Effectiveness and Toxicity of Fractionated Proton Beam Radiotherapy for Cranial Nerve Schwannoma Unsuitable for Stereotactic Radiosurgery.

Tanja Eichkorn,Klaus Herfarth,Juliane Hörner-Rieber,Dorothea Kronsteiner,Jürgen Debus,Sebastian Regnery,Rami A El Shafie,Thomas Held,Laila König

doi:10.3389/fonc.2021.772831

Abstract

PurposeIn this benign tumor entity, preservation of cranial nerve function is of special importance. Due to its advantageous physical properties, proton beam radiotherapy (PRT) is a promising approach that spares healthy tissue. Could PRT go along with satisfactory preservation rates for cranial nerve function without compromising tumor control in patients with cranial nerve schwannoma unsuitable for stereotactic radiosurgery?MethodsWe analyzed 45 patients with cranial nerve schwannomas who underwent PRT between 2012 and 2020 at our institution. Response assessment was performed by MRI according to RECIST 1.1, and toxicity was graded following CTCAE 5.0.ResultsThe most common schwannoma origin was the vestibulocochlear nerve with 82.2%, followed by the trigeminal nerve with 8.9% and the glossopharyngeal nerve as well as the vagal nerve, both with each 4.4%. At radiotherapy start, 58% of cranial nerve schwannomas were progressive and 95.6% were symptomatic. Patients were treated with a median total dose of 54 Gy RBE in 1.8 Gy RBE per fraction. MRI during the median follow-up period of 42 months (IQR 26–61) revealed stable disease in 93.3% of the patients and partial regression in 6.7%. There was no case of progressive disease. New or worsening cranial nerve dysfunction was found in 20.0% of all patients, but always graded as CTCAE °I-II. In seven cases (16%), radiation-induced contrast enhancements (RICE) were detected after a median time of 14 months (range 2–26 months). RICE were asymptomatic (71%) or transient symptomatic (CTCAE °II; 29%). No CTCAE °III/IV toxicities were observed. Lesions regressed during the follow-up period in three of the seven cases, and no lesion progressed during the follow-up period.ConclusionThese data demonstrate excellent effectiveness with 100% local control in a median follow-up period of 3.6 years with a promising cranial nerve functional protection rate of 80%. RICE occurred in 16% of the patients after PRT and were not or only mildly symptomatic.

Highlights

Schwannomas are usually benign and slowly growing nerve sheet tumors that arise from the Schwann cells lining of peripheral nerves
Vestibular schwannomas that commonly arise from the vestibular portion of the eighth cranial nerve account for most schwannomas
This study investigates the effectiveness and toxicity of fractionated proton beam radiotherapy for cranial nerve schwannomas that were unsuitable for stereotactic radiosurgery (SRS)

Summary

Introduction

Schwannomas are usually benign and slowly growing nerve sheet tumors that arise from the Schwann cells lining of peripheral nerves. They are most commonly located in the intradural extramedullary space and affect mainly cranial or spinal nerves [1]. Vestibular schwannomas ( known as acoustic neuromas) that commonly arise from the vestibular portion of the eighth cranial nerve account for most schwannomas. While sporadic schwannomas are rare, they occur frequently in patients with neurofibromatosis. Bilateral vestibular schwannomas in children with neurofibromatosis put them at risk for complete deafness [3,4,5,6]

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