Description of the condition

Vestibular schwannomas, also known as acoustic neuromas, are benign tumours of Schwann cell origin that occur on the eighth cranial nerve. They represent 5% to 10% of all intracranial tumours (House 1974), and 80% to 90% of tumours of the cerebellopontine angle (Hart 1981).

The incidence of vestibular schwannoma is 2 per 100,000 person‐years (Stangerup 2012), and the prevalence is estimated at 2 in 10,000 people (Lin 2005). Several centres have reported an increasing incidence of diagnosed vestibular schwannoma over recent years (Stangerup 2012). These tumours occur with about equal frequency in adult males and females, most frequently in patients between the ages of 30 and 60 years (Stangerup 2012). A minority of patients have central neurofibromatosis, with bilateral tumours.

Commonly presenting symptoms include hearing loss, tinnitus and balance disturbance. Tumour progression can lead to brainstem compression, cranial neuropathies and hydrocephalus (Hansasuta 2011). The diagnosis is established with magnetic resonance imaging (MRI), which can demonstrate tumours as small as a few millimetres. As imaging technology has improved, more small tumours have been diagnosed.

Management options include observation, microsurgical resection and stereotactic radiotherapy (Hansasuta 2011; Varughese 2012).

Observation with neuroradiological follow‐up is a management option for vestibular schwannoma, since many of these tumours grow slowly over years, or may not grow at all (Stangerup 2012).

The surgical resection of vestibular schwannoma has been a central feature of neurosurgical practice for more than 100 years (Ramsden 1995), from the pioneering attempts to achieve simple decompression, to the contemporary era, where the function of the eighth and adjacent cranial nerves may be preserved (Sampath 1998). However, while total tumour removal is possible with microsurgical resection in about 95% of patients, normal or near‐normal facial nerve preservation is only possible in about 80% on average (Samii 1997), and hearing may be preserved in about 35% to 65% of patients (Battaglia 2006), although these numbers depend highly on tumour size, tumour localisation and the experience of the surgical team.

Stereotactic radiotherapy is a non‐invasive technique that delivers high‐dose irradiation to small, targeted tissue volumes. The use for vestibular schwannoma was first described by Leksell (Leksell 1971), and it represents an alternative to microsurgical resection in patients with small and moderate size tumours. It has been said that tumours up to 3 cm in diameter, when including the internal auditory canal in the measurement, can be successfully controlled in the majority of patients with this technique (Lederman 1997), although most studies reporting on the control of tumour growth include patients without documented tumour growth before the initiation of treatment.

There is no international consensus regarding the optimal treatment for vestibular schwannoma.

Description of the intervention

Brain radiotherapy is a procedure in which beams of radiation are converged and targeted at a volume of tissue within the brain. Stereotaxis is an accurate targeting technique for intracranial structures that uses an external reference frame fixed to the head. Stereotactic radiotherapy therefore refers to the non‐invasive destruction, by focused irradiation, of a particular intracranial target localised stereotactically.

In 1968, the first 'Leksell gamma unit', or 'gamma knife', was designed at the Karolinska Institute in Stockholm, Sweden. The unit evolved into the commercially available Leksell gamma unit, containing cobalt‐60 sources located in a hemispherical array around a common focal point. Stereotactic radiotherapy has become an important treatment option for many patients with intracranial and spinal disorders (Sheehan 2009), ranging from arterial venous malformations to benign and malignant neoplasms (Yu 1997). Targets up to 3 cm in diameter are usually considered suitable for stereotactic radiotherapy.

A typical stereotactic radiotherapeutic procedure begins with the application of the stereotactic head frame under local anaesthesia. A magnetic resonance imaging (MRI) scan is then performed and a radiotherapeutic protocol designed to deliver the radiation dose to the defined target volume. The procedure is usually performed as an outpatient procedure and is well tolerated by the patient, with few patients developing major acute effects (Chao 2012).

How the intervention might work

Stereotactic radiotherapy works by inducing radiation necrosis in the targeted tissue volume. The desired response is long‐term growth control of the tumour (Niranjan 2004).

An observational study by Breivik and colleagues followed 237 patients with vestibular schwannoma for almost five years. One‐hundred and thirteen patients received radiotherapy and 124 were managed conservatively. There was a significant reduction in tumour volume over time in the radiotherapy group and the need for additional treatment was reduced, compared to the observed patients, suggesting that vestibular schwannomas can be controlled by radiotherapy (Breivik 2013). As the technique has developed, cranial nerve complications following stereotactic radiotherapy have been significantly reduced by modifications in dose schedules (Noren 1993).

In comparison, a meta‐analysis of observation as a management option included 26 studies with 1340 patients in total. Growth of the tumour was observed in 46% of patients, with a mean growth rate of 1.2 mm/year. Subsequent active treatment was required by only 18% of all patients (Yoshimoto 2005).

Why it is important to do this review

We are unaware of any guidelines on the effectiveness and safety of stereotactic radiotherapy in comparison to observation or microsurgical resection as a management strategy. The best candidates for stereotactic radiotherapy are usually also ideal candidates for microsurgical resection (patients with easily resectable small to moderate size tumours). This dilemma is evident in everyday clinical practice, in centres which can offer stereotactic radiotherapy as an alternative treatment. Furthermore, it is still unclear whether some of these patients can be managed by observation alone. The limited availability of radiotherapeutic equipment makes decision‐making even more complex.

This review is important to facilitate decision‐making by clinicians and patients.