A new thermal dose model based on Vogel-Tammann-Fulcher behaviour in thermal damage processes.

Thermal dose models are metrics that quantify the thermal effect on tissues based on the temperature and the time of exposure. These models are used to predict and control the outcome of hyperthermia (up to 45°C) treatments, and of thermal coagulation treatments at higher temperatures (>45°C). The validity and accuracy of the commonly used models (CEM43) are questionable when heating above the hyperthermia temperature range occurs, leading to an over-estimation of the accumulation of thermal damage. A new CEM43 dose model based on an Arrhenius-type, Vogel-Tammann-Fulcher, equation using published data, is introduced in this work. The new dose values for the same damage threshold that was produced at different in-vivo skin experiments were in the same order of magnitude, while the current dose values varied by two orders of magnitude. In addition, the dose values obtained using the new model for the same damage threshold in 6 lesions in ex-vivo liver experiments were more consistent than the current model dose values. The contribution of this work is to provide new modeling approaches to inform more robust thermal dosimetry for improved thermal therapy modeling, monitoring, and control.

Investigations of large vessel cooling during interstitial laser heating.

Interstitial laser heating of tissues is influenced by blood flow in the treatment region. Temperature gradients around large blood vessels may result in local underheating of tissues. A three-dimensional, time-dependent finite difference model of interstitial laser heating around large vessels is presented. A thermal conduction model was developed using a transport theory approximation for the energy distribution from an optical line source. Calculated transient temperature profiles and temperature reductions around 0.144 and 0.400 cm diam vessels show qualitative agreement with those measured in a series of tissue phantom studies. Experiments and calculations for a large vessel located approximately 1.0 cm from the optical source indicate that temperature reductions are less than 1 degree C at distances greater than approximately 1.0 cm from the vessel surface. The model also indicates that significant reductions in the extent of a thermal coagulation boundary can occur if a large vessel is situated inside the normal coagulation zone.

Basic optothermal diffusion theory for interstitial laser photocoagulation.

A theoretical basis for interstitial laser photocoagulation (ILP) practiced with point-emitting fiber tips has been established by solving the bioheat transfer equation, using basic Green's function methods, for steady and instantaneous point sources of both optical energy and direct heat. Three combination optical and thermal parameters have been identified that strongly influence temperature distributions during ILP. These are defined here as optothermal heat capacities and an optothermal diffusion length, all of which characterize how a thermal diffusion temperature profile is flattened and reduced when optical diffusion is added. Relevance and limitations of this theory for practical ILP are discussed. A useful result is a mathematical verification of previous empirical observations that point optical sources heat tissues less than point heat sources of the same power. A comparison of normalized theoretical temperature transients with published measurements suggests that in normal liver, blood perfusion cooling may exceed thermal conduction by a factor of 5.6 +/- 1.7.