Vestibular schwannomas in Gamma knife surgery based on microanatomy: Advantages in detection of tumor origin and its expansion using gadolinium enhanced CISS on Gamma Plan

Keywords: vestibular schwannoma, Imaging, gamma knife, anatomy, radiosurgery

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     Gamma Knife surgery (GKS) should be standard treatment ranked with surgical operation for vestibular schwannomas from the perspective of tumor control and audio-facial nerve function preservation in small and middle size (Koos stage 1-3). We have been looking forward to establish a new treatment concept and technique, which will improve shrinkage ratio, shorten the recovery term and even to recover functioning alternatively surgical resection.
     The purpose of this study was to discuss our dose-planning concepts.
     e have treated 260 cases with the above technique using robotized system “APS” at our institution in between 2003 and 2011. 182 patients could be followed more than 36 months.
     Along with the development of GKS system, we developed MRI sequences specific to this treatment as an enhanced 3D heavily T2 thinner images. Newly developed sequences have enabled us to have a clear understanding of cranial nerves distribution, especially from cisternal portion to internal acoustic meatus. A fusion image with CT scan has also helped us to understand anatomical relationship. Finnaly, we assigned this tumor as inferior and superior vestibular origin according to the findings on Gamma Plan, and we succeeded making dose planning to irradiate tumor conformally with higher selectivity with preventing overcoverage the surround vital structures such as the facial, cochelea, superior and inferior vestibular nerves, the cochlea, and the semicircular canal. In addition, we should keep higher dose (80%) line area inside the tumor as much as possible aiming at its shrinkage rather than just controlling it.
     W Tumor control rate was 98.9% and shrinkage one was 75.8% at the time of two years follow up and 80% at 36 months follow-up, facial nerve preservation rate was 100%, hearing preservation rate was 84.5%, and among them, of which five (2.8%) proved to be functioning recovery case. Transient enlargement was observed in most cases, but no severe complications were found. The patients with inferior vestibular schwannomas had a potential to be easier to suffer from hearing disturbance before and after GKS (not yet significant value).
     This was a retrospective study.
     We only have preliminary results yet, but the clinical result is surely a great breakthrough as compared to the previous treatment. 
     We presume that longer follow-up and more treatment cases will firmly establish this method as an absolute treatment option for vestibular schwannomas.


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