A method of treating a solid tumor, including the steps of localizing the tumor and the arterial vessel that supplies it, and injecting into the vessel, a vaso-occlusive collagen material adapted to produce occlusion of vessels having lumen diameters between about 10 and 150 microns. The injected material is effective to produce occlusion of the secondary and tertiary vessels supplying the tumor, distal to the collateral blood vessels which may also supply the tumor. The region of the occluded tumor is heated under conditions that produce tissue necrosis selectively in the occluded tumor tissue.