Radiosurgery and stereotactic radiation therapy for vestibular schwannoma in neurofibromatosis type 2 patients

Abstract

Neurofibromatosis type 2 (NF2) patients typically suffer from bilateral vestibular schwannoma (VS) and are at risk for developing bilateral deafness, bilateral trigeminal and bilateral facial nerve function loss. Previous reports suggest that treatment outcomes in these patients are worse compared to patients with sporadic solitary VS. Especially in NF2 patients who acquired unilateral hearing loss already, avoidance of hearing loss on the opposite side poses a challenge for stereotactic radiosurgery. We studied our treatment results in NF2 patients treated for VS with linac based radiosurgery (RS) and stereotactic radiation therapy (SRT). In 204 VS patients treated with RS or SRT in Amsterdam starting from 1992, we identified 25 NF2 patients with bilateral tumors. Indications for treatment were either tumor progression on sequential MRI and/or progressive hearing loss. Stereotactic irradiation was administered to all patients using 5 non-coplanar arcs with a single isocenter to a dose of 10–12.5 Gy in a single fraction or 20–25 Gy in 5 fractions in 1 week prescribed to the 80% isodose encompassing the tumor. In the first patients a single round collimator was used for every treatment. In subsequent patients conformal arcs were used. Dynamic conformal arcs application, resulting in superior dose distributions, is used currently. Five patients were followed less than 1 year. Of the remaining 20 patients, 5 were ipsilateral deaf prior to treatment. Consequently 15 patients were at risk for treatment related hearing loss. They showed a mean Pure Tone Average (PTA) of 51 dB (8–112) prior to treatment. On the untreated side, all patients showed hearing loss and 8 (53%) were deaf. After treatment all patients were assessed at yearly intervals including MRI and pure tone audiometry. Median follow-up time was 51 months (12–109). Local tumor control was obtained in all 20 patients and no treatment related trigeminal or facial nerve toxicity was observed. Hearing status was assessed yearly after treatment. This revealed that the mean PTA in the 15 hearing patients dropped from 51 to 77 dB (40–120). In 6 of these patients (40%) the additional PTA loss ranged from 0–15 dB, in another 6 patients (40%) it ranged from 15–45 dB and in 3 of these patients (20%) it was more than 45 dB. No additional hearing loss was observed beyond 36 months after treatment. Excellent local control rates can be obtained with RS and SRT for VS in NF2 patients, with minimal facial and trigeminal nerve toxicity. Hearing preservation, however, remains disappointing with 60% of the patients losing 15 dB or more of PTA on the irradiated side. This seems worse than our previously reported outcomes of SRS and RS in patients with solitary sporadic tumors. Improvements may be expected by application of dynamic conformal beam shaping