Stereotactic radiation therapy (SRT) is becoming more and more important in the treatment of inoperable patients with early stages of pulmonary carcinomas (T1-T2 N0M0). In certain cases, evaluation of the response is still problematical and it can be difficult to differentiate response from progression. The aim of this paper is to set out these various changes and to produce a protocol for optimal monitoring. By comparing our clinical experience with data from the literature, the main visual aspects on a CT scan are set out and illustrated for each clinical situation: radiation pneumonitis, radiation fibrosis, therapeutic response and progression. The literature was reviewed by querying the main databases and selecting papers concerning pulmonary SRT and post-therapeutic radiological appearance. CT appearance induced by SRT differs significantly from images after classic conformal radiation therapy, both morphologically and chronologically. In particular, the modifications induced by stereotactic radiation therapy are only seen in a limited volume surrounding the volume treated. Knowledge of the radiological criteria necessary to differentiate between a therapeutic response and recurrence is of major importance in the present context of increase in use of this technique.