Clinical outcomes and toxicity of proton radiotherapy for vestibular schwannomas: a systematic review

Objective Vestibular schwannomas are benign tumors that are often managed by radiotherapy. Minimizing long-term toxicity is paramount for a population that remains at normal life expectancy and at risk for loss of quality of life for years if not decades. Whereas current radiotherapy standard utilizes photon radiation, proton radiotherapy characteristics may enable a reduction of toxicity by reducing the volume of collateral irradiated healthy tissue. A systematic review was conducted to assess tumor control and short- and long-term sequelae after proton irradiation. Methods Studies that reported on treatment outcomes of proton radiotherapy in vestibular schwannoma patients were included. Results Five peer-reviewed retrospective series met the inclusion criteria. Quality of the studies varied from low to good. There were 276 unique patients described. Tumor control rates ranged from 85 to 100% (radiological median follow-up of 2.2-7.4 years). Hearing loss rates, defined as losing Gardner Robertson class I/II hearing, showed an weighted crude average 52% (depending on follow-up duration). The weighted averages for post-irradiation facial and trigeminal neuropathy were 5% and 4%, respectively. The risk of neuropathy seems to decrease with lower radiation dosages. Conclusion Proton irradiation for vestibular schwannomas achieves high tumor control rates, equivalent to photon irradiation. Reported cranial nerve preservation rates vary, partly due to an apparent selection bias with a high percentage of patients with clinical symptoms prior to treatment. Results of cranial nerve function preservation, quality of life, and cognitive functioning are currently insufficiently reported. In addition, advances in proton radiotherapy technology warrant re-evaluation of current techniques and protocols for the management of vestibular schwannomas.