Histologic evaluation of rat mammary tumor necrosis by interstitial Nd:YAG laser hyperthermia

The extent of coagulative necrosis caused by interstitial laser hyperthermia was measured for different quantities of laser energy in a rat mammary tumor model. Continuous wave Nd:YAG laser at a power level of 5 W was focused onto a 600 mu diameter bare tip quartz fiber and placed inside a 19-gauge needle, which allowed the para-axial flow of normal saline at 1 cc/min. A microthermocouple soldered to the outside of the probe continuously provided the interstitial temperature. After the probe was inserted into the tumor, it was withdrawn as laser energy was administered at a rate sufficient to maintain the temperature within 42-45 degrees C. Tumors were excised after 48 hours, fixed in formalin, cut in 3 mm slices, and the coagulated surfaces measured microscopically. Laser fiber transmission loss was 1% per 1,000 J of laser energy and the average time required to coagulate 1 cc of tumor was 2 minutes. There was a statistically significant correlation between the volume of tumor necrosis and the level of laser irradiation (r = 0.71, P less than 0.001). It is concluded that the described technique is an efficient method of tumor coagulation by interstitial laser hyperthermia and proportionally larger volumes of necrosis are created with greater amounts of laser energy.