Vestibular Schwannomas: The Role of Stereotactic Radiosurgery

Chapter 103 Vestibular Schwannomas


The Role of Stereotactic Radiosurgery



Acoustic neuromas (vestibular schwannomas) are generally slow-growing, intracranial extra-axial benign tumors that usually develop from the vestibular portion of the eighth nerve.1 Bilateral vestibular schwannomas are usually associated with neurofibromatosis 2 (NF2). Both unilateral and bilateral vestibular schwannomas may form due to malfunction of a gene on chromosome 22, which produces a protein (schwannomine/merlin) that controls the growth of Schwann cells. In NF2 patients, the faulty gene on chromosome 22 is inherited and is present in all or most somatic cells. However, in individuals with unilateral vestibular schwannoma, for unknown reasons this gene loses its ability to function properly and is present only in the schwannoma cells.2


A progressive unilateral hearing decline is the most common symptom that leads to the diagnosis of a vestibular schwannoma.3 In years past it was rare for patients to present with intact hearing, but this is becoming more common due to earlier diagnosis and higher-quality magnetic resonance imaging (MRI). Overall, three separate growth patterns can be distinguished1: no or very slow growth,2 slow growth (i.e., 2 mm/year linear growth on imaging studies),3 and fast growth (i.e., >8 mm/year). Although most tumors grow slowly, some grow quickly and can double in volume within 6 months to a year.4 Cystic vestibular schwannomas are sometimes capable of relatively rapid enlargement of their cystic component. Although rare, other tumors may hemorrhage spontaneously.5 Stereotactic radiosurgery has greatly expanded the management options since patients no longer have to choose simply between resection and observation.


We believe that early diagnosis is crucial to preventing functional decline. In addition to the three primary options of surgical removal, radiosurgery, and observation with serial imaging studies, some centers suggest conformal fractionated radiation therapy using linear accelerators or proton beam irradiation.



Observation with Serial Imaging


In some cases, usually elderly or medically infirm patients or individuals with very small tumors, it may be reasonable to “watch” the tumor for potential growth.6 Repeat MRI scans over time are used to carefully monitor the tumor for any growth.7 The object of serial observation is to obviate treatment unless signs of growth are confirmed.6,8 In our 20-year experience 70% of tumors under observation have measurable growth in 5 years and almost all by 10 years. Recent published series continue to note annual tumor growth rates in the 1 to 3 mm/year range, with some evidence that extracanalicular tumors grow at a faster rate. This could be due to an easier appreciation of volumetric growth with a larger lesion.



Considering Radiosurgery Versus Surgical Resection


Resection is indicated for larger tumors with disabling brainstem compression, hydrocephalus, intractable headache, or trigeminal neuralgia. The majority of patients have smaller tumors and do not have these problems. The three main surgical avenues include the retrosigmoid, translabyrinthine, and middle fossa approaches.9,10 As discussed elsewhere in this text, several factors help in the decision on which approach is optimal. Although the outcomes of surgical removal at centers of excellence have improved markedly over the last two decades, patients increasingly seek lesser invasive options. It is important to understand the differences between approaches.


First, preservation of facial function varies according to tumor size and the surgeon’s experience.11 When tumors are smaller than 1.5 cm, good facial nerve function can be expected (House-Brackmann grades I–II) in more than 90% of patients who have surgery at centers of excellence. Only 3.2% to 6.7% of patients with smaller tumors have poor facial nerve outcomes (House-Brackmann grades III–V). In addition to tumor size, intraoperative electrophysiologic facial nerve monitoring assists the surgeon to save the nerve.12 The overall facial nerve anatomic preservation rate is 80%.13 However, facial nerve function (grades I and II) can be preserved in only 40% to 50% of patients with large (>4-cm diameter) tumors.14 Injuries of the nervus intermedius are underestimated because this nerve is rarely assessed preoperatively.15


Second, hearing preservation rates have also improved. Depending on the criteria used for reporting successful hearing conservation, preservation has been reported in 30% to 80% of patients considered eligible for hearing preservation surgery.16 A meta-analysis performed by Gardner and Robertson in 1988 revealed an overall average success rate of about 33%.17 Delayed hearing deterioration may occur days to years after surgery in 30% to 50% of patients who originally had successful hearing preservation.1821 In various studies, serviceable hearing preservation rates vary from 8% to 57%18,22,23 using the retrosigmoid approach and from 32% to 68%23 using the middle fossa approach.


Tinnitus is a frequent symptom at presentation. It is reported to worsen in 6% to 20% of individuals after tumor removal. In the majority of individuals, tinnitus remains unchanged. Approximately 25% to 60% of patients experience a decrease in tinnitus. Of patients without preoperative tinnitus, 30% to 50% developed it in the immediate postoperative period. Tinnitus appears to mimic phantom limb pain in the sense that it may remain even in the absence of preserved hearing.


Cerebrospinal fluid leakage through either the surgical incision or the Eustachian tube and middle ear occurs in 2% to 20% of patients.20,21,2426 Although in individual published reports the cerebrospinal fluid leak rate appears higher with the retrosigmoid approach (2.9%–18%),23 a recent meta analysis suggests similar rates of cerebrospinal fluid leak for all surgical approaches (10.6% of 2273 retrosigmoid surgeries; 9.5% of 3118 translabyrinthine surgeries; and 10.6% of 573 middle fossa surgeries). The adjunctive use of endoscopy may assist the surgeon to avoid or to detect a CSF leakage.27 Other rarer perioperative complications include death (0%–3%),28,29 intracranial hematomas (1%–2%), wound hematoma (3%), cerebellar and brainstem edema, hemiparesis, meningitis (1.2%), wound infections (1.2%), abducens nerve paresis (1%–2%), and other lower cranial nerve injuries.25,29


Overall, tumor recurrence rates of 5% to 10% are found in the published literature, although a few studies report no long term recurrence after translabyrinthine approach.30 However, incomplete resection of vestibular schwannomas is associated with a significant risk of tumor progression requiring subsequent intervention.31



The Patients’ Perspective on Resection


A variety of complications have been reported after vestibular schwannoma surgery.3238 Studies from the Acoustic Neuroma Association are a good resource.39 Bateman et al., described patients’ subjective condition after vestibular schwannoma surgery as impairment (141, 51%), disability (95, 34%) or handicap (43, 15%).40 Most of the impairments were related to problems with facial nerve function. The other most common issues were “balance problems” (19/141, 13%) followed by “hearing loss” (17/141, 12%) and “difficulty with background noise” (14/141, 10%). Tinnitus accounted for 5 of 141 responses (4%). Disabilities resulting from facial nerve dysfunction accounted for most of the disabilities reported by patients. A significant number of disabilities were associated with balance problems (e.g., “unable to drive,” “problems changing direction,” “unable to swim, cycle, run, climb steps, do aerobics,” and “problems bending down”) and with hearing loss (e.g., “difficulty locating the source of sounds,” “difficulty following conversations in a crowd,” “unable to hear people to one side” and “unable to hear doorbell/telephone”). Some patients reported symptoms of social isolation after the surgery. Fifteen out of forty-three responses (35%) were “reluctance to attend large social gatherings.” Employment-related problems were also important with 7 of 43 responses (16%).



Stereotactic Radiosurgery


Vestibular schwannoma stereotactic radiosurgery using the gamma knife was first performed by Leksell in 1969.41 During the past two decades radiosurgery has emerged as an effective alternative to surgical removal of small- to moderate-sized vestibular schwannomas (Figs. 103-1 and 103-2). Long-term results have established radiosurgery as an important minimally invasive alternative to resection. Advanced multi-isocenter dose-planning software, high-resolution MRI for targeting, dose optimization, and robotic delivery reflect the evolution of this technology. Other image-guided linear accelerator (LINAC) devices (Trilogy, Synergy S, Novalis and CyberKnife) generally can be used to fractionate radiation delivery in 5 to 30 sessions. Proton beam technology is also used to deliver fractionated radiation therapy. The goals of vestibular schwannoma radiosurgery are to prevent further tumor growth, preserve neurologic function where possible, avoid the risks associated with open resection, and in selected patients to improve pre-existing symptoms.





Radiosurgery Technique for Vestibular Schwannomas


Patients with vestibular schwannomas are evaluated with high-resolution MRI (CT may be substituted in patients who cannot undergo MRI scans) and audiologic tests that include pure tone average (PTA) and speech discrimination score (SDS) measurements. Hearing is graded using the Gardner-Robertson modification of the Silverstein and Norell classification and/or the American Academy of Otolaryngology-Head and Neck Surgery guidelines, and facial nerve function is assessed according to the House-Brackmann grading system. “Serviceable” hearing (classes I and II) is defined as a PTA or speech reception threshold lower than 50 dB and speech discrimination score better than 50%. The Committee on Hearing and Equilibrium of the American Academy of Otolaryngology-Head and Neck Surgery has established guidelines for reporting vestibular schwannoma results. In this classification, hearing loss at a higher frequency (3000 Hz) is also included in calculating the PTA. “Serviceable” hearing (classes A and B) is similar to class I and II of Gardner-Robertson hearing classes. Every patient is counseled about the options and risks and benefit of microsurgical and radiosurgical management strategies. Modern reports on vestibular schwannoma outcomes should include these data.


Radiosurgery can be performed using the gamma knife, modified LINACs, or the proton beam. Techniques of head frame fixation, stereotactic imaging, dose planning, and dose delivery are different for these three modalities. In gamma knife radiosurgery the procedure begins with rigid fixation of an MRI-compatible Leksell stereotactic frame (Model G, Elekta Instruments, Atlanta, GA) to the patient’s head. Local anesthetic scalp infiltration (5% Marcaine and 1% Xylocaine) is used, supplemented by mild intravenous sedation as needed. High-resolution images are acquired with a fiducial system attached to the stereotactic frame. For vestibular schwannoma radiosurgery, a three-dimensional (3-D) volume acquisition MRI using a gradient pulse sequence (divided into 1- or 1.5-mm thick, 28–36 axial slices) is performed in order to cover the entire lesion and surrounding critical structures. A T2-weighted, 3-D volume sequence is performed to visualize cranial nerves and delineate inner ear structures (the cochlea and semicircular canals) (Figs. 103-3 and 103-4). Planning is performed on narrow-slice thickness, axial MR images with coronal and sagittal reconstructions. Centers using LINAC or proton-beam systems may use mask immobilization of the patient’s head along with image guidance and typically deliver the radiation dose in five or more fractions over many days. CT is used for planning at most LINAC sites but may be fused to MRI scans.





Radiosurgical Dose Planning


Dose planning is a critical aspect of radiosurgery. Complete coverage of the tumor and preservation of facial, cochlear, and trigeminal nerve function is given priority during dose planning. For large tumors, preservation of brain-stem function is also a consideration. Conformality and selectivity is necessary for hearing and facial nerve preservation.42 Specific gamma knife radiosurgery techniques include accurate definition of the tumor volume, use of multiple isocenters, beam weighting, and selective use of plug patterns to reduce dose to critical structures. This degree of conformality can be achieved through complex multi-isocenter planning (see all figures). Vestibular schwannoma planning is usually performed using a combination of small-beam-diameter (4- and 8-mm) collimators. For large tumors, 14-, 16-, or 18-mm collimators are used. A series of 4-mm isocenters are used to create a tapered isodose plan to conform to the intracanalicular portion of the tumor.



Dose Selection


After optimizing the plan, a maximum dose inside target is determined as well as the dose to the tumor edge. The treatment isodose, maximum dose, and dose to the margin (edge) are jointly decided by a neurosurgeon, radiation oncologist, and medical physicist and, in some centers, a neurotologist. In gamma knife radiosurgery, a dose of 12 to 13 Gy is typically prescribed to the 50% (or other) isodose line that conforms to the tumor margin. The commonest dose is 12.5 Gy, used for hearing preservation in smaller tumors. Larger tumors may receive 12 Gy, and in those with hearing loss or prior resection, 13 Gy. Dose prescription for vestibular schwannomas changed significantly during the first 10 years experience at our center. This margin dose range is associated with a low complication rate and yet maintains a high rate of tumor control as we have found in our most recent 10-years-plus experience using these doses. We suspect that further dose reduction is unwarranted. Most centers are reluctant to prescribe lower margin doses (such as 12 Gy) for vestibular schwannomas. Similar doses are also used for patients with bilateral (NF2 related) vestibular schwannomas and for patients with contralateral deafness from other causes, for whom hearing preservation is highly desirable. After prescribing the margin dose, the mean dose to the cochlea, semicircular canals, and brain stem are assessed. A mean cochlear dose less than 4.2 Gy may be important for hearing preservation (Fig. 103-5). The majority of the tumor volume is receiving a radiobiologic dose in excess of a biologically equivalent dose delivered by fractionated image-guided radiation therapy. The maximum radiosurgical dose of 25 Gy may be radiobiologically equivalent to 100 Gy of fractionated radiation. While many radiosurgical centers have evolved toward similar dose selection parameters, the doses and regimens chosen for fractionated radiotherapy continue to vary widely.



After radiosurgery, all patients are followed up with serial contrast-enhanced, gadolinium-enhanced MRI scans, which are generally requested at 6 months, 12 months, and 2, 4, 8, 12, 16, and 20 years. All patients who have detectable hearing before radiosurgery are requested to obtain audiologic tests (PTA and SDS) near the time of their follow-up MRI.



Gamma Knife Radiosurgery: Clinical Results


Long-term results of gamma knife radiosurgery for vestibular schwannomas have been documented.4348 Recent reports suggest a tumor control rate of 93% to 100% after radiosurgery.4264 Kondziolka et al. studied 5- to 10-year outcomes in 162 vestibular schwannoma patients who had radiosurgery at the University of Pittsburgh.58 In this study a long-term 98% tumor control rate was reported. Sixty-two percent of tumors became smaller, 33% remained unchanged, and 6% became slightly larger. Some tumors initially enlarged 1 to 2 mm during the first 6 to 12 months after radiosurgery as they lost their central contrast enhancement. Such tumors generally regressed in volume compared to their preradiosurgery size. Only 2% of patients required tumor resection after radiosurgery. Norén, in his 28-year experience with vestibular schwannoma radiosurgery, reported a 95% long-term tumor control rate. Litvack et al. reported a 98% tumor control rate at a mean follow-up of 31 months after radiosurgery using a 12-Gy margin dose.65 Niranjan et al. analyzed the outcome of intracanalicular tumor radiosurgery performed at the University of Pittsburgh.66 All patients (100%) had imaging-documented tumor growth control.


Flickinger et al. performed an outcome analysis of acoustic neuroma patients treated between August 1992 and August 1997 at the University of Pittsburgh. The actuarial 5-year clinical tumor control rate (no requirement for surgical intervention) was 99.4 ± 0.6%.44,50 The long-term (10–15 years) outcome of benign tumor radiosurgery also has been evaluated. In a study which included 157 patients with vestibular schwannomas, the median follow-up for the patients still living at the time of the study (n = 136) was 10.2 years. Serial imaging studies after radiosurgery (n = 157) showed a decrease in tumor size in 114 patients (73%), no change in 40 patients (25.5%), and an increase in three patients who later had resection (1.9%).47 No patient developed a radiation-associated malignant or benign tumor (defined as a histologically confirmed and distinct neoplasm arising in the initial radiation field after at least 2 years have passed). In patients under 40 years of age, with minimum 4-year follow-up, all remained employed and active.65



Hearing Preservation


Preradiosurgery hearing can now be preserved in 60% to 90% of patients, with higher preservation rates found for smaller tumors. In a long-term (5–10-year follow-up) study conducted at the University of Pittsburgh, 51% of patients had no change in hearing ability.50,58 All patients (100%) who were treated with a margin dose of 14 Gy or less maintained a serviceable level of hearing after intracanalicular tumor radiosurgery.66 Among patients treated after 1992, the 5-year actuarial rates of hearing level preservation and speech preservation were 75.2% and 89.2%, respectively, for patients (n = 89) treated with a 13-Gy tumor margin dose. However, in a longer-term assessment at a median of 6 years, the same Gardner/Robertson level was preserved in 71%, serviceable hearing in 74%, and any testable hearing in 95%. For intracanalicular tumors, these rates were 84%, 92%, and 100%.


Our recent research has shown that the mean cochlear dose is important for hearing preservation. A dose of less than 4.2 Gy was associated with better hearing,67 a finding similar to the dose of 4 Gy noted from Marseille. Age is also important, as those under 60 years old fare better.67 Long-relaxation-time (T2) volumetric images are important to identify the cochlea for dose planning.

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Jul 16, 2016 | Posted by in NEUROSURGERY | Comments Off on Vestibular Schwannomas: The Role of Stereotactic Radiosurgery

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