A system for dynamic treatment simulation for radiotherapy. Software is implemented on a computer system to perform dynamic simulation by displaying every segment of dynamic treatment beams on top of real-time fluoroscopic image sequences. Automated distortion correction of the fluoroscopic image due to the image intensifier is performed in real-time. A respiration phase indicator measures changes in circumference of the patient's chest through the movement of a radio-opaque marker that appears directly in the fluoroscopic images of the patient.