An Approach to Improving Efficacy of Cryosurgery: Numerical and Experimental (Using Gel Phantoms) Studies

Abstract

Freezing and ablating using cryosurgery is becoming a promising surgical tool for the treatment of tumours. For improving the efficiency of the cryosurgical procedure, different approaches have been implemented till now. Most of these techniques have focussed on the freezing process, without giving adequate attention to the damage to the surrounding healthy tissue. In this study, a novel concept is proposed which achieves the desired freezing while protecting the surrounding healthy tissue through the use of low thermal conductivity liquid layer (perfluorocarbons) around the interface of the tumour. Numerical modelling has been done to determine the location of the ice fronts in the presence of this perfluorocarbon layer around the boundary of the tumour. It is noticed that this method leads to a higher ablation rate substantially reducing the surgical time. Also, an optimal offset, i.e. the minimum distance between the tip of the cryoprobe and the boundary of the tumour, is identified for a given tumour radius and active length which gives maximum tumour necrosis in minimum time. It is also observed that for a 2 mm increase in the active length of the cryoprobe, the decrease in optimal offset is approximately 1 mm. Furthermore, for the tumour with different radii (between 10 mm and 15 mm), with same active length of the cryoprobe, the time taken for complete ablation of the larger tumour is nearly 2.7 times the time taken for the smaller one for every 2.5 mm increase in the tumour radius. The results also reveal that there exists an optimal thickness of the perfluorohexane layer around the tumour interface. It is also seen that among perfluorohexane, octafluoropropane and water, perfluorohexane acts as the best substitute for the formation of an insulating layer around the tumour interface. Experiments have been performed to prepare pefluorocarbon (perfluorohexane and perfluorodecalin) emulsions in varied concentration (i.e. 30%, 50%, 70% and 90% (w/v)) through probe sonication. Further, this study reports the particle size, emulsion stability, functional group analysis, thermophysical properties of both perfluorodecalin and perfluorohexane emulsions. With regard to thermal conductivity, it is observed that perfluorodecalin emulsions possess a marginally lower thermal conductivity than perfluorohexane emulsions. It is interesting to note that during cryosurgery of gel phantom in the presence of low thermal conductivity perfluorodecalin emulsion (90% (w/v)), it is observed that the freezing front is not able to penetrate the gel while in its absence, there is a temperature of 􀀀4oC at the same thermocouple location of 10 mm (in the axial direction).Cryosurgery of glycine-containing gels is carried out in presence and absence of perfluorohexane layer, and the thermal history is measured using K-type thermocouples connected to a data acquisition system. The presence of glycine causes rapid freezing during cryosurgery with an ice ball depth of 16 mm, while with a perfluorohexane layer at this gel interface, this depth is 13 mm, indicating the ability of this layer to limit freezing. In this study, alumina has also been utilised for the preparation of adjuvant containing gel phantoms. After cryosurgery, it is clearly evident that a temperature decrease is observed in the alumina consisiting gel phantoms when compared to the agarose gel phantoms. It is also noticed that with the increase in insertion depth of the cryoprobe (from 1 to 1.5 cm), there is a decrease in temperature at each thermocouple location in the gel phantoms. This study also demonstrates that in the presence of perfluorohexane layer, when the alumina consisting gel phantoms are cryosurgically cooled; even with the increase in insertion depth, the thermocouple placed axially at 10 mm which is inside the perfluorohexane solution layer indicates a temperature of 25oC. However, in its absence, the temperature is found to be 􀀀5:47oC at the same position, suggesting that the freezing is limited within the gel. Furthermore, this work also proposes a new approach that utilises glycine-alumina emulsions as an adjuvant. After cryosurgery of glycine-alumina containing gel, a substantial temperature decrease is observed at all thermocouples placed nearer to the probe, thus indicating an enhancement in freezing. In conclusion, this study proposes novel approaches to improve the cryosurgical procedure through numerical modelling and experiments in gel phantoms, thus, providing newer approaches to improve the cryosurgical outcome.