The sections show features of a moderately cellular glial tumour. In some areas (50%), the appearances are typical for those of pilocytic astrocytoma. Scattered bipolar astrocytes are seen in fibrillary background. These cells have elongated and hyperchromatic nuclei. There are many associated Rosenthal fibres. In specimen 4 (50%), the tumour forms perivascular pseudorosettes, in which the bipolar cells radiate from the central blood vessels. The background is prominently myxomatous. Tumour is also present in the subarachnoid space. The features here resemble those of pilomyxoid astrocytoma. Overall, mitoses are inconspicuous. No endothelial cell hyperplasia or necrosis is present. Cerebral cortex is present in specimens 2 and 3. The neurons appear to show normal lamination and orientation. No dysmorphic or balloon cells are present. Neoplastic ganglion cell component, as in ganglioglioma, is not seen. The tumour cells in the pilomyxoid areas are GFAP, synaptophysin, p53 and MGMT immunostains positive. The Ki-67 index is up to 3%. EMA and IDH-1 immunostains are negative.
DIAGNOSIS: Low grade astrocytoma with mixed features of pilocytic and pilomyxoid astrocytoma.
Comment: The behaviour of these intermediate tumours is unclear and cannot be predicted on histology. It is not known whether they behave more like pilocytic (grade I) or pilomyxoid (grade II) astrocytoma.