Thoracic spinal cord myelopathy from complex arachnoid web formation/exacerbation after palliative radiation therapy for spinal metastasis: illustrative case.

backgroundRadiation therapy is a common treatment for spinal metastases. However, its effects on extramedullary intrathecal structures are poorly understood. Arachnoid webs are rare structural abnormalities that can cause significant neurological symptoms due to spinal cord compression. While radiation-induced myelopathy is a rare known complication of radiation therapy, the potential relationship between radiation treatment and the formation or exacerbation of arachnoid webs has not been previously reported. OBSERVATIONS: The authors present the case of a 64-year-old female with breast cancer and spinal osseous metastases who underwent palliative radiation therapy. Six months after radiation therapy, she developed progressive myelopathy refractory to conservative treatments. Imaging revealed dorsal cord indentation and edema with findings suggestive of a compressive arachnoid web. Surgical intervention confirmed an extensive complex arachnoid web spanning T5-8. Fenestration of the web resulted in immediate intraoperative improvement in somatosensory evoked potentials and significant postoperative neurological recovery over 8 months. LESSONS: This case highlights the importance of considering structural abnormalities, such as arachnoid webs, in patients with progressive myelopathy following spinal radiation therapy, even when doses are within safe limits. Myelography may be helpful for characterizing intrathecal changes in this context. Further research is needed to explore the relationship between radiation therapy and arachnoid pathology. https://thejns.org/doi/10.3171/CASE25733.