Long-term results in proactive treatment with gamma knife radiosurgery for tiny intracanalicular vestibular schwannom

Keywords: vestibular schwannoma, natural history, gamma knife, radiosurgery, outcome

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     Management options for patients with tiny intracanalicular vestibular schwannomas include observation, surgical resection and stereotactic radiosurgery. By using Fast Imaging Employing STeady-state Acquisition (FIESTA) image of brain MRI, the fine anatomical structures of cranial nerves and tumor in the internal acoustic canal can be identified without the use of contrast enhancement. 
     We proactively treated these tiny vestibular schwannomas with Gamma Knife Radiosrugery (GKRS) and collected data prospectively to assess the long-term results.
     Thirteen (8 women and 5 men) patients were diagnosed as tiny intracanalicular vestibular schwannoma (defined as less than 0.5 cc. in tumor volume). The mean patient age was 60 years (range, 45-80). Hearing was graded using the Gardner-Robertson (GR) classification.
     Between January and December of 2004, FIESTA images were routinely added to all patients for MRI study of brain in Buddhist Tzu Chi General Hospital, Hualien, Taiwan.  Dose planning was performed on intraoperative stereotactic contrast enhanced images using multiple 4-mm isocenters. The mean tumor volume was 0.098 cc (range, 0.013-0.4). The mean margin dose was 12.4 Gy (range, 11-14) and the isodose line was set at a mean of 53.8 % (range, 50-70%).
     Twelve patients had GR Grade I or II hearing before gamma knife radiosurgery (GKRS) and GR I or II hearing was maintained in 11 patients. Facial and trigeminal nerve function was preserved in all patients. The tumor control rate was 100 % at a mean follow-up of 7.1 years (range, 6.7-7.6). One patient developed acute vertigo one day after the GKRS and it was subsided after the short term use of the steroid and has never happened during the follow up period.
     This was a retrospective study.
     With the application of FIESTA image, tiny acoustic neuromas can be early detected. Because the low dose (12-14 Gy) GKRS is save and effective for control of tumor growth with good preservation of hearing. 
     It may be worthy to use the FIESTA to detect the tiny acoustic neuromas and to treat by GKRS.


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