Purpose: Minimally invasive techniques for the thermal treatment of solid tumors, such as laser-induced interstitial thermotherapy (LITT), demand adequate monitoring and treatment planning options. Magnetic resonance thermal imaging (MRTI) is a common way of following the temperature increase in the target tissue during irradiation. A detailed knowledge, however, about the relationship between time/temperature exposure and resulting tissue damage is needed to match the size of the induced lesion to that of the tumor boundaries. Aim of this work was to assess the value of Monte Carlo simulations (MCS) of laser-tissue interaction to improve laser therapy planning and to derive criteria for LITT monitoring.
Methods and Materials: The distribution of laser optical power in the target tissue was simulated by following the random walk of individual photons undergoing elastic scattering, reflexion and absorption (λ=1064 nm). Thermal damage was modelled with an Arrhenius approach. MCS yielded true 3D distributions of the absolute temperature and thermal damage as a function of irradiation time. Several tissue types (liver, brain, muscle, kidney) were simulated. MCS results were compared with 2D MRTI data recorded during in vitro irradiation experiments on corresponding tissue samples.
Results: MCS correctly predicted the complex shape of the experimental temperature and damage distributions. The simulated and the measured lesion diameters agreed within 10%. For each simulated tissue type, the boundary of the induced lesion was well approximated by a 3D surface corresponding to a critical isotherm Tc (7% variance). The exact temperature values for Tc varied slightly with tissue type (53oC for liver, 50oC for brain, 55oC for muscle, 52oC for kidney) and were consistent with published literature results obtained during animal model LITT procedures.
Conclusion: These results show that MCS of interstitial laser coagulation can be used to derive tissue-specific parameters to correlate local tissue temperature with thermal damage and has the potential to yield refined criteria for an improved and more patient-specific irradiation planning of LITT procedures.
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