Colorectal tumors and hepatic metastases differ in their optical properties - Relevance for dosimetry in laser-induced interstitial thermotherapy

Background and Objectives: The therapeutic application of laser light is a promising alternative to surgical resection of colorectal liver metastases. The extent of tumor destruction achieved by this strategy depends primarily on light distribution in the target tissue. Knowledge about optical properties is necessary to predict light distribution in the tissue for careful irradiation planning. The aim of this study was to compare the optical behavior of healthy colon tissue with that of colorectal carcinomas and their hepatic metastases in the native and coagulated state in order to test the effect of malignant degeneration, metastasis, and thermal coagulation on optical parameters. Materials and Methods: Ninety tissue samples were taken from patients with a colorectal. carcinoma and concomitant liver metastases: healthy colon tissue (n = 30); colon carcinoma (n = 30); liver metastases (n = 30). Optical properties were measured according to the single integrating sphere principle in the native state and after thermal coagulation in the wavelength range of 800-1,100 nm and analyzed by inverse Monte Carlo simulation. Results: The highest optical penetration depth for all tissue types was obtained at the end of the spectral range investigated. The highest penetration depths of 4.13 mm (healthy colon), 7.47 mm (colon carcinoma tissue), and 4.08 (liver metastases) were at 1,060 nm, although the values decreased significantly after thermal coagulation. Comparing healthy colon-to-colon carcinoma always revealed a significantly lower absorption and scattering coefficient in the tumor tissue. This resulted in a higher optical penetration depth of the laser light in the colon carcinoma tissue (P < 0.05). A direct comparison disclosed no agreement between the optical properties of the primary tumor and the liver metastases. In the native state, colon carcinoma tissue had a lower scattering coefficient (P < 0.05), higher anisotropy factor, and optical penetration depth than liver metastases (P < 0.05). The absorption coefficient did not differ significantly. The differences in the native state were equalized by tissue coagulation. Conclusions: Colon carcinoma tissue has a higher optical penetration depth than healthy colon tissue, which speaks in favor of tumor selectivity for interstitial laser application, since large treatment volumes can be obtained in the tumor. The lack of agreement between primary tumors and their concomitant liver metastases indicates a modification of optical behavior through metastasis. Thermal coagulation of tissue leads to changes in the optical properties, which are clearly less pronounced in carcinoma tissue. The data obtained in this study clearly show that an individual irradiation schedule is necessary for effective and safe dosimetry in laser-induced thermotherapy (LITT).