Is there an optimal sequencing of endoscopic third ventriculostomy and stereotactic radiosurgery for brain metastasis with secondary obstructive hydrocephalus? Illustrative case.

backgroundSecondary hydrocephalus caused by tumor obstruction of the aqueduct leads to anatomical distortion of the third ventricle (3V) and the surrounding structures. This report quantifies and discusses the dosimetric impact of hydrocephalus in stereotactic radiosurgery (SRS) treatment and discusses the optimal sequencing between radiosurgery and endoscopic third ventriculostomy (ETV). OBSERVATIONS: The authors review a case of intracranial metastasis causing secondary obstructive hydrocephalus that underwent ETV before SRS. Dual SRS plans were made based on the MRI before and after the ETV to evaluate the impact of the ventricular volume on dosimetry. Both plans had the same treatment parameters. The time interval between the MRI studies was approximately 20 days, with a 74% increase in the tumor size. The post-ETV plan revealed a reduced ventricular volume, particularly the 3V, with a noticeable shift in normal brain structures, leading them to receive increased doses. LESSONS: The anatomical changes caused by the ventricular reduction after the ETV associated with the tumor growth were enough to produce a higher dose to the healthy tissue, exposing the patient to a greater risk of adverse radiation effects. Shortening the interval between ETV and SRS can minimize anatomical shifts and the risk of adverse effects while maintaining adequate patient management. https://thejns.org/doi/10.3171/CASE25275.