Fractionated Proton Beam Radiation Therapy and Hearing Preservation for Vestibular Schwannoma: Preliminary Analysis of a Prospective Phase 2 Clinical Trial

Abstract

Management for vestibular schwannoma (VS) achieves excellent local control, and advances have increasingly focused on long-term serviceable hearing preservation. Fractionated stereotactic radiation therapy (FSRT) is associated with better hearing preservation compared to stereotactic radiosurgery (SRS). Fractionated proton beam radiation therapy (FPRT) may be associated with greater hearing preservation due to unique dosimetric properties of proton radiotherapy.A single institution, prospective, single-arm, phase 2 clinical trial was designed for patients with VS between 2010-2019. All patients received FPRT to a total dose of 50.4-54Gy relative biological effectiveness (RBE) over 28-30 fractions. The primary endpoint was serviceable hearing preservation, defined as Gardner-Robertson (GR) score 1-2, defined as Pure Tone Average (PTA) ≤50dB and Word Recognition Score (WRS) ≥50%, at 5 years compared to a historical rate of serviceable hearing preservation of 65% in patients treated with photon radiation therapy. Other audiometric measures included Otoacoustic Emissions (OAE) and Auditory Brainstem Response (ABR). Preliminary analysis was performed for 20 patients accrued with a minimum follow up of 1.0 year.Twenty patients were accrued with median follow up of 4.0 years (range 1.0-5.0 years), with a median age of 64 years, median RT dose of 50.4 Gy (RBE). Serviceable hearing preservation at 1 year was 53%, primary endpoint was not yet met. Local control at 4 years was 100%. Median cochlea D90 was lower for patients with serviceable hearing at 1 year (median 40.5 Gy (RBE) vs 46.9 Gy (RBE), P = 0.0863). V90, Dmean, Dmin, and other cochlea dosimetric measures were also generally lower, but the associations with hearing preservation at 1 year were not statistically significant. Median PTA at 1-year worsened from 42dB at baseline to 62dB (P < 0.0001), and median WRS at 1-year worsened significantly from 85% to 48% (P < 0.0001); PTA worsening developed earlier than WRS worsening. The rate of OAE absence significantly worsened from 60% at baseline to 100% at 1-year (P = 0.0156). Other audiometric measures, including contralateral PTA, ABR, tympanometry, were not independently associated with hearing preservation. Treatment was well tolerated, with low levels of acute and late-term toxicity; one patient incurred grade one CN V dysfunction, with no grade two or higher CN dysfunction.FPRT for vestibular schwannoma results in high rates of local control with comparable serviceable hearing preservation compared to historical RT series. Audiometric analyses suggest radiation-associated hearing dysfunction is manifested as early PTA worsening followed by WRS worsening.