Predictors of hearing preservation after stereotactic radiosurgery for acoustic neuroma

Clinical article

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Object

Many patients with acoustic neuromas (ANs) have hearing function at diagnosis and desire to maintain it. To date, radiosurgical techniques have been focused on conformal irradiation of the tumor mass, with less attention to inner ear structures for which there was scant radiobiological information. The authors of this study evaluated tumor control and hearing preservation as they relate to tumor volume, imaging characteristics, and nerve and cochlear radiation dose following stereotactic radiosurgery (SRS) using the Gamma Knife.

Methods

Seventy-seven patients with ANs had serviceable hearing (Gardner-Robertson [GR] Class I or II) and underwent SRS between 2004 and 2007. This interval reflected more recent measurements of inner ear dosimetry during the authors' 21-year experience. The median patient age was 52 years (range 22–82 years). No patient had undergone any prior treatment for the ANs. The median tumor volume was 0.75 cm3 (range 0.07–7.7 cm3), and the median radiation dose to the tumor margin was 12.5 Gy (range 12–13 Gy). At diagnosis, a greater distance from the lateral tumor to the end of the internal auditory canal correlated with better hearing function.

Results

At a median of 20 months after SRS, no patient required any other additional treatment. Serviceable hearing was preserved in 71% of all patients and in 89% (46 patients) of those with GR Class I hearing. Significant prognostic factors for maintaining the same GR class included (all pre-SRS) GR Class I hearing, a speech discrimination score (SDS) ≥ 80%, a pure tone average (PTA) < 20 dB, and a patient age < 60 years. Significant prognostic factors for serviceable hearing preservation were (all pre-SRS) GR Class I hearing, an SDS ≥ 80%, a PTA < 20 dB, a patient age < 60 years, an intracanalicular tumor location, and a tumor volume < 0.75 cm3. Patients who received a radiation dose of < 4.2 Gy to the central cochlea had significantly better hearing preservation of the same GR class. Twelve of 12 patients < 60 years of age who had received a cochlear dose < 4.2 Gy retained serviceable hearing at 2 years post-SRS.

Conclusions

As currently practiced, SRS with the Gamma Knife preserves serviceable hearing in the majority of patients. Tumor volume and anatomy relate to the hearing level before radiosurgery and influence technique. A low radiosurgical dose to the cochlea enhances hearing preservation.

Abbreviations used in this paper:AN = acoustic neuroma; GR = Gardner-Robertson; IAC = internal auditory canal; PTA = pure tone average; SDS = speech discrimination score; SRS = stereotactic radiosurgery; SRT = stereotactic radiotherapy.

Article Information

Address correspondence to: Douglas Kondziolka, M.D., F.R.C.S.(C), University of Pittsburgh, Suite B-400, UPMC Presbyterian, 200 Lothrop Street, Pittsburgh, Pennsylvania 15213. email:kondziolkads@upmc.edu.

Please include this information when citing this paper: published online March 13, 2009; DOI: 10.3171/2008.12.JNS08611.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    A and B: Axial T2-weighted MR images obtained in a 67-year-old woman, showing a right AN (Koos Grade III at the time of SRS). C and D: Axial T1-weighted contrast-enhanced MR images showing a right AN. One-sided arrow indicates the well-visualized cochlea; cruciform marks indicate the center of the cochlea, or the modiolus (dose = 3.1 Gy); and the double-sided arrow shows the distance from the lateral end of tumor to the end of the IAC (3.9 mm). The radiosurgery plan is shown to deliver 12.5 Gy to the tumor margin.

  • View in gallery

    A and B: Axial T2-weighted MR images obtained in a 54-year-old man, demonstrating a right AN (Koos Grade II at the time of SRS). C and D: Axial T1-weighted contrast-enhanced MR images showing a right AN. Arrow indicates the cochlea, cruciform mark shows the cochlear modiolus (dose = 2.6 Gy), and double-sided arrow shows the distance from the lateral end of the tumor to the end of the IAC (2.5 mm). The Gamma Knife surgery plan is shown.

  • View in gallery

    A and B: Axial T2-weighted MR images obtained in a 51-year-old man, revealing a right AN (Koos Grade III at the time of SRS). C and D: Axial T1-weighted contrast-enhanced MR images showing a right AN. Arrow indicates the cochlea, cruciform mark shows the modiolus (dose = 4.1 Gy), and double-sided arrow demonstrates the distance from the lateral end of the tumor to the end of the IAC (3.2 mm).

  • View in gallery

    A and B: Axial T2-weighted MR images obtained in a 47-year-old man, showing a right AN (Koos Grade II at the time of SRS). C and D: Axial T1-weighted contrast-enhanced MR images revealing a right AN. Arrow shows the cochlea; a cruciform mark, the modiolus (dose = 8.6 Gy); and double-sided arrow, the distance from the lateral end of the tumor to the end of the IAC (0 mm), because the tumor filled the entire canal. The patient had poor hearing.

  • View in gallery

    Upper: Kaplan-Meier curves comparing the maintenance of the same GR hearing class with a radiation dose < vs ≥ 4.2 Gy in the middle of the cochlea in patients < 60 years old. A radiation dose < 4.2 Gy was significantly associated with better odds of remaining within the same GR hearing class (p = 0.032). Lower: Kaplan-Meier curves comparing the maintenance of serviceable hearing with a radiation dose < vs ≥ 4.2 Gy in the middle of the cochlea in patients < 60 years old. Again, patients who had received a radiation dose < 4.2 Gy had significantly better odds of retaining serviceable hearing (p = 0.027).

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