>Background Stereotactic radiosurgery plays a critical role in the treatment of central nervous system neoplasm and cerebrovascular malformations. This procedure is purportedly less invasive, but problems occurring later including tumor formation, necrosis, and vasculopathy-related diseases have been reported. >Clinical Presentation We report on a 65-year-old man who had experienced a de novo aneurysm in an irradiated field and an acute onset of right hemiparesis and aphasia. He had undergone gamma knife radiosurgery to treat an arteriovenous malformation 15 and 12 years prior, with 18 and 22 Gy marginal doses. At current admission, radiologic studies showed a de novo aneurysm in the irradiated field without recurrence of malformation. The aneurysm was resected. Histologic findings showed a disruption of the internal elastic lamina accompanied by fibrous degeneration. >Conclusion Stereotactic radiosurgery is a promising treatment tool, but long-term risks have not been fully researched. The treatment procedure for benign lesions should be chosen prudently.
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