Mechanism of cyst formation and enlargement following gamma knife surgery for arteriovenous malformation

Keywords: arteriovenous malformation, outcome, gamma knife, complications, cyst

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     Cysts can occur after AVM radiosurgery.
This study retrospectively analyzed the clinical characteristics of patients who presented with cyst formation following gamma knife surgery (GKS) for arteriovenous malformation (AVM), and mainly discusses the mechanism of cyst enlargement.
     Twelve male and five female patients aged 17 to 47 years (mean 28.7 years) were retrospectively identified among 750 patients who underwent GKS for AVM at our institution.
     The calculated nidus volume, prescription dose to the nidus margin, timing of occurrence of neuroimaging change, follow-up imaging of cysts, findings during surgery, and pathological findings of the cyst wall and associated granulomatous lesions were investigated.
     Expanding hematoma was associated with cyst formation in 4 patients. The mean nidus volume at the time of GKS was 10.1 ml (0.1-26.7 ml), and the mean prescription dose at the nidus margin was 20.3 Gy (18-28 Gy). Complete obliteration of nidus was obtained in 12 patients, partial obliteration in 4, and no change in 1. Cyst formation was detected at 2.6-15 years (mean 6.9 years) after GKS. Two patients underwent craniotomy for cyst opening and removal of the incompletely obliterated nidus, and two received placement of Ommaya reservoir. Spontaneous regression of the cyst was observed in one patient. Serial magnetic resonance (MR) imaging was performed in the other 15 patients because the cyst showed unchanged size or remained asymptomatic. Histological examination of cyst wall revealed linear deposits of hemosiderin with gliosis, but no evidence of fresh bleeding in the cyst wall. Histological examination of the enhanced lesion on MR imaging demonstrated degenerated nidus with infiltration of inflammatory cells and old hemorrhage, and granulation tissue with chronic hemorrhage from the newly developed capillary vessels.
     This was a retrospective review.
     Cysts developing after GKS for AVM enlarge mainly due to repeated minor bleeding from angiomatous lesions developing within the degenerated nidus or adjacent brain. 
     The optimal treatment for such cysts is wide opening with removal of the angiomatous lesion through craniotomy.


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