Local failure is a major cause for low overall survival rates in advanced non small cell lung cancer (NSCLC). Among others, radioresistant tumor clones as well as geographical miss can explain these high local failure rates. One reason for geographical miss is a change of tumor related atelectasis in the course of radiotherapy. We present the case of a patient with UICC Stage IIIb NSCLC who presented with a large tumor related atelectasis. During definitive radiochemotherapy, the atelectasis resolved, which resulted in a massive tumor shift out of the planning target volume within 2 days. Without close monitoring by cone beam CTs and prompt replanning, this would have led to a geographical miss and relevant underdosage of the tumor. Furthermore, changes in anatomy and pulmonary function during treatment had implications for organs at risk and opened windows for dose escalation. We suggest at least biweekly CBCTs in patients with poststenotic atelectasis to ensure the rapid detection of geographical changes of the target and subsequent intervention if necessary.