Delayed but Dangerous: Chronic Encapsulated Expanding Hematoma as a Reversible Cause of Steroid-Resistant Extensive Edema Following Stereotactic Radiosurgery for Cerebral Arteriovenous Malformation.

Chronic encapsulated expanding hematoma (CEEH) is a rare, delayed complication of stereotactic radiosurgery (SRS) for brain arteriovenous malformations (AVMs), often masquerading as radiation necrosis or tumor recurrence. We report the case of a 61-year-old man who developed progressive right hemiparesis and extensive steroid-resistant vasogenic edema 15 years after embolization and multisession SRS for a ruptured left parietal AVM, which had been previously confirmed as completely obliterated. Initial imaging showed a small enhancing lesion with surrounding edema, suspected to be radiation necrosis. Over the next two years, the lesion expanded with persistent edema despite corticosteroids and decompressive craniectomy. Follow-up MRI revealed classic hemorrhagic features, including a peripheral hypointense rim on T2*-GRE and SWI sequences, raising suspicion for CEEH. Surgical resection confirmed the diagnosis histopathologically and was followed by rapid clinical and radiological improvement. This case underscores the importance of considering CEEH in patients presenting with delayed symptoms and steroid-resistant edema after SRS, even when the lesion appears small. Early surgical recognition and intervention can be both diagnostic and therapeutic, preventing further morbidity.