Long-term outcome after Gamma Knife radiosurgery for acoustic neuroma of all Koos grades: a single-center study

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OBJECTIVE

The authors present long-term follow-up data on patients treated with Gamma Knife radiosurgery (GKRS) for acoustic neuroma.

METHODS

Six hundred eighteen patients were radiosurgically treated for acoustic neuroma between 1992 and 2016 at the Department of Neurosurgery, Medical University Vienna. Patients with neurofibromatosis and patients treated too recently to attain 1 year of follow-up were excluded from this retrospective study. Thus, data on 557 patients with spontaneous acoustic neuroma of any Koos grade are presented, as are long-term follow-up data on 426 patients with a minimum follow-up of 2 years. Patients were assessed according to the Gardner-Robertson (GR) hearing scale and the House-Brackmann facial nerve function scale, both prior to GKRS and at the times of follow-up.

RESULTS

Four hundred fifty-two patients (81%) were treated with radiosurgery alone and 105 patients (19%) with combined microsurgery-radiosurgery. While the combined treatment was especially favored before 2002, the percentage of cases treated with radiosurgery alone has significantly increased since then. The overall complication rate after GKRS was low and has declined significantly in the last decade. The risk of developing hydrocephalus after GKRS increased with tumor size. One case (0.2%) of malignant transformation after GKRS was diagnosed. Radiological tumor control rates of 92%, 91%, and 91% at 5, 10, and 15 years after GKRS, regardless of the Koos grade or pretreatment, were observed. The overall tumor control rate without the need for additional treatment was even higher at 98%. At the last follow-up, functional hearing was preserved in 55% of patients who had been classified with GR hearing class I or II prior to GKRS. Hearing preservation rates of 53%, 34%, and 34% at 5, 10, and 15 years after GKRS were observed. The multivariate regression model revealed that the GR hearing class prior to GKRS and the median dose to the cochlea were independent predictors of the GR class at follow-up.

CONCLUSIONS

In small to medium-sized spontaneous acoustic neuromas, radiosurgery should be recognized as the primary treatment at an early stage. Although minimizing the cochlear dose seems beneficial for hearing preservation, the authors, like others before, do not recommend undertreating intracanalicular tumors in favor of low cochlear doses. For larger acoustic neuromas, radiosurgery remains a reliable management option with tumor control rates similar to those for smaller acoustic neuromas; however, careful patient selection and counseling are recommended given the higher risk of side effects. Microsurgery must be considered in acoustic neuromas with significant brainstem compression or hydrocephalus.

ABBREVIATIONS CI = cochlear implant; GKRS = Gamma Knife radiosurgery; GR = Gardner-Robertson; HB = House-Brackmann; NF2 = neurofibromatosis type 2; SRS = stereotactic radiosurgery.

Article Information

Correspondence Brigitte Gatterbauer: Medical University Vienna, Austria. brigitte.gatterbauer@meduniwien.ac.at.

INCLUDE WHEN CITING Published online March 2, 2018; DOI: 10.3171/2017.8.JNS171281.

Disclosures None of the authors disclosed any competing interests or specific funding regarding this retrospective study.

© AANS, except where prohibited by US copyright law.

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Figures

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    Images from a case with malignant transformation after GKRS. Planning T1-weighted contrast-enhanced (A) and T2-weighted (B) MRI sequences obtained in a patient undergoing radiosurgery with a 13-Gy prescription dose targeting the 50% isodose line for acoustic neuroma. A preradiosurgery biopsy was not performed. Axial T1-weighted contrast-enhanced (C) and T2-weighted (D) images obtained 8 years after radiosurgery, showing significant tumor progression. The patient underwent microsurgery. Postoperative MRI showed total tumor resection (not shown). Histopathological analysis revealed schwannoma with a rather high MIB-1 labeling index (see Fig. 2A–D). Eight months later, the patient developed sudden hemiparesis and ataxia. Contrast-enhanced T1-weighted MRI (E and F) revealed a massive, multifocal tumor recurrence. A second microsurgical resection/mass reduction was performed. Histopathology revealed a malignant peripheral nerve sheath tumor (PNST). One month later, follow-up contrast-enhanced T1-weighted MR images (G and H) revealed another multifocal and even contralateral tumor recurrence and progression.

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    Histopathological studies from a case with malignant transformation after GKRS. Eight years after GKRS, microsurgical resection and subsequent histopathological analysis revealed a schwannoma (A and inset B) but already containing well-delineated areas (A and D, arrows) with increased cellularity, brisk mitotic activity (inset C), and a rather high MIB-1 labeling index of 35%, in retrospect indicating transformation into a malignant PNST. A second microsurgical resection was performed 8 months after the first because of clinical deterioration and multifocal recurrence. Histopathological analysis showed increased anisomorphism throughout the entire tumor tissue (E) with an MIB-1 labeling index of up to 63% (F), thus fulfilling the criteria for a malignant PNST. One month later the patient died as a result of brainstem compression. H & E (A–C, E), original magnification ×40 (A), ×200 (B, D–F), ×600 (C). Figure is available in color online only.

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    A: Kaplan-Meier analysis of the tumor control rate in a sample with a minimum of 2 years of follow-up (423 patients). Patients were censored at the last available radiological follow-up. Time is relative to the date of GKRS. Actuarial analysis revealed radiological tumor control rates of 92%, 91%, 91%, and 91% at 5, 10, 15, and 20 years, respectively. B: Bar graph of tumor control rates according to Koos acoustic neuroma grades. Tumor control rates were similar across all Koos grades.

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    Examples of patients with larger acoustic neuromas and their long-term follow-ups. Three months after initial patient presentation with tinnitus and hearing deterioration, an acoustic neuroma was radiologically diagnosed on contrast-enhanced T1-weighted MR images (A–C). At the time of GKRS, the patient had GR class II hearing. A prescription dose of 12 Gy was delivered to the 50% isodose line. The target volume was 4.7 cm3. Ten-year follow-up contrast-enhanced T1-weighted MRI showed significant tumor shrinkage (D–F). At the time of the last follow-up, the patient was experiencing nonserviceable hearing (GR class III) but had grown accustomed to the persisting tinnitus. No postradiosurgical complications were noted. Another patient experienced nonserviceable hearing (GR class III) and slight dysfunction of the facial nerve (HB grade II). Contrast-enhanced T1-weighted MRI (G–I) showed an acoustic neuroma with slight compression of the brainstem, which was radiosurgically treated in 1999. A prescription dose of 12 Gy was delivered to the 50% isodose line. The target volume was 9.1 cm3. Regular follow-up MRI showed continuous tumor shrinkage. At the 17-year follow-up, the tumor volume was significantly reduced (J–L). The patient experienced poor hearing (GR class IV), but no postradiosurgical complications had occurred. A third patient initially presented with nonserviceable hearing (GR class III). Contrast-enhanced T1-weighted MRI (M–O) showed an acoustic neuroma with moderate compression of the brainstem. A prescription dose of 12 Gy was delivered to the 45% isodose line. The target volume was 3.4 cm3. After GKRS, the patient experienced a transient tic and transient facial paresis. The patient received dexamethasone treatment and was relieved from both symptoms a few months after GKRS. At the 8-year follow-up, MRI (P–R) showed a significant reduction in tumor volume. The patient’s hearing was graded as GR class III at follow-up.

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