A retroclival endodermal cyst mimicking an arachnoid cyst: illustrative case.

backgroundIt is important to differentiate arachnoid cysts from endodermal cysts in the posterior cranial fossa because they require different surgical treatments. Arachnoid cysts generally require fenestration. Meanwhile, in endodermal cysts, complete cyst resection is preferred to prevent recurrence. Despite the use of MRI and intraoperative findings, misdiagnosis can occur. Herein, the authors report a retroclival endodermal cyst that was initially suspected to be an arachnoid cyst based on pre- and intraoperative findings but was pathologically diagnosed as an endodermal cyst postoperatively. OBSERVATIONS: A 23-year-old female patient presented with a cystic lesion on the anterior brainstem. MRI showed that the cystic contents and CSF were similar in terms of signal. The cyst was suspected to be an arachnoid cyst. However, it continued to increase in size. Craniotomy and fenestration were performed, and the intraoperative findings were consistent with an arachnoid cyst. Therefore, intraoperative pathological examination was not performed, and the cyst wall was partially resected. The diagnosis of an endodermal cyst was confirmed via postoperative pathological examination. LESSONS: This case underscores the importance of considering endodermal cysts in the differential diagnosis of retroclival lesions even if the imaging and intraoperative findings are indicative of an arachnoid cyst. https://thejns.org/doi/10.3171/CASE24826.