Abstract: Apparatus and method for locally generating hyperthermia-induced coagulation necrosis in tissue masses located deeply within the body of a subject. High energy laser radiation is introduced into the target tissue mass using an optical fiber inserted into a metallic cannula through which a suitable fluid is coaxially flowed. A combination of the fluid flow and the slow withdrawal of the cannula/fiber combination during therapy protects the optical fiber tip, thereby preserving its energy transmission characteristics. After establishment of a small fluid bolus in front of the fiber tip, fluid flow which is approximately proportionate to the laser energy employed has been found to be most effective in this regard.