Large Vestibular Schwannoma Treated By Gamma Knife Radiosurgery ? Long Term Outcome

Keywords: vestibular schwannoma, radiosurgery, gamma knife, skull base, outcome

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     Radiosurgery is well accepted for treatment of small to medium-sized vestibular schwannomas (VS). Nevertheless, its application to treat large VS remains controversial to most neurosurgeons due to potential unfavorable swelling and compression of brain stem. 
     The authors report experience of Gamma knife radiosurgery (GKS) for large VS and analyze possible factors affecting outcome by 15 years follow up.
     Large VS was defined as a tumor bigger than 3cm in diameter (tumor volume > 12.7mL). Since 2003, the authors treated 21 patients with large VS; the mean tumor volume was 17.3mL (12.7-25.2). 17 had undergone prior craniotomies to remove tumor one to two times. Four patients were fresh case choosing GKS due to patients’ preference or poor condition which precludes microsurgery under general anesthesia.
     The average dose prescribed to tumor margin was 11.9Gy (11-14). Mean dose ranged from 15 to 17.5Gy. The average follow-up period is 55 months (6.5-155).
     20 and 14 cases were followed up more than 12 and 24 months respectively. All had significant compression of brain stem or cerebellum. After GKS, 10/12(83%), 4/4(100%), 1/1(100%) follow up more than 5, 10 and 15 years was found tumor regression or stable without clinical deterioration. Two patients received VP shunt for hydrocephalus. One patient needed aspiration to decompress the enlarging tumor cyst. Two patients had to receive craniotomy to decompress the tumor due to progressive and intolerable brain stem compression. One of them had tumor controlled for 5.5 years. She received craniotomy to excise the tumor after VP shunt performed because of rapid tumor growing. The pathology turned out to be a malignant peripheral nerve sheath schwannoma. The other patient was proved to have a common schwannoma. Among possible factors influencing tumor control, multiple cystic component of tumor with minimal solid portion and postoperative status were two favorable factors to radiosurgery.
     This is a retrospective study.
     Radiosurgery is not only a practical treatment for patients with small to medium-sized VS but also larger tumor up to 25 ml. Tumor with multiple cysts and postoperative status seemed to benefit most from GKS, even with evident brain stem compression and ventricular dilation.
     The adverse radiation effect and tumor swelling prompt to microsurgical removal is rare. After long-term follow up, we suggest by careful selection of patient, GKS plays a good role in treating large vestibular schwannomas.


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