Using cone beam CT to investigate the local geometrical uncertainties during head and neck radiation therapy

High quality head and neck (H&N) intensitymodulated radiation therapy (IMRT) requires comprehensive data on geometrical changes of target and critical structures during treatment course. In this study, we developed a method to quantitatively investigate the spatial head displacements, head rotation and neck movement by using sequential cone beam CT (CBCT) images. Five patients undergoing radiation therapy received weekly CBCT scans under the head and shoulder thermoplastic masks (MedTech®, Orange City, USA). CBCT images were registered to the planning CT at the clivus and occipital bones. Eight visually distinguishable bony landmarks, representing position of the head, were identified. The displacements of the landmarks relative to the planning CT were computed. Three non-linear bony points were employed and the head rotation was derived from these points. For the cord movement, the spinal canal was contoured and the geometrical centroid line served as the cord surrogate. The cord movement relative to the planning CT was quantified by the variations of the centroid lines. The five patient data show that the head bony landmarks displaced maximal 0.4 cm. The head rotated in the range of 4 degrees on the sagittal and coronal planes and 2 degrees on the axial plane. The cord offsets were minimal superiorly with the mean between –0.1 to 0.1 cm and increased to the range of –0.3 to 0.4 cm inferiorly. Our preliminary results show spatial uncertainties and indicate the possibility of local treatment margins.