In embodiments of the invention, a radiosensitizer or other agent is relatively uniformly accumulated in the cells of a tumor. An external monochromatic x-ray beam, which is tuned to a predetermined energy level associated with k-edge effects in the agent, is then directed to the tumor. The monochromatic x-ray beam activates the release of additional localized radiation from the agent. The additional radiation destroys the DNA of at least some tumor cells, making those tumor cells incapable of reproduction and repair. In embodiments of the invention, a single monochromatic x-ray beam source is used for both imaging and radiotherapy.