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Stereotactic body radiotherapy (SBRT) is an ablative method for lung malignancies. Here, the definition of the gross target volume (GTV) is subject to interobserver variation. In this study, we aimed to evaluate the interobserver variability during SBRT and its dosimetric impact, as well as to introduce a semi‐automated delineation tool for both planning computer tomography (P‐CT) and cone beam CT (CBCT) to help to standardise GTV delineation and adaptive volume‐change registration. The interobserver variation of GTV manual contours from five physicians was analysed in 15 patients after lung SBRT on free breathing (FB) P‐CT (n= 15) and CBCT (n= 90) before and after each fraction. The dosimetric impact from interobserver variations of GTV based on the original treatment plan was analysed. Next, the accuracy of an in‐house easy‐to‐use semi‐automated‐segmentation algorithm for pulmonary lesions was compared with gold standard contours in FB P‐CT and CBCT, as well as 4D P‐CT of additional 10 patients. The interobserver variability in manual contours resulted in violations of dose coverage of the planning target volume (PTV), which, in turn, resulted in compromised tumour control probability in contours from four physicians. The validation of the semi‐automated delineation algorithm using thorax phantom led to a highly reliable accuracy in defining GTVs. Comparing the unsupervised auto‐contours with the gold standard delineation revealed high equal high concordance for FB P‐CT, 4D P‐CT and CBCT, with a DSC of 0.83, 0.76 and 0.8, respectively. The supervised use of the semi‐automated delineation tool improved its accuracy, with DSCs of 0.86, 0.86 and 0.8 for FB P‐CT, 4D P‐CT and CBCT, respectively. The use of the algorithm was associated with a significantly shorter working time. The semi‐automated delineation tool can accurately register volume changes in CBCTs. The segmentation algorithm provides a reliable, standardised and time‐saving alternative for manual delineation in lung SBRT in P‐CT and CBCT. The Interobserver variability for target volume delineation in the SBRT of pulmonary lesions ‐ could have major negative dosimetric consequences, which could threaten the tumour control probability. In this study, we introduce an easy‐use semi‐automated delineation tool for peripheral lung lesions in free breathing (FB) and four‐dimensional (4D) planning CT and cone beam CT (CBCT). The Dice similarity coefficient (DSC) showed the reliability of the tool alone or better in a supervised application. The semi‐automated delineation tool can accurately track the interfraction and intrafraction changes or deformations of tumours through the therapy in CBCT, opening the door for its use in adaptive planning.