrosis in different types of tissue have yet to be determined. However, a critical isotherm is commo nly adopted by cryosurgeons as a boundary of lethality for tissue. Locating this isotherm within an iceball is problematic due to the limitations of M RI, ultrasound and CT imaging modalities. This paper describes a time-depen dent two-dimensional axisymmetric model of iceball formation about a single cryoprobe and extensively compares it with experimental data. Thermal hist ories for several points around a CRYOprobe(TM) are predicted to high accur acy (5 degrees C maximum discrepancy). A realistic three-dimensional probe geometry is specified and cryoprobe temperature may be arbitrarily set as a function of time in the model. Three-dimensional temperature distributions within the iceball, predicted by the model at different times, are present ed. Isotherm locations, as calculated with the infinite cylinder approximat ion, are compared with those of the model in the most appropriate region of the iceball. Infinite cylinder approximations are shown to be inaccurate w hen applied to this commercial probe. Adopting the infinite cylinder approx imation to locate the critical isotherm is shown to lead the user to an ove restimate of the volume of target tissue enclosed by this isotherm which ma y lead to incomplete tumour ablation.