Spinal intradural extramedullary atypical neurofibromatous neoplasm of uncertain biological potential: illustrative case.

backgroundAtypical neurofibromatous neoplasm of uncertain biological potential (ANNUBP) is a rare type of peripheral nerve sheath tumor that exhibits at least two of the following features: atypia, loss of neurofibroma architecture, hypercellularity, and a mitotic index of > 1 per 50 high-power fields (HPF) and < 3 per 10 HPF. OBSERVATIONS: The authors report the first case of a spinal intradural ANNUBP, the first from the Philippines, and the second in a patient without neurofibromatosis type 1 (NF1). The patient was a 50-year-old man who presented with a 1-year history of back pain and progressive bilateral lower extremity weakness and sensory loss. His motor strength was 0/5 in both lower extremities, and the last normal sensory level was at T7. Imaging showed a T6-7 intradural extramedullary lesion with a contrast-enhancing rim. The patient underwent laminectomy and gross-total tumor excision, but his neurological function did not improve. Histopathological and immunohistochemical testing revealed the diagnosis of ANNUBP. LESSONS: This report describes a rare case of a spinal intradural extramedullary ANNUBP, a lesion typically located outside the CNS. This case adds to the limited body of literature on ANNUBP, emphasizing its potential to occur outside typical locations and in non-NF1 patients. https://thejns.org/doi/10.3171/CASE26258.