Mathematical analysis of the temperature field during ex vivo and in vivo experimental laser interstitial thermotherapy (LITT) in breast tissue models

Purpose: Laser interstitial thermotherapy is a promising minimally-invasive technique for the treatment of small cancers of the breast that are currently removed surgically by lumpectomy. The purpose of this work was to analyze in-situ temperature fields recorded with stainless-steel thermocouple probes during experimental laser interstitial thermo-therapy (LITT). Methods: Both a CW Nd:YAG laser system emitting 20 W for 25 to 30s and a 980 nm diode laser emitting 10 to 20 W for up to 1200s delivered through a fiber-optic probe were used to create localized heating in fatty cadaver pig tissue and milk as phantoms. To quantify an artifact due to direct heating of the thermocouple probes by laser radiation, experiments were also performed in air, water and intralipid solution. The temperature field around the fiber-optic probe during laser irradiation was measured every 0.3 s or 1 s with an array of up to fifteen needle thermocouple probes. The effect of light absorption by the thermocouples probes was quantified and the time-dependence of the temperature distribution was analyzed. Results: After removal of the thermocouple artifact, the temperature was found to vary exponentially with time with a time constant of 600 to 700 s. Conclusions: The time-dependence of the interstitial temperature can be modeled by exponential functions both during ex vivo and in vivo experiments.