Relevance in Standardizing Hearing Apparatus Contours

Abstract

The potential for sensorineural hearing loss (SNHL) is an unfortunate reality with patients who receive radiation therapy (RT) to the head and neck (H&N) region. A review of the literature seems to indicate there is a lack in standards for contours associated with SNHL. The range of contoured structures reported by institutions include the tympanic cavity, Eustachian tube, mastoid process, cochlea, vestibule, inter auditory canal (IAC), and various combinations of the aforementioned items. At our institution, we have historically contoured the cochlea, vestibule, and a portion of the inner auditory canal as a single volume for each side (i.e. the bony labyrinth of the inner ear) and used that structure for optimization and dosimetric analysis. This retrospective study examines how much dose the cochlea, vestibule, and IAC individually received when the bony labyrinth of the inner ear was contoured and used as an avoidance structure during planning. Twenty consecutive patients who received IMRT for H&N cancer in 2015 at our institution were selected for this study. A Phillips Brilliance Big Bore 16 slice CT scanner was used for the RT simulation and images were acquired at a slice thickness of 3 mm. Contouring was done on a Phillips Pinnacle treatment planning system (TPS) and planned on an Accuray TomoTherapy TPS. The dose was transferred from the TomoTherapy TPS to the Pinnacle TPS where dose statistics were collected. A paired t-test analysis was performed with 40 data points of each structure (20 entries from each anatomical side) using SPSS software. Mean dose to our planning bony labyrinth organs at risk (OAR) was 12.95 Gy (standard deviation (SD) 9.96; standard error mean (SEM) 1.58). Cochlea mean dose = 13.66 Gy (10.42 SD; 1.65 SEM); Vestibule mean dose 12.05 Gy (9.57 SD, 1.51 SEM); and IAC mean dose = 14.24 Gy (11.35 SD, 1.80 SEM). Statistically significant differences were seen as we compared mean doses between our bony labyrinth to the cochlea (P=.0168) and the vestibule (P = .0001), but not to the IAC (P = .0877). Our results showed significant differences between the mean dose of the bony labyrinth versus the mean doses to the cochlea and vestibule but not with the IAC when the bony labyrinth of the inner ear is contoured as a single structure instead of having separate delineation of its components. Considering that there is ongoing research investigating the potential differences in tolerances for the various hearing structures, it is important to keep in mind that non-standard or ill-defined contouring of these structures may result in variations of dosimetric data between studies. This may lead to false correlations between dose given to OAR’s and clinical effect.