Experimental study of laser application in medical oncology Development and optimization of a thermal model for liver ablation and analysis of optical fiber sensors in temperature monitoring

"Thanks to advances in cancer prevention, early detection, treatment, and support for those facing the disease, more people than ever before have reason to hope" [1]. Laser thermal ablation is one of the most common minimally-invasive tumor ablation techniques, along with RadioFrequency ablation, Microwave ablation, and Cryoablation. It is practised when a traditional surgical treatment of cancer would be too invasive, or when it would be too risky to access some tumor regions, as it often happens for liver cancers. Laser ablation exploits the heating of the tissue to induce coagulative necrosis of the molecules and destroy the tumoral cells "in-situ". The response of the tissue strongly depends on the temperatures reached during the treatment. These temperatures must be monitored in real-time to have a feedback-based adjustment of the procedure. Optical sensing fibres are the preferential method for non-invasive temperature monitoring; indeed, as they are non-conductive, it is feasible to handle these fibres under Magnetic Resonance without the risk of experiencing the artifacts that could arise from a high thermal conductivity. In particular, in this work of thesis, Fibre Bragg Gratings (FBGs) have been used as multi-point measurement sensors to detect the temperature in different positions simultaneously. The FBGs working principle is based on the reflection of a single wavelength occurring when an incident light goes across the gratings; this "Bragg wavelength" undergoes a shift when a temperature or strain perturbation occurs. MicronOptics interrogator for optical fibres together with Matlab® computing environment have been used to characterize the sensors and derive temperature measurements during ex-vivo bovine liver ablation. As stated, the shift of the Bragg wavelength can also be induced by a mechanical stress; experiments have therefore been performed on FBGs sensors to evaluate the influence of the strain. In addition to the evaluation of temperature rises during laser ablation through experimental tests, a thermal model has been developed to predict the temperatures reached by the tissue in order to help and find feed-forward adjustments of the ablation therapy. The model uses the Partial Differential Equation Toolbox in Matlab® to solve the differential equations of heat diffusion matters using the Finite Elements Method. The 2D-geometry of a cylinder section has been modeled, where the delivery probe connected to the laser is considered to be on the rotation axis of the cylinder; boundary conditions have been imposed in compliance with the experimental conditions. The model described above helps with predicting the temperatures in specific nodes of the geometry that can be compared with the measured values in the corresponding geometric positions. An optimization code has finally been implemented to minimize the error between predicted and measured temperatures and to evaluate the optimal values for both thermal and optical parameters of the tissue such as thermal conductivity, absorption coefficient and scattering coefficient.