Long Exposure Growth of In-Vivo Interstitial Laser Photocoagulation Lesions

Abstract. An investigation of the temperature response and growth of thermal lesions resulting from in vivo, interstitial laser photocoagulation at long exposures was conducted to assess extended lesion growth characteristics and test the applicability of first order unimolecular rate kinetics (Arrhenius theory) to thermal lesion growth. Irradiations were performed in vivo in rabbit muscle using a continuous 805 nm diode laser source operating at 1.0 W coupled to an optical fibre with a precharred tip (i.e. point heat source). Temperature responses were measured using a linear array of five microthermocouples. Each temperature–time profile was fitted to a solution of the Weinbaum–Jiji bioheat transfer equation (W–J BHTE). Lesions were resected 48 h post-irradiation and the necrosis boundaries were determined histologically. Numerical integration of the Arrhenius damage integral using temperature–time data at the lesion boundary produced corresponding pairs of activation energy and pre-exponential factor (Ea, α) consistent with reported values for various other end-points and tissue types. Lesion radii were 6.0±0.6, 8.7±0.4 and 9.7±0.5 mm for 10, 20 and 30 min irradiations respectively. Thermal lesion growth predicted from Arrhenius theory was consistent with experimental results and is non-asymptotic by 30 min. Thermal parameters generally assumed to be constant when solving the W–J BHTE were found to vary with radial distance from the source, presumably due to a temperature dependence.