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Stereotactic Radiosurgery for Vestibular Schwannomas

Vestibular schwannomas, also referred to as acoustic neuromas, are relatively rare tumors, representing < 10% of primary brain tumors. The incidence of vestibular schwannomas is estimated to be 10 per 1,000,000 people per year, representing ~2000 to 2500 new cases diagnosed each year in the United States. With the advent and widespread availability of magnetic resonance imaging (MRI), vestibular schwannomas are diagnosed more frequently and at earlier stages of presentation. The patients are generally 40 to 59 years old, with a slight female preponderance. Histologically, vestibular schwannomas result from benign proliferations of Schwann cells arising from the myelin sheath, mostly of one of the vestibular branches of the eighth cranial nerve (CN VIII). These tumors develop either in the internal auditory canal or in the cerebellopontine angle. Although their histological characteristics are benign and their behavior is noninvasive, vestibular schwannomas create a problem because of their proximity to the brainstem and multiple cranial nerves. Mass effect exerted on these structures results in the common presenting symptoms of hearing loss (95–98% of cases), tinnitus (63–70%), disequilibrium (61–67%), facial numbness (9–29%), facial weakness (5–10%), otalgia (9%), and altered taste sensation (2–6%). Deafness leads to the diagnosis of vestibular schwannoma in 95% of cases, and the most frequently obtained audiologic curve shows a major hearing loss in the 1000 to 4000 Hz frequency, sometimes rising to 8000 Hz. Several hypotheses have been proposed to explain the loss of hearing: (1) direct compression of nerve fibers by the tumor, creating a conduction block, followed by degeneration of nerve fibers; (2) compression or thrombosis of the internal auditory artery, causing deafness of the endocochlear type, sometimes of sudden origin; (3) biochemical modifications of endo- and perilymphatic fluids; and (4) destruction of the cochlea by the tumor (rare and mostly seen in cases of neurofibromatosis).¹

Evolution of Radiosurgery Technique

In 1971 Dr. Lars Leksell published the indications for and technique of vestibular schwannoma radiosurgery, as first performed in a patient in 1969.² In the application of his initial concept of radiosurgery described in 1951,³ Leksell used the first generation of the Gamma Knife (Elekta Instruments AB, Stockholm, Sweden) unit with 179 cobalt 60 radiation beams to target the tumor using air or contrast encephalography. A comprehensive evaluation of the initial Swedish series of 14 patients was reported by Noren et al in 1983.⁴ The modern era of vestibular schwannoma radiosurgery was ushered in at the University of Pittsburgh under Dr. L. D. Lunsford using a 201-source Gamma Knife unit and high-resolution imaging techniques. In 1988, Kamerer and Lunsford published their early experience of nine patients treated with this technique.⁵

In the early 1990s, radiosurgical techniques evolved considerably. These changes included improvements in four different fields:

1. Stereotactic imaging improved significantly. Tumor imaging with computed tomography (CT), and later MRI, replaced pneumencephalography and angiography, which allowed precise visualization of the whole tumor by today’s standards. The intracanalicular portion of the vestibular schwannoma was seen and could be targeted accurately.
   
2. Dosimetric planning progressed considerably with the use of computers. Because treatment-planning computer programs were faster and fully integrated with imaging, highly conformal multiisocenter treatment plans could be performed. Later years saw the introduction of inverse treatment planning in the treatment-planning computer program, wherein the computer itself was programmed to create a dosimetric planning based on three-dimensional tracing of the tumor volume.
   
3. Improvement in the mechanical precision and automatization of beam delivery. The new generations of radiosurgical devices included robotic systems for patient positioning during radiation treatment.⁶ These robotic systems increased the accuracy of beam delivery according to the planned dose distribution.⁷ Moreover, automatization of the changes in patient positioning has facilitated the use of highly conformational planning using a large number of small-diameter isocenters.
   
4. Reduction of the prescription doses for vestibular schwannoma radiosurgery. Initially, minimum tumor doses of 16 to 20 Gy were prescribed, according to tumor volume. To reduce the incidence of facial paresis and increase preservation of hearing, prescription doses were lowered slowly. The most commonly used prescription dose today is in the range of 12 to 13 Gy, with a lower morbidity and no compromise in tumor control so far.^8,9

Management Options for Vestibular Schwannomas

Patients with vestibular schwannomas have several treatment options, including observation, microsurgical resection, stereotactic radiosurgery, and fractionated radiation therapy. The decision can be difficult for some patients and easier for others; each therapeutic procedure has its own indications, limitations, results, and morbidity. Individual treatment decisions should take into account the patient’s age, past medical history, tumor size, hearing status, and extent of symptoms. We believe that observation can be a good option only for very old patients or for patients with asymptomatic and small-sized tumors. Hence, a significant tumor growth is found in the majority of patients with vestibular schwannomas who choose observation.¹⁰ Microsurgical resection is mainly indicated for patients with large tumors (Koos grade IV) that have caused neurologic deficits from brainstem compression or when pathologic examination is required because tumor imaging is not characteristic of vestibular schwannoma. The consequence of surgery is often total loss of unilateral hearing and partial or total loss of unilateral facial nerve function. Within this context, stereotactic radiosurgery provides an attractive alternative to surgical resection. Radiosurgery is used for small or medium-sized vestibular schwannomas with the goals of preserved neurologic function and prevention of tumor growth. The long-term outcome of Gamma Knife radiosurgery has proven its role in the primary or adjuvant management (e.g., after subtotal removal or recurrence) of this tumor.^8,9 Fractionated radiotherapy has been suggested as an alternative treatment option for selected patients with larger tumors for whom microsurgery may not be feasible or for some patients in an attempt to preserve cranial nerve function.¹¹

The Brussels Gamma Knife Center Experience

The Brussels Gamma Knife Center was inaugurated in January 2000. Between January 2000 and December 2007, 300 patients with a vestibular schwannoma were treated by Gamma Knife radiosurgery in our center located on the campus of the University Hospital Erasme, Brussels, Belgium (Table 12.1). The median patient age was 54 years (standard deviation [SD] 15 years; range 11–88 years). There were 147 women and 153 men. The tumor was located on the left side in 162 patients and on the right side in 138 patients. For 267 patients (89%), radiosurgery was the first treatment of their vestibular schwannoma; 33 patients (11%) were treated by Gamma Knife for a residual or recurrent tumor after microsurgery. Hearing status before radiosurgery was assessed the day before treatment for all patients, using the Gardner-Robertson classification:¹² 168 patients (56%) had serviceable hearing (100 patients in class 1 and 68 patients in class 2), and 132 patients (44%) had nonserviceable hearing (60 patients in class 3, 4 patients in class 4, and 68 patients in class 5).

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