[en] Complete text of publication follows: Purpose: PET tumour segmentation is currently an active research topic in the field of radiotherapy planning and multi-parametric data quantification. Although being efficient on homogeneous spheroid-shaped tumours, classical threshold-based approaches are of limited value for heterogeneous or complex-shaped tumours, still making expert-based manual delineation the reference standard for tumour imaging, despite several limitations. In this context, the aim of this study was to assess the performances of an active contour-based approach for the PET segmentation of complex-shaped lung tumours, in comparison to an optimized expert-based manual reference standard. Subjects and Methods: Seventy-five thoracic tumours were segmented using the same graphical user interface (GUI) ITK snap software. For each tumour, an optimized expert-based reference standard was generated from the set of six independent expert-based manual segmentation results using the Simultaneous Truth And Performance Level Estimate (STAPLE) algorithm. In addition, four raters semi-automatically segmented the 75 PET tumours twice using the active contour based-procedure of the GUI software, with a delay time of one week between two segmentation sessions. For the 75 tumours, accuracy of the semi-automatic segmentations against the optimized expert-based reference standard was assessed using the DICE similarity coefficient (DSC). Inter-rater and intra-rater reliability analyses were performed using the intra class correlation coefficients (ICC) estimates of the output volumes, along with their 95% confidence intervals (two-way mixed-model, individual-rating, absolute-agreement). For all tumours segmentation procedures, average time per procedure was also estimated. Results: Overall accuracy of the semi-automatic procedure was excellent with a DSC of 0.835 (95%CI = 0.775-0.895). Inter-rater reliabilities provided the following results: ICC = 0.941 (95%CI = 0.913-0.961) for the first session and ICC = 0.935 (95%CI = 0.906- 0.956) for the second session. Intra-rater reliabilities provided the following results: ICC = 0.993 (95%CI = 0.990-0.996) for the rater 1; ICC = 0.987 (95%CI = 0.976-0.993) for the rater 2; ICC = 0.972 (95%CI = 0.956-0.982) for the rater 3; and ICC = 0.977 (95%CI = 0.964-0.985) for the rater 4. Average time was 631 seconds for manual segmentation procedure and 130 seconds for active contour-based. Conclusions: Compared to the state of the art expert-based manual segmentation, the GUI-based active contour procedure provided excellent accuracy and reliability, with a mean procedure duration almost five-times faster than the manual reference procedure. ITK snap software is robust, fast, and easy enough to be routinely applied as a powerful alternative to the manual reference standard in this setting