The patient was hospitalized because of unbearable headache and was examined for intracranial occupancy, brain tumor accompanied by hydrocephalus. The family members were confused; why would hydrocephalus appear in a brain tumor? Can hydrocephalus be cured? Can a brain tumor be cured? The secretion and absorption of normal cerebrospinal fluid is often in a dynamic balance. In pathological conditions, excessive intracranial cerebrospinal fluid accumulates in the ventricular system, or sometimes in the subarachnoid space, mostly with increased intracranial pressure, called hydrocephalus. In question 19, we have talked about the production and circulation of cerebrospinal fluid. Hydrocephalus occurs when the cerebrospinal fluid circulatory system is infarcted at any site, with hydrocephalus caused by brain tumors being the most common. The areas prone to hydrocephalus are: Interventricular foramen: Cerebrospinal fluid from the ventricles enters the third ventricle through this foramen. When a tumor occurs in this area, this foramen becomes blocked and one of the lateral ventricles becomes enlarged. The common tumor is glioma. The third ventricle: It is the necessary pathway for cerebrospinal fluid circulation in both lateral ventricles. When obstruction occurs, I will see symmetrical enlargement of both ventricles. The common tumors are glioma and craniopharyngioma. Pineal region: The cerebrospinal fluid in the third ventricle flows into the fourth ventricle through the conduit of the midbrain, and the pineal region is located just behind the conduit. Usually both lateral ventricles and the third ventricle are enlarged at the same time. Common tumors include gliomas and germ cell tumors. Posterior cranial fossa: Includes the fourth ventricle, cerebellopontine angle, occipital foramen, and slope. It can cause enlarged effusion in the ventricles above the fourth ventricle. Sensory tumors include glioma, auditory neuroma, chordoma, meningioma, etc.