• Takashi Shuto, Shigeo Inomori, Hideyo Fujino, Hisato Nagano, Naoki Hasegawa, and Yukio Kakuta.
• Department of Neurosurgery, Yokohama Rosai Hospital, Yokohama, Kanagawa, Japan. shuto@yokohamah.rofuku.go.jp
• J. Neurosurg. 2005 Jan 1; 102 Suppl: 134-9.

ObjectThe authors conducted a study to evaluate the clinical significance of cyst formation or enlargement after gamma knife surgery (GKS) for intracranial benign meningiomas.MethodsThe medical records of 160 patients with 184 tumors were examined for those with follow-up data of more than 2 years among 270 patients who underwent GKS for intracranial meningiomas between February 1992 and November 2001. Cyst formation or enlargement following GKS was observed in five patients, one man and four women (mean age 61.2 years). The tumor location was the sphenoid ridge in one case, petroclival in two, tentorium in one, and parasagittal region in one. All patients underwent surgery before GKS. The mean tumor volume was 10.5 cm3, the mean margin dose was 13.4 Gy (median 14 Gy), and the mean maximum dose was 27.5 Gy (median 24.1 Gy). At the time of GKS three tumors were associated with cyst, of which two enlarged after radiosurgery. Three cysts developed de novo after GKS. Three of the five patients needed surgery to treat the cyst formation or enlargement. Histological examination demonstrated various findings such as tumor necrosis, proliferation of small vessels, vascular obliteration, and hemosiderin deposits.ConclusionsNew cyst formation following GKS for benign intracranial meningioma is relatively rare; however, both preexisting and newly developed cysts tend to enlarge after GKS and often require surgery.

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