Pilocytic astrocytoma

Case contributed by A.Prof Frank Gaillard


Recent immigrant from Africa. Headaches and disequilibrium.

Patient Data

Age: 30 years
Gender: Male

A peripherally enhancing lesion in the right lower pons, extending into the cerebellopontine angle abuts but does not appear to involve the right cerebellar hemisphere; the right vertebral artery passes anteriorly to it. There is a small amount of adjacent hypoattenuation in the pons, in keeping with edema. There is localized mass effect with a small degree of effacement of the right pre medullary cistern secondary to the mass. Normal fourth ventricle caliber. 


Within the right cerebellopontine angle a heterogeneous peripherally enhancing multilobulated mass. Centrally there is non-enhancement and increased T2 signal without diffusion restriction and only partial flair suppression. Some areas demonstrate low T2 signal. The mass is clearly separate from the cisternal seventh and eighth cranial nerves although does abut their nerve root entry zone. The ninth and tenth cranial nerves are closely related to this lesion and difficult to separate from it. What is probably a loop of the posterior inferior cerebellar artery is seen related to the lateral surface of the mass. The adjacent brain stem and inferior cerebellar peduncle demonstrate high T2 signal. This appears to have positive mass effect and is somewhat mass like in appearance.

Along the superior surface of the left cerebellar hemisphere is a linear region off volume loss with widening of the cerebellar folia which is quite striking on axial imaging but of uncertain significance.

Conclusion: Right cerebellopontine angle mass is uncertain etiology. The main differential is between an exophytic enhancing tumor, such as a pilocytic astrocytoma, or a tuberculoma. Although the latter is more likely in this demographic (the patient is from Africa) the overall morphology favors a tumor.


Sections show a 2mm piece of mildly hypercellular tumor composed of mildly enlarged, oval, hyperchromatic nuclei with small nucleoli. There are scattered Rosenthal fibers and eosinophilic granular bodies present. A single mitosis is identified. No necrosis or microvascular proliferation is seen.

Immunohistochemical results show tumor cells stain:

  • GFAP: Positive
  • Nestin: Positive
  • NogoA: Negative
  • IDHl R132H: Negative
  • ATRX: Positive
  • p53: Positive
  • p16: Positive
  • Co99: Positive
  • EMA: Negative
  • BRAFV60oE: Negative
  • Topoisomerase proliferation index: 1%

BRAF testing

  • BRAF mutation c.1799T>A (V60oE) NOT DETECTED
  • BRAF-KIAA1549 translocation NOT DETECTED
  • H3F3A (H3.3A) MUTATION STATUS Codons 27 and 34 of the H3F3A gene are sequenced by pyrosequencing.

DIAGNOSIS: pilocytic astrocytoma (WHO Grade I). 

Case Discussion

After having carefully evaluated the location of a mass, which often is the most crucial part of interpretation (e.g. in this case, realizing that the mass is not related to vestibulocochlear nerve excludes and acoustic schwannoma form the differential), one is left with a variety of diagnoses and other factors come into play. How much weight to put on demographic information (e.g. prior medical history, ancestry, travel) and how much to rely on more generic stats is a challenge and tests Bayesian thinking.  In this case, the fact that the patient was a recent immigrant from Africa was a red-herring. 

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Case information

rID: 59524
Published: 22nd Apr 2018
Last edited: 14th Aug 2019
Inclusion in quiz mode: Included

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