Cerebellar medulloblastoma with multiple differentiation in a dog.

A 4-year-old female, spayed Border Collie Dog was brought to the Veterinary Medical Teaching Hospital for evaluation of a progressive head tilt and ataxia that were unresponsive to therapy. Neurologic examination localized a right-sided lesion. The owner refused additional diagnostic tests, and necropsy was performed after euthanasia. Gross findings included atrophy of the temporal muscles and a moderately well delineated, 2.5- x 1.5- x 1.0-cm, gray soft-tissue mass compressing the right cerebellar hemisphere and dorsal hindbrain, resulting in massive dilatation of the lateral, third, and fourth ventricles and hydrocephalus. Histologic examination revealed two distinct features: undifferentiated, primitive, polygonal to fusiform cells with typical morphologic characteristics of medulloblastoma and interspersed areas containing myelinated axons and cells with glial and neuronal differentiation. Immunohistochemical examination confirmed the presence of primitive neuroepithelium and cells with glial and neuronal differentiation.