title: How to distinguish tumor growth from transient expansion of vestibular schwannomas following Gamma Knife radiosurgery.
abstract: BACKGROUND
  Typically, vestibular schwannomas (VS) react to Gamma Knife radiosurgery (GKRS) with a transient increase of tumor volume owed to tumor swelling at about 6 months followed by a reduction of tumor volume owed to tumor shrinkage at about 18 months. It is important to distinguish this transient tumor expansion (TTE) from tumor growth. We undertook this study to see if there is a typical time interval in the follow-up of VS following GKRS, which may indicate tumor growth rather than TTE.
  
  
  METHODS
  We retrospectively reviewed the patient charts of patients who underwent GKRS for unilateral sporadic VS at the Gamma Knife Center Zurich from 1994-2009 and who were treated by J. Siegfried or one of the authors (TM). Tumor progression was defined as an increase of tumor volume of ≥ 20 % as compared to the initial tumor volume at the earliest 2 years following GKRS. This time interval of ≥ 2 years was chosen in order to distinguish TTE from genuine tumor progression. Whenever tumor enlargement was suspected on follow-up MRI at ≥ 2 years following GKRS, tumor volumes were measured using custom software.
  
  
  RESULTS
  From 1994-2009, 235 patients underwent GKRS in Zurich for unilateral sporadic VS. Tumor progression with a volume increase of ≥ 20 % occurred in 21/235 (8.9 %) patients at 3.4 ± 0.9 years following GKRS. Seventeen out of 235 (7 %) patients had a clinically relevant tumor progression requiring microsurgery or repeat radiosurgery.
  
  
  CONCLUSIONS
  According to our data, time may be a good parameter distinguishing tumor progression due to tumor growth from TTE due to tumor swelling in VS following GKRS. Tumor growth seems to occur at about 3-4 years following GKRS for VS as opposed to TTE, which seems to be present at about 6-18 months following GKRS for VS.
authors.affiliation: Neurosurgery, Klinik Im Park, Seestrasse 220, 8027, Zurich, Switzerland, tmindermann@hin.ch.
authors.forename: Thomas
authors.initials: T
authors.lastname: Mindermann
authors.forename: Ines
authors.initials: I
authors.lastname: Schlegel
mesh_terms: D000328:Adult; D000368:Aged; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009364:Neoplasm Recurrence, Local / Q000473:pathology* / Q000601:surgery; D009464:Neuroma, Acoustic / Q000473:pathology* / Q000601:surgery; D016634:Radiosurgery; D012189:Retrospective Studies; D013997:Time Factors; D047368:Tumor Burden