DIAGNOSIS AND DISCUSSION
Diagnosis: Cerebellar neurocytoma (WHO Grade I).
Discussion: Neurocytic neoplasms are generally centered around the lateral ventricle and have a good prognosis upon complete resection. The most prevalent example, central neurocytomas, were first described in 1982  as a tumor that typically grows from the foramen of Monro or the septum pellucidum and involves the lateral or third ventricle; often with calcification. This benign tumor affects young adults and is relatively uncommon; comprising less than 1% of all central nervous system tumors.
Histologically, central neurocytomas are characterized by small round neuronal cells with salt and pepper chromatin and an empty appearing halo which is similar to that seen with oligodendrogliomas. There is modest cytoplasm and an eosinophilic fibrillar background. Immunohistochemical staining is usually positive for synaptophysin and neuron specific enolase and negative for GFAP with a low Ki67 value. Ultrastructurally, they are characterized by dense core vesicles and microtubules and lack intermediate filaments.
Since central neurocytomas are usually intraventricular or periventricular growths, they often present with signs and symptoms of obstructive hydrocephalus such as headache, seizures, nausea, vomiting, papilledema, and memory and visual disturbances. A review of their location demonstrates a large percentage in the lateral ventricle as well as a significant portion in the third ventricle . Other sites have included the spinal cord  as well as the cerebral hemispheres, thalamus, pons, amygdala, pineal gland, and retina .
Neurocytic lesions in the cerebellum are less common and this variant is a more recently described entity. A majority of the neurocytic lesions in the cerebellum are liponeurocytomas. These were first described in 1978  and contain neuronal and glial differentiation with lipomatous areas. To our knowledge, there have been only two reports of neurocytomas (without lipomatous areas) in the cerebellum [2,3].
In a study of 20 cerebellar liponeurocytoma patients , the mean age was 51 years with a range of 24 to 77 years with no sex predilection. The clinical course after surgery was good for the majority of patients with six receiving postoperative radiotherapy (50-59 Gy) and with five tumors recurring 8 to 12 years after the first surgery. The cerebellar liponeurocytomas demonstrated a monotonous pattern of isomorphic round cells with focal accumulations of lipid-laden cells. All of the specimens were positive for synaptophysin with GFAP being positive only focally. The MIB-1 proliferation index was between 1 and 6% for most of the cases.
Brandis and colleagues  and Enam and colleagues  have reported their experience with similar cerebellar neurocytomas that lacked lipomatous areas. Their cases involved a male infant and a 46 year old woman respectively. These tumors exhibited similar histologic features and immunohistochemically demonstrated synaptophysin positivity, only scattered GFAP positivity, and a low MIB-1 labeling index (5% or less). There was no mention of a recurrence in the 46 year old woman and there was no recurrence in the infant after 6 years.
The tumor from our case is particularly interesting, and distinct, for several reasons. It is a neurocytoma located in the cerebellum and, unlike previously reported liponeurocytomas in the cerebellum, it lacks a lipomatous component. Additionally, this lesion has recurred twice with no cytological atypia and very little change in morphology and immunophenotype. Also, this neurocytoma was originally diagnosed as a pilocytic astrocytoma. Although both lesions can present in pediatric patients and have good prognoses with complete resection, they represent very different entities physiologically.
We would like to emphasize that neurocytomas should be in the differential for lesions in the posterior fossa. These tumors are characterized by small round cells with salt and pepper chromatin with modest cytoplasm with an eosinophilic fibrillary background. There are often clear halos surrounding these cells. As such, neurocytomas can be misdiagnosed as clear cell ependymomas, lymphomas, oligodendrogliomas, or neuroblastomas.
Acknowledgments The authors would like to thank Dr. Linda Gray for help with the radiology interpretation, Steve Conlon for help with the images, and Jim Burchette for assistance with immunohistochemistry.
Contributed by Gregory T. Ray, MD, PhD, Shanti S. Agarwal, MD, Roger E. McLendon, MD