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Delayed mass effect (DME) is a rare yet significant long-term complication following endovascular coiling of intracranial aneurysms, often mimicking intracranial neoplasms and leading to diagnostic challenges. This report presents a distinctive case of DME that occurred 3 years after coiling of a posterior inferior cerebellar artery (PICA) aneurysm, characterized by a prolonged latency period, imaging findings suggestive of an evolving mass lesion, and concurrent pulmonary pathology that compounded diagnostic complexity. A 59 year-old female underwent microcatheter-assisted coiling for a PICA aneurysm. Two-year post-procedural angiographic follow-up revealed aneurysm neck recanalization. At 3 years post-embolization, organized intra-aneurysmal thrombosis resulted in a prominent peribrainstem mass effect, manifesting as progressive brainstem compression symptoms. This case highlights that DME, as a rare but serious long-term complication of aneurysm coiling, warrants heightened clinical awareness. Its management necessitates early detection and personalized treatment strategies based on a comprehensive assessment of the patient's overall condition and the specific morphological features of the aneurysm.