Keywords

Stereotactic radiosurgery and conventionally fractionated radiotherapy have well-established track records with high local control rates and robust long-term follow-up data; recent studies utilizing FSRT in patients with large tumors or those abutting critical OARs have emerged. This chapter summarizes hypofractionated radiotherapy techniques, including SRS and FSRT, for vestibular schwannomas.

1 Pearls

  • Incidence is estimated at 0.6–0.8 per 100,000 person-years and is increasing over time.

  • Increased incidence is due (at least in part) to incidental diagnosis in asymptomatic patients in the setting of widespread MRI and CT imaging—as vestibular schwannomas are identified on 0.2% of MRIs in asymptomatic patients.

  • Comprise 8% of adult intracranial tumors, 80–90% arise within the cerebellopontine angle, with more than 90% being sporadic and unilateral.

  • The median age of diagnosis is 50 years; rare in children with the exception of patients with NF2.

  • Both sporadic and NF2-associated vestibular schwannomas are routinely associated with biallelic inactivating mutations of the tumor-suppressor gene NF2 (located on 22q12).

    • Bilateral vestibular schwannomas are pathognomonic for NF2 and patients with NF2 commonly manifest symptoms by 20–30 years of age.

  • For sporadic lesions, the estimated average growth rate is 1–2 mm per year, while for NF2-associated lesions it is 3 mm per year.

  • The cystic schwannoma subtype displays a more aggressive growth pattern, but malignant transformation is rare.

  • When tumors are symptomatic, the most common symptoms include hearing loss (95% objective, 66% subjective; usually gradual in tempo—but a subset present with sudden hearing loss), tinnitus (63%), imbalance or vertigo (61%, generally mild-to-moderate unsteadiness with ambulation, tilting, or veering, with true spinning vertigo unusual), facial paresthesias or pain (17%, typical onset more than 2 years since presence of hearing loss), facial paresis or taste disturbance (6%), and less commonly cerebellar symptoms or lower cranial nerve deficits.

  • Local anatomy:

    • The cerebellopontine angle is bounded by the temporal bone laterally, the brainstem medially, the cerebellum superiorly and posteriorly, and the inferior cranial nerves inferiorly (CN IX-XI).

      • Additional structures within the cerebellopontine angle include CN VII and the anterior inferior cerebellar artery.

    • The vestibular and cochlear nerve roots arise from the vestibular and cochlear apparatus, respectively, which together form the vestibulocochlear nerve, which travels through the internal auditory canal to the cerebellopontine angle.

    • The majorities of vestibular schwannomas arise within the internal auditory canal from the superior or inferior branches of the vestibular nerve, and rarely arise from the cochlear nerve.

    • The natural history is characterized by progressive growth within the internal auditory canal, extending to the cerebellopontine angle with associated compression of nearby cranial nerves—most notably the facial and trigeminal nerves—as well as the brainstem.

  • Medical workup:

    • History and physical, including assessment of performance status, with emphasis on preceding neurological symptoms (e.g., hearing loss, tinnitus, imbalance, facial paresthesias, or weakness) and thorough neurologic examination including detailed examination of cranial nerves, balance, and ambulation.

      • Weber and Rinne testing may suggest asymmetric sensorineural hearing loss.

      • Romberg and Hall-Pike maneuvers are typically normal.

    • Audiometry: Initial screening test of choice, as 95% of patients will have an abnormal test, most commonly revealing asymmetric sensorineural hearing loss, preferentially at higher frequencies with impaired speech discrimination scores out of proportion to the degree of hearing loss.

    • Vestibular testing: Not commonly performed as a screening modality given decreased sensitivity, but may show decreased or absent caloric response on the involved side.

    • Brainstem-evoked response audiometry is less commonly performed.

  • Imaging workup:

    • CT: Appear as a well-defined isodense, contrast-enhancing mass within the internal auditory canal with variable extension into the cerebellopontine angle, and rarely harbor calcifications (as opposed to meningiomas).

    • MRI: Gold standard imaging modality; typically appear iso- or hypointense to the pons on T1-weighted images, heterogeneously hyperintense on T2-weighted images, and strongly and homogenously contrast enhancing.

      • Purely intracanalicular vestibular schwannomas are usually round or oval in shape, while those extending into the cistern have a spherical extra-internal auditory canal component with a taillike taper into the internal auditory canal.

      • Post-contrast T1-weighted images with thin (1 mm) sectioning through the internal auditory canal are ideal. High-resolution constructive interference in steady state (CISS) or 3D fast imaging employing steady-state acquisition (FIESTA) sequences can show enhanced visualization of structures surrounded by CSF, thereby assisting in delineation of the tumor and cranial nerves.

      • High-resolution CT with and without contrast can be used as an alternative in patients who cannot tolerate MRI.

  • Management options include surveillance, surgical resection, SRS, FSRT, or conventionally fractionated radiotherapy.

  • Goals of therapy are to maximize local tumor and preservation of function (i.e., minimizing hearing loss and other cranial nerve deficits such as facial or trigeminal nerve dysfunction).

  • Surgical resection is performed via a suboccipital (retrosigmoid), middle fossa, or translabyrinthine approach. Hearing preservation rates for suboccipital and middle fossa approaches range from 20 to 71% with smaller tumor size and extent of preoperative hearing level of variable prediction for hearing preservation; the general indications and limitations for each are as follows [1,2,3]:

    • Suboccipital (retrosigmoid):

      • Indications: Any tumor size, can attempt hearing preservation, lower risk of facial nerve injury.

      • Limitations: Increased incidence of headache and CSF leak, incomplete visualization of the internal auditory canal fundus.

    • Middle fossa:

      • Indications: Small tumors ≤1.5–2 cm and hearing preservation can be attempted (highest rates of hearing preservation among surgical approaches).

      • Limitations: Increased risk of facial nerve damage, incomplete visualization of the internal auditory canal fundus.

    • Translabyrinthine:

      • Indications: Non-serviceable hearing in affected ear, any tumor size, and complete visualization of the internal auditory canal.

      • Limitations: Hearing is inevitably sacrificed.

2 Staging, Grading, and Other Classifications

Vestibular schwannomas are divided into four grades based on size and location according to the Koos grading system (Table 6.1, [4]).

Fig. 6.1
figure 1

A 1.9 cc right vestibular schwannoma; GTV target delineation in red. (a) Simulation CT (left), post-contrast T1-weighted MRI (right). (b) Treatment plans with prescription IDL in green, effective normalization 97%. HSRT, 5 Gy × 5 fx (25 Gy total) with 6 MV photon using VMAT. Simulation CT axial (left) and coronal (right). IDL isodose line

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Table 6.1 Koos Grading System for Vestibular Schwannomas
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3 Patient Selection for SRS or FSRT

  • Factors influencing treatment recommendations include patient age, medical comorbidities, cranial nerve deficits, tumor size and/or growth rate, presenting symptoms, competing symptoms (i.e., contralateral hearing loss), and proximity to critical organs at risk (OAR, such as brainstem, cochlea).

  • For single-fraction SRS, targets should generally be <3 cm.

  • For FSRT, tumors may be larger (>3–4 cm), in closer proximity to or involving OARs.

  • Patients with non-serviceable hearing (typically <50% speech discrimination at >50 dB) may not benefit from therapeutic approaches to preserve hearing [5].

Table 6.2 Treatment planning considerations
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4 Treatment Planning Considerations (Table 6.2)

5 Commonly Used Dose/Fractionation Schemes

Commonly utilized dose/fractionation schemes for SRS and FSRT are described in Table 6.3.

Table 6.3 Commonly utilized dose/fractionation schemes for SRS and FSRT
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6 Normal Tissue Tolerances

Improved serviceable hearing preservation has been reported in patients with vestibular schwannomas treated with GammaKnife SRS who received a central cochlea dose <4.2 Gy [10] (Table 6.4).

Table 6.4 Normal tissue tolerances
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7 Patient Management Considerations

  • Premedication/prophylactic medication:

    • There is no standard premedication regimen.

    • Consideration of steroids is dependent on severity and tempo of symptoms or neurologic deficit(s), treatment volume, number of fractions, as well as patient age and/or medical comorbidities.

  • Acute toxicity: Treatment is generally well tolerated, transient dizziness reported in ~17% [13].

  • Late toxicity: Hearing loss (29–68%), CN V/VII neuropathy (<5%), dizziness (2%) [13, 14].

8 Follow-Up

  • H&P every 6–12 months.

  • Yearly imaging (ideally MRI, CT with contrast if non-tolerant, or MRI contraindicated) for 4–5 years, then every 2 years.

  • Audiometry and vestibular testing as needed.

9 Relevant Literature

  • The treatment of vestibular schwannomas with stereotactic radiosurgery and conventionally fractionated radiotherapy is well characterized with excellent local control rates and extensive long-term follow-up.

  • In recent years FSRT has emerged as a promising treatment technique in patients with large tumors or those in close proximity to or involving critical OARs. However, more mature data are required for adequate evaluation of long-term local control rates as well as associated toxicity profiles for FSRT compared to SRS or conventionally fractionated radiotherapy (Table 6.5).

Table 6.5 Relevant literature
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