Continuous changes in the optical properties of liver tissue during laser-induced interstitial thermotherapy

Background and Objective: Laser-induced thermotherapy (LITT) is a promising treatment for irresectable liver tumors. To predict the effects of laser a pplications and to optimize treatment planning in LITT, it is essential to gain knowledge about light distribution in tissue, tissue optical propertie s (absorption, scattering, anisotropy, penetration depth), and their contin uous changes during therapy. Study Design/Materials and Methods: Measurements of optical properties were performed with a double integrating-sphere system and a laser diode (830 n m). Porcine liver tissue samples were examined in a native state (35 degree sC) and after exposure to different temperatures (45 degreesC to 80 degrees C). Results: Rising temperature was accompanied by a decrease in the absorption coefficient and anisotropy factor and an increase in the scattering coeffi cient. These changes were only significant in the temperature range of 50 d egrees to 65 degreesC (P < 0.01). The optical penetration depth decreased f rom 3.1 mm in the native state to 1.7 mm at 65<degrees>C (P < 0.01). Above 65<degrees>, there was no significant change in the tissue optical properti es. Conclusion: The optical properties of liver tissue change significantly und er the influence of tissue heating, resulting in a decreased optical penetr ation depth. These changes occur mainly in the temperature range of 50 degr eesC to 65 degreesC, corresponding to protein denaturation. To ensure a saf e and effective procedure, an adjustment of the laser power to the actual p enetration depth is recommended during therapy. Lasers Surg. Lasers Surg. M ed. 28:307-312, 2001. (C) 2001 Wiley-Liss, Inc.