The use of image-guided radiotherapy (IGRT) for lung cancer improves target coverage and potentially reduces the risk of treatment-related toxicity. Four-dimensional radiotherapy (which refers to the explicit inclusion of temporal changes in anatomy during imaging) treatment planning and treatment delivery are important components of IGRT for lung cancer. Four-dimensional CT (4DCT) scanning is a major breakthrough that has transformed radiotherapy planning for lung cancer. Individualized internal target volumes can be directly obtained from these 4DCT scans, e.g. using maximum intensity projections. Color intensity projections of 4DCT scans provide temporal information on organ mobility within a single composite image. As only a minority of patients are likely to benefit from respiratory gating, patient-specific assessment of tumor mobility is essential.