Abstract
Radiosurgery is now considered as the first line management option for the majority of small to medium sized acoustic (vestibular schwannomas) patients who desire facial and auditory nerve preservation. The goals of VS radiosurgery are to control tumor growth and preserve cranial nerve function. Preservation of auditory and facial nerve function is related to various factors that include dose planning and dose delivery technique, margin dose, tumor volume, and cranial nerve function at the time of the procedure. In Gamma Knife radiosurgery (GKRS) a dose of 12–12.5 Gy is typically prescribed to the isodose line that conforms to the tumor margin. With current techniques useful hearing preservation can be preserved in majority of patients and facial nerve preservation in almost all patients. For patients with large tumors with symptomatic mass effect, partial tumor resection followed by planned radiosurgery offers the best chance of preserving facial and cochlear function in such patients. Cystic VS often respond with significant tumor regression. GKRS remains the best procedure to perform for patients with small to moderate size VS. This report describes the current radiosurgery techniques and clinical outcomes.
Keywords
stereotactic radiosurgery, Gamma Knife, vestibular schwannoma
Highlights
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Auditory nerve function can be preserved in the majority of patients regardless of tumor volume. Rates of serviceable hearing preservation range from 49% to 79% in published studies.
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With the current Gamma Knife radiosurgery technique, facial nerve function can be preserved in almost all (99%) patients.
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Vestibular schwannoma radiosurgery leads to improved quality of life.
Background
During the last three decades, stereotactic radiosurgery (SRS) has become a widely preferred surgical strategy to manage patients with small- to moderate-size vestibular schwannomas (VSs). Long-term results from multiple international studies have established the Leksell Gamma Knife ® (GK) method to perform SRS as a more effective and safer method to manage VS, compared with the older techniques that require craniotomy and attempt tumor removal. This has become especially important now that such tumors are frequently detected earlier. Patients with episodic disequilibrium, asymmetric hearing, episodes of vertigo, or tinnitus should now get an appropriate magnetic resonance imaging (MRI) with and without contrast agents with sufficient resolution of the internal auditory canal to exclude the possibility of a VS. Ironically, since the introduction of SRS, patients with early detection are now often counseled to simply wait and observe their tumor until the tumor volume has progressed significantly. Unfortunately, such treatment delay is often associated with worsened outcomes, regardless of the interventional strategy.
The goals of VS radiosurgery are to control tumor growth, preserve cranial nerve function, and rapidly return the patient to work. With current techniques, useful hearing is preserved in the majority of patients, and the facial nerve are preserved in almost all patients. Preservation of auditory and facial nerve function is related to various factors that include dose planning and dose delivery technique, margin dose, tumor volume, and cranial nerve function at the time of the procedure. It is important to understand the published outcome data as a means to refute the disinformation often provided to patients with VS who are evaluating SRS as an option: (1) It will damage your hearing and face. (2) It will fail, and your tumor will grow. (3) When it fails, your tumor will be stuck and your nerves will be cut when it requires removal. (4) Even if it works for a brief period of time, the risk of getting cancer caused by radiation of the brain is too high. This report will describe the current outcomes patients and their referring doctors may expect after VS Gamma Knife stereotactic radiosurgery (GKRS).
Anatomic Insights
The radiosurgery dose plan should be highly conformal, especially at the anterior margin of the VS, because the facial and cochlear nerve complexes are generally considered to be stretched along the anterior-superior and anterior-inferior sides of the tumor, respectively ( Fig. 27.1A and B ). Although there is no clear consensus on the sensitivity of the cochlea, it is important to minimize the amount of radiation falloff on the cochlea.
Prevention
Conformal Gamma Knife Radiosurgery Technique to Prevent Cranial Nerve Complications
Complete 3-D coverage of the tumor with sparing of facial and cochlear nerves and the cochlea is given priority during dose planning. This is achieved with the use of multiple small-volume isocenters, beam weighting, and beam plug patterns to spare the cochlea and brainstem. A series of 4-mm isocenters are used to create a tapered isodose plan to conform to the intracanalicular portion of the tumor. Current GK models (LGK Icon ® and LGK Perfexion ® ) allow for selective beam channel sector (24 beams) blocking that can be used to enhance dose falloff (selectivity) and to create sharp falloff of the energy deposited in the cochlea. Planning is typically performed by the neurosurgeon in consultation with a radiation oncologist and medical physicist. The oncologist often performs tumor contouring, and the physicist can calculate the conformality (how well the 3-D plan conforms to the tumor margin at the prescription isodose) as well as the selectivity (how rapidly the dose falls off outside of the tumor). It is thus both conformality and selectivity that set the GK planning apart from other radiation tools and that allows the surgical procedure to be performed in a single session. Technologies that fractionate radiation therapy to such tumors do so because poor conformality and selectivity require dividing dose delivery into multiple sessions to maintain safety.
In Gamma Knife ® radiosurgery (GKS), a dose of 12 to 12.5 Gy is typically prescribed to the isodose line that conforms to the tumor margin. This margin dose is associated with a low complication rate and a high rate of tumor control. After the margin dose is prescribed, the dose falloff on the cochlea and brainstem are checked to keep them below tolerance level. The dose–volume histograms of tumor, cochlea, and brainstem are evaluated to document minimum tumor dose and to check the volumes of the cochlea and brainstem. At our center we attempt to keep the cochlear dose below 4.2 Gy in patients with preserved hearing.
- 1.
Large Tumors: Large tumors with symptomatic mass effect on the brainstem are not candidates for radiosurgery as the primary management. Partial tumor resection followed by planned radiosurgery offers the best chance of preserving facial and cochlear function in such patients.
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Cystic Tumors: There is a misconception among some physicians that cystic tumors do not respond to radiosurgery and therefore should not be offered radiosurgery. On the contrary, the fact is that cystic VS often responds with significant tumor regression.
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Higher Margin Doses: Margin doses higher than 13 Gy are not needed for VS radiosurgery. Higher doses are associated with higher risk of facial and auditory nerve complication.
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Tumor Expansion: Follow-up MRI shows some tumor expansion after radiosurgery in about 5% of patients. These patients should be followed using serial MRIs and should not be rushed to surgery.
Clinical Results
Hearing Preservation
Published VS radiosurgery studies show that rates of serviceable hearing preservation range from 49% to 79% ( Table 27.1 ). In a study by Flickinger et al., 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. Kano et al. evaluated factors related to hearing preservation in 77 VS patients. At a median of 20 months after SRS, serviceable hearing was preserved in 71%. Among the patients who had Gardner-Robertson (GR) Class I hearing before radiosurgery, 89% retained serviceable hearing. Significant prognostic factors for serviceable hearing preservation were GR Class I hearing, a patient age younger than 60 years, an intracanalicular tumor, and a smaller tumor volume. A cochlear dose of less than 4.2 Gy to the central cochlea was significantly correlated with better hearing preservation of the same GR class.
| First Author | Year | Marginal Doses | Follow-up Months, Median/Mean (Range) | Serviceable Hearing Preservation % |
|---|---|---|---|---|
| Flickinger | 2004 | 13 (12–13) | 24 (12–115) | 78.6 |
| Pollock | 2006 | 12.2 | 42 (12–62) | 63 |
| Chopra | 2007 | 13 (12–13) | 68 (12–143) | 57 |
| Regis | 2007 | 12 | ≥24 | 60 |
| Niranjan | 2008 | 13 (10–18) | 28 (12–144) | 64.5 |
| Kano | 2009 | 12.5 (12–13) | 20 (6–40) | 71 |
| Myrseth | 2009 | 12 | 24 | 68 |
| Tamura | 2009 | 12 (9–13) | 48 (36–132) | 78 |
| Kim | 2010 | 12 (11–15) | 36 (9–81) | 68 |
| Delbrouck | 2011 | 12 | ≥12 | 66 |
| Kim | 2011 | 12 (12–13) | 25 (6–48) | 61 |
| Massager | 2011 | 12 | 43 (24–96) | 79 |
| Boari | 2014 | 13 (11–15) | 60 (36–1530) | 49 |
| Lipski | 2015 | 11.5 (11–12) | 48 (24–84) | 77 |
| Mousavi | 2015 | 12.5 (12–13) | 65 (12–183) | 67 |
| Horiba | 2016 | 11.9 (11–12) | 56 (24–99) | 57 |
| Akpinar | 2016 | 12.5 (11.5–13) | 59 (10–168) | 79 |
Boari et al. reviewed 152 VS patients with 10-year audiologic follow-up after GKRS using a median margin dose of 13 Gy (range, 11–15 Gy). In patients with GR Class I hearing, hearing preservation was 71% and reached 93% among cases of GR Class I hearing in patients younger than 55 years. This study suggests that younger GR Class I patients have a significantly higher probability of retaining functional hearing even at the 10-year follow-up.
Baschnagel et al. studied the effect of cochlear dose on serviceable hearing preservation. Forty patients with VS with serviceable hearing were treated using GKS with a median marginal dose of 12.5 Gy (range, 12.5–13 Gy) to the 50% isodose volume. The median cochlear maximum and mean doses were 6.9 and 2.7 Gy, respectively. The 1-, 2-, and 3-year actuarial rates of maintaining serviceable hearing were 93%, 77%, and 74%, respectively. Patients who received a mean cochlear dose less than 3 Gy had a 2-year hearing preservation rate of 91% compared with 59% in those who received a mean cochlear dose of 3 Gy or greater. In this study a mean cochlear dose less than 3 Gy was associated with higher serviceable hearing preservation.
Lipski et al. analyzed 126 patients with VS who were treated with GKRS using a mean marginal dose of 11.5 Gy (range, 11–12 Gy). Impairment of hearing compared with its pretreatment level was revealed in 12%, 13%, and 16% of patients at 1 year, 2 years, and 3 years after radiosurgery, respectively. Overall, 77% of patients with pre-GK serviceable hearing maintained it 3 years after GKRS. Klijn et al. treated VS patients with GKRS using a median marginal dose of 11 Gy. Preservation of serviceable hearing was evaluated in a subgroup of 71 patients with serviceable hearing at baseline. Actuarial 3- and 5-year hearing preservation rates were 65% and 42%, respectively. In a recent study Schumacher et al. used low-dose (11.0 Gy) GKRS in 30 patients with VSs. At a median follow-up time of 42 months, serviceable hearing was preserved in 50% of patients. Only higher mean and maximum dose to the cochlea significantly decreased the proportion of patients with serviceable hearing. Horiba et al. evaluated the outcome of low-dose GKS for VSs. Patients underwent treatment with a mean marginal dose of 11.9 Gy (range, 11–12 Gy). The doses for the cochlea were kept below 4 Gy. Out of 49 patients with serviceable hearing before GKS, 28 (57%) demonstrated its preservation at the time of the last follow-up.
Mousavi et al. studied the impact of pre-GK hearing status on hearing preservation rates after GKRS. These authors analyzed 68 VS patients with GR class I hearing. Twenty-five patients had no subjective hearing loss (group A), and 43 patients reported subjective hearing loss (group B) before GKRS. Three years after GKRS, serviceable hearing (GR grade I or II) was preserved in 100% of patients in group A compared with 81% at 1 year, 60% at 2 years, and 57% at 3 years after GKRS for group B patients. Patients without subjective hearing loss had higher rates of grade I or II hearing preservation. To identify the candidates with a high chance of hearing preservation, Mousavi et al. retrospectively analyzed 166 patients who had pre-GK audiograms showing GR class I hearing. These patients were subclassified into Class I-A (no subjective hearing loss, 53 patients) and class I-B (subjective hearing loss, 113 patients). Class I-B was further divided into two groups: class I-B1 (56 patients) if the difference in pure tone average (PTA) in the affected ear was ≤10 dB compared with the unaffected ear, and class I-B2 (57 patients) if the difference in PTA was >10 dB. The 5- and 10-year rates of serviceable hearing preservation for Class I-A were 100% and 92%, respectively. The 5- and 10-year rates of serviceable hearing preservation for Class I-B1 were 71% and 57%, respectively. The 5- and 10-year rates of serviceable hearing preservation for Class I-B2 were 26% and 26%, respectively. This study suggested that hearing preservation in patients with small VSs with pre-GK normal hearing (GR class I) was significantly better if SRS was performed before subjective hearing loss was reported. In patients who reported subjective hearing loss, the difference in PTA between the affected ear and the unaffected ear was an important factor in long-term hearing preservation.
In a recent study Akpinar et al. studied the impact of an observation period in patients with a small VS who present with normal hearing. The 5- and 10-year preservation rates of serviceable hearing for patients treated early were 89% and 86%, respectively. The 5- and 10-year preservation rates of serviceable hearing for patients treated later were 66% and 66%, respectively. This study suggested that early intervention using GKS results in better hearing preservation rates.
Yang et al. reviewed the published literature on the GKS for VS patients. These authors evaluated 45 articles that represented 4234 patients. At a median follow-up of 35 months, an overall hearing preservation rate was 51%, regardless of radiation dose, patient age, or tumor volume. Lower margin doses (≤13 Gy) were associated with better (60.5%) hearing preservation rates. Patients with smaller tumors (average tumor volume ≤1.5 cc) had a higher hearing preservation rate (62%) compared with patients harboring larger tumors.
Yomo et al. compared the post-SRS hearing deterioration with the natural course of hearing deterioration due to the tumor itself. A group of 154 patients with unilateral VS was conservatively monitored for more than 6 months and then treated with GKS. The annual hearing decrease rate was measured before and after radiosurgery. The mean dose prescribed to the tumor margins was 12.1 Gy. The mean annual hearing decrease rates before and after GKS were 5.39 dB/year and 3.77 dB/year, respectively (p > 0.05). A maximum cochlear dose of less than 4 Gy was found to be the only prognostic factor for hearing preservation. This study demonstrated the absence of an increase in annual hearing decrease rate after radiosurgery as compared with the natural history.
Overall long-term studies suggest that hearing can be preserved in the majority of patients who present with small- to medium-size tumors and good or normal hearing.
Facial Nerve Preservation
With the current technique, facial nerve function can be preserved in almost all patients ( Table 27.2 ). In an analysis of VS radiosurgery using current techniques (MR based dose planning, a 13 Gy or less tumor margin dose), Flickinger et al. reported a 0% risk of new facial weakness numbness (5-year actuarial rates). A higher margin dose (>14 Gy) was associated with 5-year actuarial rates of 2.5% risk of new onset facial weakness. None of the patients who had radiosurgery for intracanalicular tumors developed new facial or trigeminal neuropathies.
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