A Protective Cap: Fundal Fluid Cap Facilitates a Reduction in Inner Ear Radiation Dose in the Radiosurgical Treatment of Vestibular Schwannoma

Objective: Assess inner ear radiation dose magnitude as it relates to fundal cap length and hearing outcomes in the radiosurgical treatment of vestibular schwannoma. Study Design: Retrospective case series. Setting: Tertiary neurotology referral center. Patients: Patients treated with Gamma Knife radiosurgery for vestibular schwannoma between March 2007 and March 2017 were considered for this study. Exclusion criteria included pretreatment pure-tone average (PTA) >90 dB, neurofibromatosis type II, history of previous surgical resection, and follow-up less than 1 year. Main Outcome Measure: (s): Hearing function was assessed by maintenance of class A/B hearing level and maintenance of baseline hearing (<= 20 dB change in PTA following Gamma Knife radiosurgery). Results: Lower radiation doses delivered to the inner ear were associated with longer fundal cap lengths: mean cochlear dose (r = -0.130; p = 0.184), mean labyrinth dose (r = -0.406; p < 0.001), max cochlear dose (r = -0.326; p = 0.001), and max labyrinth dose (r = -0.360; p < 0.001). Kaplan-Meier analysis with log-rank testing revealed that patients with a mean labyrinth dose < 3 Gy achieved higher rates of preserving baseline hearing (<= 20 dB change in PTA) following radiosurgery, compared to patients with a mean labyrinth dose >= 3 Gy (p < 0.001). A fundal fluid cap length of 2.5 mm was associated with the 3 Gy mean labyrinth dose threshold. Conclusions: We report that fundal cap presence facilitated the creation of treatment plans with a lower dose delivered to the labyrinth. By affording this dose reduction, a fundal cap may be associated with a slight improvement in hearing outcomes.