Development of an undifferentiated sarcoma following internal carotid artery aneurysm stenting: causality or coincidence? Illustrative case

BACKGROUND: Undifferentiated soft tissue sarcomas are rare tumors lacking identifiable differentiation. Primary sarcomas associated with aneurysm sacs are extremely rare, typically involving large vessels. Intracranial involvement is exceptional and poorly documented. OBSERVATIONS: The authors report the case of a patient treated in 2009 for a left paraclinoid internal carotid artery aneurysm (18 × 10 mm) by endovascular stenting (Silk 4.5/30). Eleven years later, he developed headache, somnolence, visual blurring, left ptosis, and anisocoria. CT and CTA showed the known thrombosed aneurysm and an adjacent hypervascular midline lesion. Angiography confirmed complete exclusion with a patent stent and no new aneurysms. MRI revealed a suprasellar-retrosellar multilobulated enhancing mass causing marked brainstem compression. Surgery identified a solid lesion seemingly arising along the thrombosed aneurysm sac, and partial resection was performed. The patient initially recovered but subsequently deteriorated with rapid tumor regrowth and died during hospitalization. Histological analysis showed a grade 3 undifferentiated sarcoma (vimentin +, MDM2+, focal smooth muscle actin -, CAM5.2-, GFAP-). Postmortem examination confirmed origin within the aneurysm sac adjacent to the stent. LESSONS: To the authors' knowledge, this is the first reported undifferentiated sarcoma arising from a stent-treated carotid aneurysm. Although causality cannot be established, it parallels rare stent-associated sarcomas in large vessels and highlights the diagnostic challenges of atypical masses in treated vessels. https://thejns.org/doi/10.3171/CASE26659.