Radiotherapy is a common treatment for lung cancer. One Challenge of delivering radiation treatment to lung tumors accurately is tumor movement which occurs as a patient breathes. In some situations, tumors move enough during breathing so that some or all of the tumor may be missed by a radiation treatment. One way to decrease the amount a lung tumor moves during radiotherapy treatments is for patients to held their breath briefly during a radiation treatment. By doing this, a patient's lung tumor may not move as much as it would during regular breathing. In this study, the investigators aim to study patients with lung cancers which move during breathing. Patients will be asked to hold their breath after inspiration while a CT scan of their lung tumor is obtained. The purpose of this study is to study how much less patients' lung tumors move when they hold their breath

Inclusion Criteria:

• biopsy confirmed primary lung cancer
• undergoing radiotherapy or chemoradiotherapy
• able to perform adequate deep inspiration breath hold
• patients of childbearing potential must practice adequate contraception
• signed study-specific informed consent form

Exclusion Criteria:

• unable to perform adequate deep inspiration breath hold
• prior tumor resection
• prior chest or neck RT
• pregnant

patients with any primary lung cancer who are undergoing radiotherapy or chemoradiotherapy