title: Delayed cyst formation after gamma knife radiosurgery for brain metastases.
abstract: OBJECTIVE
  Gamma knife radiosurgery (GKRS) is occasionally a useful tool for maintaining good brain status in patients with brain metastases (METs). Conversely, we recently experienced patients with delayed cyst formation (DCF) several years after GKRS, a complication not previously reported. Herein we assessed the frequency and characteristics of DCF after GKRS for METs.
  
  
  METHODS
  Eighty of 1209 patients with METs treated with GKRS maintained good brain status for more than 3 years without regrowth of tumorous lesions in the brain. In this study, DCF was defined as secondary cyst formation more than 3 years after the first GKRS in patients with METs who did not have cysts at the start of MET treatment. The 80 patients were divided into 2 groups (DCF group and non-DCF group) for assessment of the frequency and characteristics of DCF. Of the patients with cystic METs at the start of MET treatment, 16 were included in the latter group.
  
  
  RESULTS
  Among these 80 patients, 8 had DCF after GKRS (DCF group), detected by magnetic resonance imaging from 37 to 121 months after the first GKRS (median interval of 53 months). Of these 8 patients, 7 were symptomatic, and surgical treatments including Ommaya reservoir placement were needed in 5. A comparison of the non-DCF and DCF groups revealed that a higher number of GKRS treatments was a risk factor for DCF. Moreover, patients surviving more than 5 years after the initial GKRS are at risk for DCF.
  
  
  CONCLUSIONS
  Although DCF is not a widely recognized complication of GKRS for METs, we advocate careful follow-up, with surgical intervention for DCF if necessary, for frequently irradiated and long-surviving patients with METs treated with GKRS.
authors.affiliation: Department of Neurosurgery, Institute of Clinical Medicine, Graduate School of Comprehensive Human Sciences, University of Tsukuba, Tsukuba, Japan. e-ishikawa@md.tsukuba.ac.jp
authors.forename: Eiichi
authors.initials: E
authors.lastname: Ishikawa
authors.forename: Masaaki
authors.initials: M
authors.lastname: Yamamoto
authors.forename: Atsushi
authors.initials: A
authors.lastname: Saito
authors.forename: Yuji
authors.initials: Y
authors.lastname: Kujiraoka
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Iijima
authors.forename: Hiroyoshi
authors.initials: H
authors.lastname: Akutsu
authors.forename: Akira
authors.initials: A
authors.lastname: Matsumura
mesh_terms: D001921:Brain / Q000473:pathology / Q000503:physiopathology / Q000528:radiation effects; D001932:Brain Neoplasms / Q000556:secondary* / Q000601:surgery*; D002277:Carcinoma / Q000556:secondary* / Q000601:surgery*; D020863:Central Nervous System Cysts / Q000209:etiology* / Q000473:pathology / Q000503:physiopathology; D004660:Encephalitis / Q000209:etiology / Q000473:pathology / Q000503:physiopathology; D005260:Female; D006801:Humans; D015918:Infusion Pumps, Implantable; D008279:Magnetic Resonance Imaging; D008875:Middle Aged; D010051:Ovarian Neoplasms / Q000473:pathology*; D011183:Postoperative Complications / Q000209:etiology / Q000473:pathology / Q000503:physiopathology; D016634:Radiosurgery / Q000009:adverse effects* / Q000379:methods; D013995:Time; D013997:Time Factors; D016896:Treatment Outcome