Megavoltage imaging method and apparatus are provided to monitor a target volume of a patient in real time. Structure and steps are provided for generating an image on a detector (having a layer of photoreceptor material deposited on a layer of a high density substrate) by passing a photon beam first through the target volume of the patient and subsequently through the detector. The layer of the high density substrate is on the side of incidence of the photon beam for intensifying photon quanta and filter-scattered radiation from the patient induced by the photon beam. Thus, the photons impinging on the high density substrate are converted to electrons, and the images generated by the electrons subsequently traversing the photoreceptor material to cause the target volume of the patient to be monitored in real time.