Contouring the middle and inner ear on radiotherapy planning scans

The purpose of this study was to establish guidelines that help radiation oncologists contour the inner and middle ear on treatment planning scans. The radiotherapy computed tomography (CT) scans of 15 previously treated patients were reviewed for the ability to identify 3 separate auditory structures. The middle ear, the cochlea, and the vestibular apparatus were identified and contoured on each scan using anatomic landmarks. The volume and maximum axial dimension of each contour were calculated. The middle ear, cochlea, and vestibular apparatus were identified on all scans. The middle ear was defined by the tympanic membrane laterally and by the interface between air and the temporal bone in all other directions. The plane of the internal auditory canal through the temporal bone was the landmark distinguishing the vestibular apparatus from the cochlea. The mean volume of the middle ear, vestibular apparatus, and cochlea were 0.58 cm3, 0.44 cm3, and 0.14 cm3, respectively. The maximum axial dimension across the contour averaged 1.57 cm for the middle ear, 1.10 cm for the vestibular apparatus, and 0.69 cm for the cochlea. A reference atlas was constructed that shows the contour of each structure on 5 consecutive CT images. Accurate identification of the middle ear and inner ear structures on radiotherapy planning scans is possible and is necessary if critical auditory organs are to be spared during radiotherapy of targets that are located near the base of the skull. The information generated in this study will help radiation oncologists contour auditory structures accurately.