A 5-year-old Caucasian boy is brought to the physician's office with recent-onset gait instability and gait ataxia. MRI of the brain shows a midline posterior fossa mass. Biopsy of the mass reveals sheets of primitive cells and many mitotic figures. This patient most likely suffers from which of the following?

Question

A 5-year-old Caucasian boy is brought to the physician's office with recent-onset gait instability and gait ataxia.  MRI of the brain shows a midline posterior fossa mass.  Biopsy of the mass reveals sheets of primitive cells and many mitotic figures.  This patient most likely suffers from which of the following?

Quizplus · Accepted Answer

Gait instability and limb ataxia are symptoms of cerebellar involvement. The mass described indicates that a brain tumor is causing the cerebellar symptoms in this child. Because both pilocytic astrocytomas and medulloblastomas arise in the cerebellum, microscopic findings must be relied on to determine which type of tumor this patient has. Sheets of primitive cells with many mitotic figures described in the vignette indicate a medulloblastoma, not a pilocytic astrocytoma. Medulloblastomas are part of a group called "primitive neuroectodermal tumors" (PNETs). PNETs are composed of sheets of small cells with deeply basophilic nuclei and scant cytoplasm (small, round, blue cells). Abundant mitoses are also seen. Medulloblastomas are undifferentiated and aggressive tumors. The cerebellar vermis is the most common location of a medulloblastoma. Symptoms include signs of increased intracranial pressure (morning headaches, vomiting, and lethargy). Cerebellar dysfunction occurs as the tumor compresses adjacent structures. (Choice A) Pilocytic astrocytoma is the most common brain neoplasm of childhood. Although both pilocytic astrocytomas and medulloblastomas are often located in the cerebellum, they can be differentiated by histology. On microscopic examination of a pilocytic astrocytoma, pilocytic astrocytes and Rosenthal fibers are seen. Pilocytic astrocytomas are low-grade tumors and have a better prognosis than medulloblastomas. (Choice D) Ependymomas are the third most common brain neoplasm found in children. They arise in the walls of the ventriculi and can hamper CSF flow and cause hydrocephalus. On microscopic examination, ependymal cells form gland-like structures called "rosettes." Educational Objective: Medulloblastoma is the second most common brain neoplasm of childhood. It is located in the cerebellum, often at the vermis, and consists of sheets of small, blue cells. Like other "PNET" tumors, medulloblastomas are poorly differentiated and have a bad prognosis.

A 5-Year-Old Caucasian Boy Is Brought to the Physician's Office