Intraoperative Neuropathology for non-Neuropathologist
Case 3 -
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|
|Ganglion cell tumors|
|Diffuse ||vs ||localized|
|Fibrillary astrocytoma ||PXA|
|Lymphoma ||Ganglion cell tumor|
| ||Ependymoma, subependymoma or choroid plexus tumor|
|Reactive gliosis (particularly around infarcts or slow growing tumors)|
|Ganglion cell tumors|
|Occasionally seen in ependymoma/subependymoma|