A monitor for pulsed high energy radiation therapy using a radiation beam passing through a treatment zone, the radiation of 0.2 MEV or greater; has a camera for imaging Cherenkov light from the treatment zone; apparatus for preventing interference by room lighting, the camera synchronized to pulses of the radiation beam; and an image processor adapted to determine extent of the beam area on the patient skin from the images. Additionally an image processor determines cumulative skin dose in the treatment zone from the images. In embodiments, the processor uses a three-dimensional model of a subject to determine mapping of image intensity in images of Cherenkov light to radiation intensity in skin, applies the mapping to images of Cherenkov light to verify skin dose delivered, and accumulates skin dose by summing the maps of skin dose.