Stereotactic Radiosurgery for Acoustic Neuromas Using the CyberKnife

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Stereotactic Radiosurgery for Acoustic Neuromas Using the CyberKnife

Gordon T. Sakamoto John Sinclair Iris C. Gibbs John R. Adler Jr Steven D. Chang Abstract

Initially, intracranial stereotactic radiosurgery required the use of rigid immobilization using a stereotactic frame to achieve treatment precision and accuracy. However, the introduction of image-guided radiosurgery with the CyberKnife® in 994 removed the need for this rigid patient immobilization. Although the CyberKnife does not require the use of a stereotactic frame, its accuracy is comparable to current frame-based stereotactic radiosurgical treatment methods. In addition, it is also possible to fractionate or stage CyberKnife treatments over several days with the goal of decreasing the risk of radiation injury to the adjacent brainstem and cochlea. This chapter outlines the rationale and treatment protocols developed at Stanford University for the treatment of acoustic neuromas using the CyberKnife.

Acoustic neuromas are misnamed in that they are actually schwannomas of the vestibular nerve. The vast majority of these tumors are usually benign and slow-growing. Based on that fact, conservative management of these tumors can be a viable alternative to treatment. For patients who choose treatment, options include partial resection, total resection and/or stereotactic radiosurgery. There remains some debate as to how to best manage these tumors. Historically, these tumors were treated with surgical resection. However, loss of hearing and facial nerve injury were not uncommon complications. Additionally, over the last few decades, stereotactic radiosurgery has emerged as a safe and efficacious treatment modality.2–0 In fact, stereotactic radiosurgery can also be used as an adjuvant treatment for a residual tumor or a recurrence of a previously resected acoustic neuroma.

Introduction Acoustic neuromas were first described in 777 by Sandifoot. More detailed accounts were published

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in 830 by Bell and in 835 by Cruveilieber. The first attempted surgical resection occurred in 89 by Charles McBurney. Thomas Annandale performed the first successful surgical resection of an acoustic neuroma in 895. Harvey Cushing ushered in a new era of surgical technique and greatly lowered the mortality rate associated with acoustic neuroma surgery. Walter Dandy further refined Cushing’s techniques, but the mortality rate associated with surgical resection in the 950s was still approximately 20%. Despite this fact, surgical resection was the only treatment option for many years. Refinement of surgical and anesthetic techniques over the last 20 years has greatly improved the mortality and morbidity rates associated with acoustic neuroma surgery, but modern surgical series still report high levels of post-operative facial nerve weakness and hearing impairment. Ryzenman et al recently reported a 45% rate of post-operative facial weakness in ,595 patients following acoustic neuroma resection. Significantly, 72% of the patients experiencing post-operative dysfunction in this series demonstrated permanent facial nerve weakness during the clinical follow-up period.2 Sampath et al also reported a high rate of post-operative facial weakness with 39% of patients experiencing facial weakness following resection of tumors less than 2.5 cm in diameter.3 The rates of hearing preservation following surgical resection have also been less than optimal and highly dependent on pre-operative tumor volume. A 6% rate of hearing preservation was reported by Hecht et al following resection of tumors larger than .5 cm in diameter. Significantly, a 50% rate of hearing preservation was reported in patients following resection of tumors less than .5 cm in diameter.4 Lars Leksell is credited with treating the first acoustic neuroma with radiosurgery in 969 at the

Karolinska Institute in Stockholm, Sweden. Leksell later published his initial experience with acoustic neuroma radiosurgery using the Gamma Knife in 97.5 Since then, radiosurgery has proven to be a safe and effective treatment option for the management of acoustic neuromas. Numerous radiosurgical series exist in the literature detailing excellent tumor control rates ranging from 92% to 00% in the first several years after treatment. 3,6–8 Additionally, studies have confirmed excellent long-term control rates at five and 0 years after treatment.6 Over the last decade, interest has been directed to improving the hearing preservation rates following radiosurgery and reducing other treatment-related morbidities. Earlier clinical studies reported hearing preservation rates that ranged from 5% to 60%.6-8 However, improvements in conformal radiation treatment delivery and utilization of lower marginal prescription doses have steadily improved hearing preservation rates to the current rate of between 7% and 73%.3, 9,0 More recently, attention has been directed to staging radiosurgical treatment with the goal of eliminating or reducing the risk of injury to adjacent critical structures such as the brainstem, cranial nerves, and cochlea.6-2 However, the experience with dose staging using an interfraction interval of 24 hours has been severely limited, as most modern radiosurgical centers still utilize frame-based techniques that require invasive rigid patient immobilization.6 The introduction of the CyberKnife removed the need for rigid patient immobilization. Consequently, it became possible to deliver highly conformal radiation to acoustic neuromas and stage the dose delivery as required. The treatment of the first acoustic neuroma with the CyberKnife at Stanford University occurred in 999. Since then, over 300 acoustic neuromas have

C H A P T E R 1 3 : Stereotactic Radiosurgery for Acoustic Neuromas Using the CyberKnife

been treated at Stanford using the CyberKnife via a standard three-day staging protocol.

Patient Selection CyberKnife radiosurgery is offered to patients with unilateral or bilateral acoustic neuromas that are generally