20.1 Introduction

The management of vestibular schwannomas (VSs) centers on maximizing tumor control while minimizing toxicity to adjacent normal structures including cranial nerves V, VII, and VIII. Because VSs are benign tumors and the majority grow slowly if at all,s. Literatur many patients initially choose surveillance with serial imaging. Factors that influence the decision to undergo active treatment versus observation include patient age, tumor size, presence and severity of symptoms, genetic conditions including neurofibromatosis 2, rate of tumor growth, and status of ipsilateral and contralateral hearing. The two main treatment options include radiotherapy (RT) or surgical resection. RT is often the modality of choice in patients with significant medical comorbidities or in those who decline surgery.

For patients undergoing RT, treatment schedules range from conventionally fractionated RT, which involves the delivery of small doses (1.8–2 Gy) daily over 5 to 6 weeks, to stereotactic radiosurgery (SRS) in which a high dose of RT is given in a single treatment. Hypofractionated stereotactic radiotherapy (HSRT) represents a hybrid between these two treatment extremes in which a moderately high dose per fraction of RT, generally 5 to 7 Gy, is given over three to five fractions. Each of these treatment schedules can be delivered via photon or proton beams. Studies of SRS using photons demonstrate excellent tumor control with 0 to 5% tumor progression rates,s. Literatur ^,​ s. Literatur however, with high rates of V and VII nerve injury of upward of 30% in initial reports.s. Literatur ^,​ s. Literatur Strategies including reduction of the prescription doses. Literatur ^,​ s. Literatur and the introduction of stereotactic hypofractionated or conventionally fractionated radiotherapy (SRT)s. Literatur ^,​ s. Literatur ^,​ s. Literatur have decreased the rates of cranial neuropathy. Another approach to potentially reduce toxicities while maintaining high tumor control rates is through the use of proton RT.

20.2 Dosimetric Benefit of Protons

Charged particles such as protons have intrinsic physical properties that result in superior dosimetric distribution compared to photons. Specifically, proton beams are able to deposit nearly all their radiation dose in the target with maximal sparing of adjacent normal tissue. This is due to the Bragg peak effect of the proton beam in which there is a low entrance dose followed by a uniform maximum dose in a modulated narrow zone (Bragg peak) and then a sharp decrease to zero dose immediately distal to the peak (Fig. 20‑1 ). With this low entrance dose and nonexistent exit dose, nontargeted tissues both upstream and downstream to the tumor are preferentially spared.

Several dosimetric studies have demonstrated an advantage of protons over photons for intracranial tumors.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur For example, one study compared five treatment planning techniques (3D conformal RT with photons, stereotactic arc therapy with photons, intensity modulated RT with photons, spot scanning with protons, and passive scattering with protons) for 12 intracranial tumors of various histologies.s. Literatur The analysis showed that while all techniques resulted in acceptable target coverage, proton techniques were always superior to photon approaches in terms of normal tissue sparing. There was an increasing comparative advantage for larger tumor volumes. For VS, this benefit would be expected to translate clinically into a decreased risk of cranial neuropathy. Another theoretical benefit from reduced normal tissue exposure is a decreased risk of secondary tumors with proton RT, which has also been shown in dosimetric modeling studies.s. Literatur Lastly, because tumor coverage in proton plans is accomplished via superimposition of the Bragg peaks from multiple proton beams, the dose distribution in the tumor is more homogeneous with protons than with photons. A comparison of proton versus photon planning for a typical VS case demonstrates the difference in dosimetric distribution (Fig. 20‑2 ).

20.3 Proton Stereotactic Radiosurgery

The primary nonoperative treatment for VS is SRS, which involves the delivery of a high single dose of RT to the tumor. Results with use of proton RT were first reported in 2002 in a prospective series of 68 patients from Massachusetts General Hospitals. Literatur and then updated in 2003 with a total of 88 patients.s. Literatur This endeavor was motivated by photon SRS studies that resulted in cranial nerve V and VII injury rates of greater than 30% despite highly conformal treatment planning and delivery techniques.s. Literatur ^,​ s. Literatur ^,​ s. Literatur The marginal doses used in those early series were 16 to 20 Gy in a single session. To determine a proton RT dosing strategy that minimized the risk of cranial neuropathy and brainstem injury, the single fraction equivalent of the standard brainstem tolerance limit of 54 Gy at 1.8 Gy daily fractions was determined using an alpha/beta ratio of 2 for the brainstem. This calculation resulted in a single fraction dose of 12 Gy (relative biological effectiveness [RBE]: 1.1), which was chosen as the dose to the tumor margin. A total of 88 patients were treated between 1992 and 2000 and were followed with neurologic evaluation and serial brain imaging (magnetic resonance imaging [MRI], or contrast-enhanced computed tomographic [CT] scans when MRI was contraindicated) at 6 months and 1 year after SRS, then annually over the next 2 years, and then once every 2 years thereafter. The median follow-up was 38.7 months. Tumor control was excellent with actuarial 2- and 5-year tumor control rates of 95.3 and 93.6%, respectively. Five-year cranial nerve V and VII function preservation rates were high at 89.4 and 91.1%, respectively. The authors did find an association between increasing prescribed dose, maximal dose, and inhomogeneity coefficient with long-term cranial nerve VII neuropathy on univariate analysis. In this cohort, 21 patients (24%) had functional hearing at baseline (Gardner–Robertson grades I and II) and only 7 (33.3%) retained serviceable hearing at last follow-up. The authors therefore proposed administering conventionally fractionated RT in patients with baseline serviceable hearing.