[en] Aim of the study. - In the case of lung tumor treatment, to adjust 3D helical computed tomography (CT) acquisition parameters using a dynamic phantom and compare to the theory the volumes of a moving object. Materials and methods. - Three helical CT acquisitions were compared using a Big Bore CT scan: an 'initial' 3D CT scan (constructor parameters), an 'optimized' 3D CT scan which parameters are chosen to obtain an axial slow scan like acquisition and a 4D CT scan. We used a phantom composed by a ball filled with water set on a dynamic platform moving in the antero-posterior or cranio-caudal direction with a 14 mm amplitude and a 4 s period. For each acquisition and modality (static and dynamic), we quantified the ball volume by automatic contouring and we estimated relative errors. Results. - For an antero-posterior displacement, the volume of the moving ball is under estimated by 14.1 % with the 'initial' scan, by 0.2 % with the 'optimized' scan and over estimated by 0.8 % with the averaged 4D scan. For a cranio-caudal displacement, it is under estimated by about 22 % with the 'initial' scan and by about 1 % with the 'optimized' scan and the averaged 4D scan. Conclusion. - Volume measurements performed with the dynamic phantom allowed us to validate the 'optimized' 3D CT scan parameters because it accurately reflects the volume of a moving object. Radiotherapy departments without 4D CT should adapt scan parameters for internal target volume definition. (authors)