Measurement of thermal effects on the optical properties of prostate tissue at wavelengths of 1,064 and 633 nm

Background and Objective: The extent of thermal injury during laser prostat ectomy is dependent on the light distribution in laser-irradiated tissue. A s tissue is irradiated, the optical properties change as a function of temp erature due to an alteration of molecular and cellular structure. The purpo se of the present study was to determine how the exposure of both fresh and previously frozen canine prostate tissue to elevated temperatures affects the optical properties. Study Design/Materials and Methods: Optical properties were measured by usi ng a double integrating sphere spectrophotometer with an inverse adding-dou bling algorithm. Measurements were made at two wavelengths (1,064 nm and 63 3 nm) on samples heated in a waterbath in 5 degrees-10 degrees increments f or 10 min through a 50 degrees C temperature range. Results: Upon coagulation, the absorption coefficient of fresh tissue decre ased from the baseline measurement for both wavelengths (0.027 +/- 0.003 to 0.019 +/- 0.002 for lambda = 1,064 nm; 0.073 +/- 0.007 to 0.061 +/- 0.006 for lambda = 633 nm). However, the scattering coefficient increased sharply from the baseline measurement following coagulation (3.06 +/- 0.26 to 6.05 +/- 0.29 for lambda = 1,064 nm; 4.89 +/- 0.23 to 7.22 +/- 0.30 for lambda = 633 nm). Thermal coagulation occurred during exposure to temperatures bet ween 60 degrees C and 70 degrees C, Conclusion: Data obtained in this study indicate that thermal coagulation o f tissue alters the optical properties. The extent to which these changes o ccur was found to be dependent on wavelength and freshness of tissue. These results are significant because they suggest how thermally induced changes in the optical properties may limit the depth of light penetration in tiss ue thus compromising treatment. Lasers Surg. Med. 24:38-47, 1999. (C) 1999 Wiley-Liss, Inc.