Segmental arachnoiditis after posterior fossa operation: differentiation from metastatic tumor deposit

A 16-year-old boy was noted to have an apparent intradural sacral mass 6 weeks after the excision of a 4th ventricle ependymoma. Although spinal axis radiation was considered, it was decided to make a definitive tissue diagnosis. At sacral laminectomy, only segmental arachnoiditis was found. The implications of this finding are the subject of this case report.