Characterization Of Patterns Of Behavior Of Vestibular Schwannomas After Gamma Knife Radiosurgery

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

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     The evaluation of the result of treating vestibular schwannomas with Gamma Knife radiosurgery is based mainly on imaging, usually MRI. Since these tumors in general are slow growing and slow shrinking, reported interval changes regarding size frequently are few or none at all.
     We decided to study size changes of vestibular schwannomas treated with Gamma Knife radiosurgery more in detail.
     A total of 139 patients with vestibular schwannomas were randomly selected for this study.
     The tumor size was defined and measured in a consistent way based on maximum sagittal, transverse and craniocaudal diameters, from which a mean diameter was calculated. All measurements were plotted in diagrams, which were updated after each new MRI. Only tumors followed for a minimum of one year were included. With few exceptions, the tumors were treated with a prescription dose of 12 Gy to 50%.
     Six different main patterns of response to the radiosurgery were defined. A: gradual shrinkage 46%; B: temporary increase followed by gradual shrinkage 16%; C: more protracted increase followed by shrinkage or permanent increase without further signs of progression 9%; D: stable size without increase or decrease 8%; E: gradual increase 4%; F: Other unusual patterns 17%.
     This is a retrospective study.
     The vast majority of vestibular schwannomas respond with shrinkage to Gamma Knife radiosurgery, sometimes with initial temporary increase reflecting a brisk transient intratumoral response to the radiosurgery. 
     Other unusual patterns include initial shrinkage followed months or even years later by increase and again shrinkage.


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