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 tumour 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 fibres. No significant nuclear pleomorphism, mitotic figures, necrosis or vascular proliferation is seen.
Immunohistochemistry results show tumour 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 labelling index: approximately 1-2%
FINAL DIAGNOSIS: 1-3. Brain, posterior fossa lesion: Pilocytic astrocytoma (WHO Grade I).