Delayed cyst formation following ventral intermediate nucleus thalamotomy by Gamma Knife radiosurgery for essential tremor: illustrative case

BACKGROUND: While deep brain stimulation (DBS) is an effective therapy for essential tremor (ET), Gamma Knife radiosurgery (GKRS) offers a noninvasive alternative for patients who are poor surgical candidates or who experience DBS-related complications. Although GKRS ventral intermediate nucleus (VIM) thalamotomy is generally safe, very delayed adverse effects are uncommon and not well characterized. OBSERVATIONS: A 69-year-old woman with medically refractory ET underwent right VIM DBS, but the device was later explanted due to infection. Because she was high-risk for reimplantation, she proceeded with GKRS VIM thalamotomy. Eight years later, she developed progressive left-sided weakness and gait deviation. MRI revealed a new right thalamic cystic lesion precisely colocalized with the prior GKRS target. Stereotactic aspiration demonstrated acellular, proteinaceous fluid without evidence of malignancy or infection, consistent with a delayed postradiosurgical cyst. Neurological function improved following drainage. LESSONS: Delayed cyst formation may occur many years after GKRS VIM thalamotomy and should be recognized as a rare but clinically significant late complication. Given the typical 7- to 10-year latency observed across radiosurgical cystogenesis, long-term but low-frequency MRI surveillance, such as a 1-year posttreatment baseline followed by imaging every 2-3 years or sooner with new neurological symptoms, may facilitate earlier detection and intervention. https://thejns.org/doi/10.3171/CASE25725.