Gamma Knife Surgery For Abducens Nerve Schwannomas

Keywords: schwannoma, gamma knife, cranial nerve, outcome, skull base

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     Abducens nerve schwannomas are extremely rare tumors, consisting approximately only 1% of all intracranial schwannomas. In most cases, the tumors are located within cavernous sinus, which makes it difficult to remove surgically without neurological deficit. Therefore, most of neurosurgeons recommend observation follow-up for asymptomatic cases.
     Our team regards Gamma knife surgery as a safe and effective method to deal with the schwannomas before any symptom appears due to tumor growth. Therefore we proactively tackle with them with Gamma Knife based on knowledge of microanatomy and also from microsurgical point of view.
     From January 2003 to September 2009, a total of 5105 patients were treated by Gamma Knife at our institute, among which 4 cases were considered abducens nerve schwannoma. .
     Surgical resection was not performed in any case. Gadolinium-enhanced axial CISS images were used in all cases to delineate the tumors inside the cavernous sinus as well as surrounding nerves to target only the tumor. Center of each isocenter was gathered at the tumor and 80% higher isodose area was placed as wide as possible within it. To avoid excess radiation to peritumoral healthy tissue such as oculomotor nerve or internal carotid artery, we targeted each isocenter to be placed within tumor membrane. The treatment was performed by 12Gy to the 50% isodose line
     Image findings suggested 3 prominent anatomical types; 1) the transitional part between orbit and cavernous sinus, 2) circumscribed within cavernous sinus, and 3) the transitional part between cavernous sinus and posterior fossa. Only 1 case had already presented abducens nerve palsy whereas the other 3 cases presented the symptom after Gamma Knife surgery (within 3 days, at 2 months and at 18 months, respectively), and were clinically estimated as abducens nerve schwannoma. Tumor growth control was achieved in all cases at an average of 18 months follow-up (range: 3-30). Complications were observed in 3 cases (transient oculomotor nerve palsy type 2 and 3 and transient complete blindness type 1) but all of them resolved eventually.
     This is a retrospective study.
     Abducens nerve schwannomas are extremely rare and only 18 cases have been reported in the literature. They are all within cavernous sinus, orbit and posterior fossa, unlike our cases with transitional parts. We experienced Gamma Knife treatment for 4 brain tumors considered abducens nerve schwannoma. Each and every case showed considerable change after irradiation.
     Therapeutic indications for the abducens nerve schwannoma should be carefully examined, but we would recommend Gamma knife surgery at an early phase if informed consent is obtained.


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