Inherent Functional Dependence Among Cochlear Dose Surrogates for Stereotactic Radiosurgery of Acoustic Schwannomas

Abstract

Various cochlear dose surrogates such as the mean dose, maximum point dose and central modiolus dose have been reported as associated with hearing outcome in studies of stereotactic radiosurgery (SRS) for vestibular schwannomas. In this study, we investigated whether an inherent functional relationship exists among these dose surrogates and its analytical formalism and expressions. A cohort of 97 serial patient cases treated with single-fraction stereotactic radiosurgery at our institution from 2007-2013 was reviewed and 85 cases were found to be analyzable, excluding cases such as surgically absent cochlea, incomplete visualization of the structure, etc. For all the cases, the mean prescription dose was 12.5 ± 0.3 Gy (range, 12-13 Gy) and mean target volume 1.32 ± 1.51 mL (range, 0.80-8.77 mL). The mean cochlea volume was 0.078 ± 0.016 mL (range, 0.048-0.131 mL; median, 0.076 mL). Correlation analysis among mean cochlear dose, point maximum dose and modiolus dose was performed and also parameterized with new variables such as the Effective Dose Radius (EDR) as derived from a general dose fall-off model (Ma et al., IJROBP, 76(3) 943-8, 2010). Weak correlation via linear regression was found between the point maximum dose and the mean cochlear dose (R2 = 0.719) as well as the modiolus dose (R2 = 0.568). However, when parameterized with EDR, a near perfect correlation via linear regression was found between the EDR for the point maximum dose and the EDR for the mean cochlear dose (R2 = 0.996), and with the EDR for the modiolus dose (R2 = 0.993). Such a result led to a functional formula relating these dose surrogates, yielding dose equivalence results such as 12 Gy point maximum dose is equivalent to mean cochlear dose of 5.6 ± 0.1 Gy (95% CL) or modiolous dose of 6.0 ± 0.2 Gy (95% CL). An inherent functional relationship exists among point maximum, modiolous, and mean cochlear doses for stereotactic radiosurgery of vestibular schwannomas. As such, clinical hearing outcome can be interchangeably analyzed or reported via any of these dose surrogates.