MEDizzy - A 5-year-old boy presents with projectile vomiting and

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Nervous System

A 5-year-old boy presents with projectile vomiting and progressive ataxia. Workup finds obstructive hydrocephalus due to an infiltrative tumor located in the posterior fossa and originating from the midline of the cerebellum. The histologic report shows a tumor composed of a patternless proliferation of small cells with little cytoplasm. What is the best diagnosis?

ExplanationA tumor composed of a patternless proliferation of small cells with little cytoplasm is a characteristic histologic appearance of a medulloblastoma. Sometimes the tumor cells may have rosettes (Homer Wright rosettes) around a central fibrillary core. A medulloblastoma is a poorly-differentiated malignant tumor of children that is exclusively found in the cerebellum. They usually arise in the midline of the cerebellum (the vermis) but in adults, where the incidence is much less than in children, they are more apt to arise in the cerebellar hemispheres in a lateral position. Medulloblastomas grow by local invasive growth and may block cerebrospinal fluid circulation (CSF block) via compression of the fourth ventricle. Recently, aggressive treatment with the combined modalities of excision, radiotherapy, and chemotherapy has improved survival rates. A medulloblastoma is considered to be a type of primitive neuroectodermal tumor (PNET), which refers to a group of malignant embryonal tumors that can be found at sites within or outside of the central nervous system. An example of a PNET located outside of the CNS is Ewing sarcoma of bone.