Lung cancer is the most common cancer worldwide and accounts for the most cancer related deaths. The United States 2006 cancer statistics showed that lung cancer is the second most common cancer for men and women but the number one cancer killer in both sexes. Radiotherapy is crucial in the treatment of lung cancer and is required in 40% of patients with lung cancer. Image-guided radiation therapy provides a tool to improve target accuracy, reduce normal tissue toxicity while increasing radiation dose to tumor that would increase therapeutic ratio of thoracic radiation therapy. Respiratory correlated 4D and breath-hold CT image data sets and those of standard free-breathing CT, are required to quantify the extent of intra-fractional motion and to design treatment plans for various respiration-correlated treatment techniques.