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.