title: Proposed mechanism for cyst formation and enlargement following Gamma Knife Surgery for arteriovenous malformations.
abstract: OBJECTIVE
  The authors retrospectively studied the mechanism of cyst formation and enlargement after Gamma Knife surgery (GKS) for arteriovenous malformations (AVMs).
  
  
  METHODS
  Eighteen patients in whom cyst formation developed following GKS for AVM were retrospectively identified among 775 patients who underwent GKS for AVM at Yokohama Rosai Hospital. The study group was composed of 12 male and 6 female patients ranging in age from 17 to 47 years.
  
  
  RESULTS
  Chronic encapsulated expanding hematoma was associated with the cyst in 5 patients. The AVM nidus volume at the time of GKS ranged from 1.9 to 36 cm(3), and the prescription radiation dose was 18-25 Gy. Complete obliteration of the AVM nidus was obtained in 13 patients and partial obliteration in 5 patients. Cyst formation was detected between 2.6 and 15 years after GKS. Craniotomy was performed in 10 patients, including 2 patients in whom the incompletely obliterated nidus was removed at the same time, and an Ommaya reservoir was placed in 2 patients. Spontaneous regression of the cyst was observed in 1 patient. Serial MR imaging was performed in the other patients because the size of the cyst was stable or the lesion was asymptomatic. Histological examination of the cyst wall revealed linear hemosiderin deposits with gliosis. The nodular lesion, which was enhanced on MR images, contained granulation tissue with chronic hemorrhage from newly developed capillary vessels.
  
  
  CONCLUSIONS
  Cysts developing after GKS for AVM enlarge mainly due to repeated minor hemorrhages from a reddish nodular angiomatous lesion that develops within an adjacent brain area. Thus, the optimal treatment is wide opening of the cyst with removal of the associated angiomatous lesion by craniotomy.
authors.affiliation: Department of Neurosurgery, Yokohama Rosai Hospital, Yokohama, Kanagawa, Japan. shuto@yokohamah.rofuku.go.jp
authors.forename: Takashi
authors.initials: T
authors.lastname: Shuto
authors.forename: Makoto
authors.initials: M
authors.lastname: Ohtake
authors.forename: Shigeo
authors.initials: S
authors.lastname: Matsunaga
mesh_terms: D000293:Adolescent; D000328:Adult; D001921:Brain / Q000000981:diagnostic imaging / Q000473:pathology* / Q000601:surgery; D003399:Craniotomy; D003560:Cysts / Q000000981:diagnostic imaging / Q000209:etiology* / Q000601:surgery; D005260:Female; D006801:Humans; D002538:Intracranial Arteriovenous Malformations / Q000000981:diagnostic imaging / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011859:Radiography; D016634:Radiosurgery / Q000009:adverse effects* / Q000295:instrumentation*; D012189:Retrospective Studies