Statistical Analysis: In a previous study comparing DIBH-gated CT-scanning to free-breathing CT-scanning performed at this institution, it was found that a sample size of 10 patients was appropriate to illustrate both a statistically significant reduction in PTV and a statistically significant reduction in V20 (the volume of lung treated to a dose of =20Gy). For the current study, given that the patient population is essentially the same as the last one, and the fact that PET and CT are comparable imaging modalities for NSCLC in terms of their sensitivity, a total sample size of 10 patients would be an appropriate number to assess the difference between breath-held PET and free-breathing PET (i.e. the first objective). This difference, in terms of both PTV and NTCP, will be assessed for statistical significance with a paired Student's t-test.