Case 3 -

Pilocytic Astrocytoma Timothy Smith and Cynthia Welsh

Tumors in children are most commonly located in the posterior fossa (70% in the posterior fossa versus 30% above the tentorium). The most common glioma in children is the pilocytic astrocytoma (a.k.a juvenile pilocytic astrocytoma) of the cerebellum. The tumor also occurs in the diencephalon, optic nerves and uncommonly spinal cord. The tumor rarely also occurs in adults. They are very low grade tumors, WHO grade 1, and tend not to recur following surgery or may take many years (up to 20) to recur. Clinically the tumors produce symptoms appropriately related to site, such as ataxia or visual symptoms, and are often adjacent to a macrocyst ("cyst with mural nodule" on scans) in the cerebellum. Pilocytic astrocytomas in other areas cause a swollen optic nerve, or spinal cord, or an exophytic lesion on the side of the medulla. They are distinct in border with surrounding tissue, enhance with contrast (unlike low grade fibrillary astrocytomas), and often display high T2 signal intensity. The surgeon may find a distinct plane between tumor and adjacent neuropil. At the time of frozen, our first clue is the age of the patient, usually under 20, often under 10 years. In children, in the cerebellum, the chief diagnostic choices are pilocytic astrocytoma, medulloblastoma, and less commonly ependymoma. The pilocytic tumor smears well and cuts well. Smears show a tumor with prominent fibrillar, hair-like astrocytic processes and background, and often Rosenthal fibers and/or eosinophilic granular bodies. The cellularity and architectural arrangement of cells is abnormal on both squash preps and frozen sections. The tumor cells have elongate bipolar cytoplasm which may be obscured on the frozen section, and variably sized nuclei. Mitoses are usually difficult to find, but finding mitoses in the presence of the other typical features does not void the diagnosis. The cells can be arranged in elongate fascicles and loosely around vessels. Increased vasculature (usually hyalinized) and some limited nuclear atypia are expected in this tumor, so don't fall into the trap of calling it high grade. The tumor is variably biphasic, having solid elongate patterns and microcysts. Rosenthal fibers are found in the solid areas. The microcysts are easily dismissed as artifact if you have problems with ice crystals. Reactive, gliotic, and inflammatory processes can superficially resemble a low grade astrocytoma. This is a recognized trap that has captured every well trained neuropathologist at least once. For example, the gliotic zone around an abscess or metastasis, and gliosis adjacent to a craniopharyngioma or hemangioblastoma, can have prominent piloid processes, nuclear prominence and Rosenthal fibers that can be very confusing. The MRI appearance and other clinical information can help avoid pitfalls. In the frozen section and squash prep, the uniform cell density and foci of inflammation can help guide one toward a reactive process. If you find you have more Rosenthal fibers than cells, you should think of gliosis before tumor. Cyst with mural nodule Pilocytic Hemangioblastoma Ganglion cell tumors PXA Diffuse vs localized Fibrillary astrocytoma PXA Oligodendroglioma Pilocytic Lymphoma Ganglion cell tumor Ependymoma, subependymoma or choroid plexus tumor DNET Hemangioblastoma Neurocytoma SEGA Biphasic tumors Pilocytic Gliosarcoma Schwannoma Rosenthal fibers Reactive gliosis (particularly around infarcts or slow growing tumors) Pilocytic Ganglion cell tumors Occasionally seen in ependymoma/subependymoma