Giant cauda equina neuroendocrine tumor with superficial siderosis: illustrative case.

backgroundCauda equina neuroendocrine tumor (CENET) is a rare spinal neuroendocrine tumor, and its coexistence with superficial siderosis (SS) is even more rare. CENETs are usually small and most commonly located below L2, and their imaging often overlaps with that of myxopapillary ependymomas or schwannomas, thus complicating their preoperative differentiation. The coexistence of SS and intratumoral fluid-fluid level (FFL) in CENET has not been documented. OBSERVATIONS: The authors describe a giant thoracolumbar CENET (T12-L3) with intracranial SS in a patient with progressive paraparesis and bladder dysfunction. MRI revealed a hypervascular intradural extramedullary mass with an intratumoral FFL and neuraxial hemosiderin deposition, suggesting recurrent hemorrhage. Because of dense cauda equina adhesions, function-preserving subtotal resection was performed under intraoperative neurophysiological monitoring, resulting in marked neurological improvement. LESSONS: In CENETs, the coexistence of SS, intratumoral FFL, and marked hypervascularity on preoperative imaging may indicate a chronic hemorrhagic phenotype with severe adhesions to the cauda equina, particularly in large multisegmental tumors. In such cases, nerve-sparing subtotal resection planned with preoperative vascular assessment and performed under intraoperative neurophysiological monitoring can be an appropriate strategy to balance tumor control with preservation of neurological function when safe dissection planes are limited. https://thejns.org/doi/10.3171/CASE251015.