A method and system for treating a subject with a disorder which provides within the subject at least one photoactivatable drug for treatment of the subject, applies initiation energy from at least one source to generate inside the subject a preferential x-ray flux for generation of Cherenkov radiation (CR) light capable of activating at least one photoactivatable drug, and from the CR light, activating inside the subject the at least one photoactivatable drug to thereby treat the disorder.