Experimental and numerical study for skin tumour ablation using cryotherapy

Abstract

Cryotherapy is a surgical technique that uses a freezing temperature to ablate the skin tumours. Liquid nitrogen is mainly used as the cryogen during cryotherapy because of its lower boiling temperature and risk-free to the human being. Cryogen spray cooling depends on various parameters such as nozzle diameter, spraying distance and angle of cryogen spray. The main idea is to provide guidelines for the ablation of an unwanted tumour while protecting the healthy tissue. The destruction of skin tissue mainly depends on the lethal temperature obtained in the lesion region; which specifically depends on the diffusion of lower temperature within the tissue. There are many other parameters like duration of freezing and thawing, cooling rate, type of tumour, number of freeze-thaw cycles etc. that affect the cryoablation of tumour.

In the present study, an experimental model has been designed to investigate the heat sink ability in a tissue model using liquid nitrogen spray device. The numerical simulation is carried out to evaluate critical properties that are difficult to determine experimentally and the results have been validated from the experimental results. The transient temperature history and ice front propagation are determined experimentally in the tissue simulating gel. The ablation volume, lethal front propagation and gap are quantified numerically which are difficult to determine using experimental techniques; where gap represents the distance between the lethal front and the ice front.

The experimental study uses 0.6% (w/v) agarose gel to mimic the thermal properties of the human cutaneous tissue. Nozzle diameter of 0.8 mm, 0.6 mm and 0.4 mm is selected for liquid nitrogen spray cooling with the spraying distance of 27 mm, 18 mm and 9 mm. Thermocouples (K-type) are used to acquire the temperature data via a data acquisition system. The cryofreezing procedure consists of a single freeze-thaw cycle where freezing consists of 120 s followed by 130 s of passive thawing. For the numerical study, a two dimensional axisymmetric cylindrical domain is used to solve the enthalpy equation. Single block with orthogonal grid is used for plain gel phantom while multi-block non-orthogonal grid is used for nodular gel phantom. The partial differential equation is solved using finite volume method.

A parametric study has been performed with liquid nitrogen spray cooling on the gel phantom for the application of superficial skin tumour ablation. The transient temperature history at a certain depth from the plain gel surface and ice front propagation are calculated numerically and validated with the experimentally measured data. Further, the ablation volume and lethal front characterised by -50oC and -25oC are calculated numerically. The GapB (difference between -25oC and 0oC) and GapM (-50oC and 0oC) are also evaluated numerically for various parameters. Furthermore, the correlations are provided for calculating the ablation volume and ice front which are the function of nozzle diameter, spraying distance and freeze duration.

The influence of liquid nitrogen spray cooling on a nodular gel phantom is studied for the application of nodular skin tumour. The thermal parameters such as temperature history and ice front are evaluated numerically in a nodular gel phantom for two different configurations; the first configuration of nodular gel has a radius and depth of 5.5 mm and 4.5 mm respectively while for the second configuration it is 4 mm each respectively. The numerically determined thermal parameters are corroborated with the experimentally measured data. The ablation volume enclosed by -50oC and -25oC are evaluated numerically along with lethal front for both the nodular gel configurations. Further, GapM and GapB are predicted numerically in the nodular gel phantom. In addition, the comparative study is performed between plain and nodular gel phantom.
Finally, the effect of the adjuvant in the 0.6% (w/v) of agarose gel is also studied for different concentration of NaCl (0%, 5%, 10% and 20% (w/v)) during cryotherapy. The thermal characteristics such as the temperature isotherms and the iceball are evaluated numerically for various adjuvant mixed gels and are confirmed with the experimentally measured values. The comparative study is performed for ablation volume, lethal front and gap in the adjuvant mixed gels.

The results of the present study indicate that the numerical models designed for the purpose of parametric analysis show a good agreement with the experimental results. The maximum ablation volume of 108 mm3 and 49 mm3 can be used to treat benign and malignant superficial tumour respectively using open spray technique with mentioned spray parameters. Malignant nodular tumour less than 4.5 mm depth and 5.5 mm radius can be effectively treated with 0.8 mm nozzle diameter with spraying distance up to 27 mm. Similarly, benign nodular tumour up to same figure can be treated with 0.6 mm and 0.8 mm nozzle diameter with spraying distance of 9 mm and 27 mm respectively. The largest ablation zone and lesser gap are found in the case of 20% NaCl-gel system. Accordingly, the study may be useful for surgeons and clinicians to predict the spray conditions required for a particular diameter of tumour, although it will be advisable to perform animal trials before conducting any clinical trials.