[Scan acquisition parameter optimization for the treatment of moving tumors in radiotherapy]

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 4s 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.