Hypofractionated Cyberknife Stereotactic Radiosurgery for Acoustic Neuromas with and without Association to Neurofibromatosis Type 2

Objective: Cyberknife stereotactic radiosurgery (CKSRS), an image-guided frameless stereotactic radiosurgery system, has been proven effective in the treatment of intra-cranial and spinal lesions. With evidence-proved accuracy, high conformality and homogeneity, CKSRS is feasible and effective not only in tumor control but also in vital organ sparing. Concerning in using CKSRS to treat patients with acoustic neuromas (AN), the preservation of hearing, trigeminal nerve, and facial nerve is critical and important. The results of CKSRS to preserve hearing function are amenable comparing to other irradiation treatment modality. In this study, we report our experience to treat acoustic neuromas using hypofractionated CKSRS.
Methods: We collected 21 patients with AN in past 2 years. Among these 21 patients, 15 patients were unilateral AN; and 6 patients were bilateral. 7 patients were neurofibromatosis type 2 (NF2) and 14 patients were non-NF-2. Among the 7 patients with NF2, one patient was unilateral AN, and the other six were bilateral. Neurological evaluation of cranial nerves V, VII and VIII were performed preoperatively and postoperatively in all the patients according to the Gardner-Robertson grading on hearing function, House-Brackmann grading on facial nerve function, and semi quantitative scale on trigeminal nerve function. All patients were evaluated with audiograms before treatment and at 3, 6 and 12 months following treatment. Magnetic resonance (MR) image was obtained at 3 months intervals after treatment. The dosimetry indices of Cyberknife treatment, including tumor coverage, homogeneity index (HI), conformality index (CI), and new conformality index (NCI) of these patients were calculated to evaluate the precision of radiosurgery treatment planning. Mean follow-up was 12 months (range: 2-22 months)
Results: Tumor volume ranged from 0.13 to 24.8 cm^3 (mean 5.4 cm^3) with the mean marginal dose 1820 cGy (range 1800-2000 cGy)/3 fractions. Tumors were treated with 80% to 89% isodose line (mean 83%) and mean 97.9% tumor coverage. For the non-NF-2 patients, useful hearing (Gardner-Robinson Class 1-2) was preserved in 9 (81%) of 11 patients at the last follow-up. For the NF-2 patients, 60% of patients (3/5) retained useful hearing. No new facial and trigeminal dysfunction developed in all patients. No patients experienced brainstem toxicity or cerebellar edema. Followed up MR images showed that tumor regression in 20 patients (95%) and stationary in one patient (5 %), and achieved 100% tumor control rate.
Conclusions: Our 2 years experience strongly support that hypofractionated CKSRS provided not only significant tumor control for AN but also excellent hearing preservation rate in non-NF-2 patients. On the contrary, in NF-2 patients, although tumor control was remarkable, hearing preservation rate is not so ideally as non-NF-2 patients.