Gamma Knife surgery for skull base meningiomas: Advantage of the 4D dose planning based on microanatomy and microsurgery

Keywords: meningioma, Imaging, gamma knife, anatomy, skull base

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     Complete surgical removal of skull base meningiomas without neurological deficit is very difficult. Hence, many neurosurgeons recommend simple observation for medium-size meningiomas. In contrast, we deal with them proactively with Gamma knife surgery before they grow and present symptoms. .
     Our treatment is performed based on knowledge of microanatomy and also from microsurgical point of view. Our purpose was to review imaging-based planning techniques.
     From January 2003 to September 2011, 584 patients with meningiomas were treated by Gamma Knife surgery with robotized system at our institute. Among them, 184 cases were skull base meningiomas. Seventy eight cases with more than 3 years of follow-up were enrolled for this study. Gadolinium-enhanced axial CISS images were used in all cases. We tried to clarify dural attachment of the tumor, and to delineate the surrounding peritumoral nerves as much as possible. Finally, we could strongly suspect tumor origin and the way of tumor expansion 4 dimensionally in Gamma Plan. And then center of each isocenter should be gathered at the dural attachment and 80% higher isodose area should be placed as wide as possible within the tumor. To avoid excess radiation to peritumoral healthy tissue, we placed each isocenter within tumor membrane. The treatment was then performed by 12Gy to the 50% isodose line.
At an average of 42 months follow-up, tumor control rate was 98.7% and tumor shrinkage rate was 73.1%. Treatment complication (transient abducens nerve palsy) was observed in 1.3%, and brain edema in 2.6% which was, on the image, considered as a result of radiation injury to the brain stem. Evaluation of unit energy (tumor energy per unit volume of tumor) showed significantly high value in shrinkage group (p=0.03).
     This was a retrospective study.
     Advanced MR imaging sequences can help with precise definition of the important anatomy for gamma knife planning.
     We hope that others will validate the value of these imaging tools.


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