Usefulness of smears in intra‐operative diagnosis of newly described entities of CNS

The recent edition of World Health Organisation (WHO) Classification of Tumours of the Central Nervous System has incorporated a substantial number of important changes. It has recognised several new entities, many of which are rare. Intra‐operative diagnosis of these tumours can be difficult with the freezing artefact that often cripples brain frozen sections. In many instances intra‐operative smears are extremely useful adjuvants in neuropathological diagnosis. In this article, we describe intra‐operative smear findings of three of the newly described tumours. Their characteristic cytologic features are illustrated along with differentiating features from the common mimics, together with a general approach to brain smears. The entities we discuss here are papillary glioneuronal tumour, papillary tumour of the pineal region and angiocentric glioma. All three tumours share at least focal pseudo‐papillary/vasculocentric architecture. Smears from papillary glioneuronal tumour demonstrated dual population of cells in a neuropil background, whereas papillary tumour of the pineal region and angiocentric glioma comprise a single population of cells. These two tumours can further be differentiated based on their cell morphology and background.