Retiform endothelial hyperplasia with growing hematoma within a vestibular schwannoma 30 years after Gamma Knife radiosurgery: histopathological evaluation of the lesion and the tumor. Illustrative case.

backgroundGamma Knife radiosurgery (GKRS) is an established therapeutic modality for vestibular schwannomas. However, various late-onset complications, including mass lesions resembling cavernous malformations, have been reported. Retiform endothelial hyperplasia (RFEH), a distinct vascular pathology, has recently been identified as a potential late-onset complication of GKRS for arteriovenous malformations. This report describes the first case of RFEH developing within a vestibular schwannoma 30 years after GKRS, necessitating surgical enucleation. OBSERVATIONS: A 54-year-old man was diagnosed with a right-sided vestibular schwannoma at age 23 years and underwent GKRS with a dose of 13.6 Gy (40% isodose). The tumor exhibited long-term regression; however, at the 28-year follow-up, MRI revealed a progressively enlarging low-intensity T2-weighted ring within the tumor. At 30 years post-GKRS, resection was performed, revealing a well-circumscribed, encapsulated, hematoma-like lesion within the yellowish schwannoma. Histopathological analysis showed an old hematoma with irregular endothelium-lined cavities lacking fibrous or smooth muscle layers, confirming the diagnosis of RFEH. The surrounding tumor cells were S-100 protein-positive with a Ki-67 labeling index of 3%. LESSONS: RFEH can develop within a vestibular schwannoma decades after GKRS, extending its known association beyond arteriovenous malformations. Recognizing this unique complication is critical for optimal patient management and surgical decision-making. https://thejns.org/doi/10.3171/CASE2537.