This review analyzes PET images in radiotherapy treatment planning for lung cancer patients and discusses the most controversial current issues. Computed tomography images are commonly used to assess location and extension of target volumes and organs at risk in radiotherapy treatment planning. Although PET is more sensitive and specific, contouring on PET images is difficult because tumor margins are indistinct, due to heterogeneous (18)fluorodeoxyglucose uptake distribution and limited spatial resolution. The best target delineation criteria have not yet been established. In non-small-cell lung cancer, PET appears to improve sparing of organs at risk and reduce the risk of toxicity; prescribed doses can be increased. Data are scarce on small-cell lung cancer.