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BACKGROUND The 2016 WHO update of CNS tumours defines oligodendroglioma as a diffusely infiltrating glioma with IDH 1 or IDH 2 mutation and co-deletion of chromosomal arms 1p and 19q. We evaluated the role of 1p19q testing by Fluorescence in situ hybridization (FISH) in the diagnosis and correct classification of CNS tumours in accordance with WHO 2016 classification and exclusion of morphological mimickers of oligodendrogliomas. MATERIAL AND METHODS A retrospective analysis of CNS tumours was carried out that were tested for 1p19q co-deletion by FISH over a period of fourteen months at our institute. Correlation with tumour morphology as well as other molecular tests (IDH mutation and MGMT) where available was then done. A tumour was considered to have 1p or 19q deletion when the 1p probe to 1q probe ratio (1p/1q) or the 19q probe to 19p probe ratio (19q/19p) was &lt;0.80. RESULTS A total of 125 cases underwent FISH testing for 1p19q co-deletion over a period of last fourteen months at our institute with 74 male patients and 51 women. Thirty-eight of the 125 evaluated patients demonstrated 1p19q co-deletion on FISH. These included 25 oligodendrogliomas, 9 astrocytomas (Grade II/III) and 4 cases of oligoastrocytomas by morphology. Out of these cases, 21 cases had a co-existent IDH mutation, while two cases were IDH wild type and for 15 cases the status was unknown. Rest of the 87 patients were negative for 1p19q co-deletion and included cases of oligodendroglioma, other gliomas (pilocytic astrocytoma, oligoastrocytoma, glioblastoma) and few glioneuronal tumours (ganglioglioma, dysembryoplastic neuroepithelial tumour and central neurocytoma). CONCLUSION 1. Analysis of 1p19q co-deletion by FISH plays an important role in correctly identifying cases of oligodendroglioma as shown in our study. This is important as it is both a predictive as well as prognostic biomarker 2. In cases with co-deletion but IDH wild type status, the 1p19q status should be confirmed by other methods such as comparative genomic hybridization to confirm the presence of whole chromosome loss.3. FISH analysis also helps in excluding morphologic mimics of oligodendroglioma such as glioneuronal tumours and pilocytic astrocytomas in young patients. 4. However, in a resource limited country like ours, a good morphological diagnosis should still form the basis for FISH and other molecular tests to avoid unnecessary testing.

neuro-oncology

P11.53 Genotype trumps phenotype: Oligodendroglioma and oligodendroglioma like tumors- A 1p19q fluorescence in situ hybridization based study at a tertiary referral centre in north India