An apparatus, method, system, and means to detect motion of a subject by direct imaging on a treatment plane during a radiotherapy treatment, the method includes delivering a radiotherapy treatment beam to a volume of interest of the subject during a treatment time, acquiring image data during the treatment time associated with the delivery of the radiotherapy treatment beam by a direct imaging of a projection of the treatment volume of interest, providing a real-time display of the acquired image data, determining the occurrence of a motion in the volume of interest during the treatment time, determining the motion exceeds a pre-determined threshold, and outputting an indication the determined motion exceeds the pre-determined threshold during the treatment time.