Retrospective kV x-ray 4DCT treatment planning for lung cancer MV linac treatment is becoming a standard-of-care for this widely used procedure for the largest cancer cause-of-death in the US. It currently provides the best estimate of a fixed-in-time but undulating and closed 3D shell to which a minimum curative-intent radiation dose should be delivered to provide the best estimated patient survival and the least morbidity, usually characterized by quantitative dose-volume-histograms (DVHs). Unfortunately this closed shell volume or internal target volume (ITV) currently has to be increased enough to enclose the full range of respiratory lesion motion (plus set-up etc. uncertainties) which cannot yet be accurately determined in real time during treatment delivery. With accurate motion-tracking, the planning target volume (PTV) or outer "shell" may be reduced by up to 40%. However there is no single 2D plane that precisely follows the reduced-PTV-volume's 3D respiratory motion, currently best estimated by the retrospective hand contouring by a trained and experienced MD radiation oncology MD using the full 3D-time information of 4DCT. Once available, 3D motion tracking in real time has the potential to substantially decrease DVH doses to surrounding organs-at-risk (OARs), while maintaining or raising the curative-intent dose to the lesion itself. The assertion argued here is that, the 3D volume-rendered imaging of lung cancer lesion-trajectories in real-time from TumoTrak digital x-ray tomosythesis, has the potential to provide more accurate 3D motion tracking and improved dose delivery at lower cost than the real time, 2D single slice imaging of MRI-guided radiotherapy.