The technique employed in radiotherapy to counteract the effect of tumour motion due to breathing is known as gating. In gated radiotherapy, beam is turned on only during a ceain phase of respiratory cycle, generally at the end of expiration. Respiration is monitored using external markers placed on the patients chest, which are assumed to move similarly to the tumor. This is done in patients who can breathe regularly and reproducibly. Advantage of this technique is that, it decreases the internal target volume and reduce the volume of normal tissue that is irradiated. Disadvantage of this approach is prolonged treatment time and lack of direct imaging of tumor position during treatment. Ref: Lung Cancer By Kemp H. Kernstine, Page 209; Biomedical Physics in Radiotherapy for Cancer By Barry Allen, 2012