Which posterior fossa tumor is typically a midline solid tumor with hydrocephalus and is more malignant, possibly involving the cerebellar peduncles?

• A. Cerebellar astrocytoma
• B. Medulloblastoma
• C. Pilocytic astrocytoma
• D. Ependymoma

Answer: (B)

The important idea here is recognizing how pediatric posterior fossa tumors differ by location, growth pattern, and behavior. A midline, solid mass in the cerebellar vermis that causes hydrocephalus and can extend along the cerebellar peduncles fits medulloblastoma. This tumor arises from primitive neuroectodermal tissue in the vermis, so its location is right at the midline. Its aggressive nature explains the hydrocephalus from fourth-ventricle compression and the tendency to invade nearby brain structures such as the cerebellar peduncles. It can also spread through cerebrospinal fluid, which is another hallmark of its malignant behavior.

In contrast, cerebellar astrocytomas and pilocytic astrocytomas are typically cystic lesions with a mural nodule, often located in the cerebellar hemispheres and generally less aggressive. Ependymomas arise from the lining of the fourth ventricle and tend to present with ventricular involvement rather than a midline vermian solid mass that invades peduncles. So the described profile most strongly points to medulloblastoma.