Spinal endodermal cyst resembling an arachnoid cyst in appearance: Pitfalls in intraoperative diagnosis of cystic lesions

Background: Surgical treatment of endodermal cysts requires total removal of the cyst wall during the first operation to prevent recurrence. Therefore, intraoperative pathological diagnosis plays an important role in determining the optimal surgical strategy. We present a rare case of a spinal endodermal cyst and discuss its diagnostic difficulty during the intraoperative pathological examination. Case Description: An 18-year-old male presented with progressive paraparesis and precordial oppression. Magnetic resonance (MR) imaging revealed an intradural extramedullary cystic mass having the same signal intensity as cerebrospinal fluid (CSF) without gadolinium enhancement at the T1-T2 level. The preoperative diagnosis was an endodermal or arachnoid cyst. The patient underwent surgery. An intraoperative frozen section showed a cyst wall consisting of loose, thin, fibrous tissue intermittently covered by flattened epithelium. The diagnosis was an arachnoid cyst. Accordingly, partial resection of the cyst wall was performed to create CSF communication between the cyst and subarachnoid space. However, the postoperative pathological diagnosis from permanent sections was an endodermal cyst, which was lined with ciliated columnar epithelium that was immunopositive for cytokeratin and epithelial membrane antigen. Subsequent paraffin embedding and immunostaining of the intraoperative frozen sample also confirmed patchy cytokeratin expression by all flattened epithelial cells. The patient's cyst had refilled 10 months after surgery, and he subsequently underwent fenestration of the cyst wall and placement of a cyst-subarachnoid shunt. Conclusion: Examination of multiple samples from multiple sites or intraoperative immunostaining of frozen sections is recommended for accurate intraoperative diagnosis of endodermal cysts.