Water‐jet‐cooled Nd:YAG laser coagulation of experimental liver metastases: Correlation between ultrasonography and histology

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Abstract
To establish the value of ultrasonography in imaging laser coagulation of tumor and surrounding tissue, the relation between measurements on ultrasound and histology was determined in a rat tumor model. A piece of colon carcinoma CC531 was implanted in the liver of 21 Wag/Rij rats; 20 days later, tumors (mean diameter 5.3 mm) were treated with a water‐jet‐cooled Nd:YAG laser at 10 W and either 150 J, 300 J, 600 J, 1,200 J, 1,700 J, or 2,400 J. Ultrasonography was done just pre‐ and immediately post‐laser treatment. The animals were sacrificed and livers removed for light microscopical evaluation. Depth and width of coagulation were measured directly on ultrasound, and on histological samples by computer‐assisted image analysis. Laser treatment did not change the echogenic aspect of the tumor on ultrasound. However, liver damage appeared hypoechoic compared to normal liver. A significant correlation was found between the total size of the lesion on ultrasound and histology (P = 0.015, r = 0.57 for depth; P = 0.012, r = 0.58 for width), suggesting that laser induced tumor destruction may be derived from the amount of surrounding hepatic damage on ultrasound.