The present invention provides a method for activating a pro-drug in vivo comprising the steps of: (a) administering a pro-drag to a subject; (b) locating a target site which has been at least partially dosed with the pro-drug at a predetermined concentration; and (d) exposing the target site to X-ray radiation. The step of exposing the target site to X-ray radiation is characterized by providing a converging X-ray of a controllable waist, substantially uniform at the target site, sufficient to convert the pro-drug to an active drug, while the X-ray photon dosage at the target site is higher than the X-ray photon dosage at an adjacent non target site.