Concept of treating volumes of the conventional RT: the figure shows a centrally located right-side lung tumor (gross tumor volume-GTV, white contour). In order to address the high-risk of subclinical/microscopic disease spread, the clinical target volume (CTV, green contour) is contoured and considered as target for irradiation. Additionally, in order to account for the tumor motion due to respiration, the internal target volume (ITV, orange contour) will also be drawn taking into consideration tumor motion in 4D-CT. Finally, to account for the set-up error(s) related to the patients daily (re)positioning, an additionally larger volume known as the planning target volume (PTV, red contour) is also drawn as the final treating volume for radiation treatment. The yellow arrows indicate the definitive diameter of the final treatment volume in comparison to the macroscopic tumor (GTV). Significant amount of the surrounding peri-tumoral healthy tissue between GTV and PTV will be exposed to the full radiation dose (blue lines), the same that will be delivered to the tumor.