Presentation
Ataxia
Patient Data
A well circumscribed rounded FLAIR hyperintense lesion is centered just to the right of midline in the cerebellar vermis involving the roof of the fourth ventricle. The T2 signal is slightly high, higher than grey matter. Prominent vessel traverses the right lateral aspect of the lesion and demonstrating intrinsic high T1 signal/low T2 signal within the lesion but normally enhancing outside the lesion. No intralesional enhancement. No cystic changes. No intralesional signal loss on gradient-echo sequence to suggest calcification or hemorrhage. There are patchy areas of mildly elevated/intermediate CBV within the mass. MR spectroscopy demonstrates elevated choline and depressed NAA, but no large lactate peak.
Conclusion: Vermian non-enhancing solid lesion is unusual in appearance. The differential is primarily between a medulloblastoma (lack of enhancement suggests genetic group 4) and pilocytic astrocytoma (lack of enhancement is unusual).
Paraffin sections show a single fragment of glial tissue which has a mild increase in cellularity with occasional atypical astrocytes which in some areas show a prominent fibrillary cytoplasm. No mitotic figures or significant nuclear pleomorphism or necrosis is seen.
There are other fragments of tissue which have neoplastic tissue which has an increase in cellularity. In some areas the tumor cells have an oligodendroglial-like appearance and in other areas the cells show varying cellularity with a fibrillary appearance and some areas suggestive of Rosenthal fibers. No significant nuclear pleomorphism, mitotic figures, necrosis or vascular proliferation is seen.
Immunohistochemistry results show tumor cells stain:
- GFAP: Positive
- Nestin: Positive (low)
- NogoA: Negative
- IDH-1 R132H: Negative (non-mutated)
- ATRX: Positive (non-mutated)
- p53: Negative
- p16 CDKN2A: Positive
- BRAF: Negative
- Topoisomerase labeling index: approximately 1-2%
FINAL DIAGNOSIS: 1-3. Brain, posterior fossa lesion: Pilocytic astrocytoma (WHO Grade I).
Case Discussion
This is a unusual case in that the tumor shown to be a pilocytic astrocytoma is non-enhancing. It shows how difficult it can be, in atypical cases, to come to a pre-operative diagnosis.
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