The sections show a densely cellular tumour. It forms diffuse sheets as well as some perivascular pseudorosettes. Homer-Wright rosettes are absent. The tumour cells have increased N/C ratio with enlarged round nuclei, small nucleoli and scanty cytoplasm. There is no nuclear moulding. Small numbers have shrunken nuclei. No necrosis is seen. There are about 10 mitoses per 10 high power fields. The tumour cells are synaptophysin positive. There is very focal staining for neurofilament. The Ki-67 index is about 20%. GFAP and EMA are negative.
FINAL DIAGNOSIS:
Pineal parenchymal tumour with intermediate differentiation (WHO Grade III).