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Brain tumors are the second most common childhood cancer (after leukemia) and the most common solid organ tumors of childhood.  This child with a heterogeneous, well-circumscribed cerebellar mass most likely has a pilocytic astrocytoma, the most common type of brain tumor in children.

Pilocytic astrocytomas are low-grade gliomas that occur most frequently in the cerebellum.  Patients typically have headaches and cerebellar findings (eg, loss of balance, incoordination).  Imaging (shown above) reveals a well-demarcated lesion comprised of cystic and solid components.  Microscopic examination shows pilocytic astrocytes with bundles of glial fibrillary acidic protein (GFAP)-positive hairlike processes and classic Rosenthal fibers (eosinophilic intracytoplasmic inclusions).  Due to the slow-growing (benign) nature, the majority of patients can be treated with surgical resection and have a favorable prognosis.

(Choice A)  Ependymomas are rare tumors arising from the ependymal lining of the ventricle.  They commonly occupy the 4th ventricle and can cause hydrocephalus by obstructing the flow of cerebrospinal fluid.  Perivascular pseudorosettes are present on microscopy.

(Choice B)  Glioblastoma multiforme, a tumor arising from GFAP-positive astrocytes, occurs most frequently in adults and is typically located within the cerebral hemispheres (often crossing the corpus callosum).  Pseudopalisading pleomorphic cells with areas of necrosis are seen on microscopy.

(Choice C)  Medulloblastoma is the most common malignant childhood brain cancer.  It is located exclusively in the cerebellum; however, it is not cystic like pilocytic astrocytoma.  Imaging reveals a solid lesion that can compress the 4th ventricle, causing hydrocephalus.  It is composed of small cells with hyperchromatic nuclei that form Homer Wright rosettes (groupings of cells surrounding the neuropil).

(Choice D)  Oligodendrogliomas, rare in children, are frequently calcified, well-circumscribed masses located in the frontal lobe.  Microscopy shows uniform cells with a "fried egg" appearance (ie, round nuclei with clear cytoplasm) surrounded by anastomosing capillaries arranged in a "chicken-wire" pattern.

Educational objective:
Pilocytic astrocytomas are the most common brain tumors in children.  They frequently arise in the cerebellum and can be differentiated from medulloblastomas by the presence of both cystic and solid components on imaging.