Fishing For 1p19q Codel In Oligodendroglioma

Together with isocitrate dehydrogenase (IDH) mutation, co-deletion of 1p19q (1p19q codel) is a prerequisite for diagnosis of oligodendroglioma, making it imperative that histopathology laboratories introduce testing for 1p19q codel. To date there is still no consensus reference range and cut-offs that confirm deletion of 1p or 19q. We embarked on determining our reference range in 11 formalin-fixed, paraffin-embedded non-neoplastic brain tissue using fluorescence in situ hybridisation (FISH) with the Vysis 1p36/1q25 and 19q13/19p13 FISH Probe Kit (Abbott Molecular Inc., USA). At same time we attempted to validate our methodology in 13 histologically-confirmed IDH-mutant oligodendrogliomas. For 1p, percentage cells with deletion (range=8-23%; mean +/- SD = 15.73 +/- 5.50%) and target: control (1p36:1q25) ratio (range = 0.89-0.96; mean +/- SD = 0.92 +/- 0.03) in non-neoplastic brain, differed significantly (p<0.000) from oligodendroglioma (percentage cells with deletion: range = 49-100%; mean +/- SD = 82.46 +/- 15.21%; target:control ratio range:0.50-0.76; mean +/- SD = 0.59 +/- 0.08). For 19q, percentage cells with deletion (range = 7-20%; mean +/- SD = 12.00 +/- 3.49%) and target:control (19q13/19p13) ratio (range:0.90-0.97; mean +/- SD = 0.94 +/- 0.02) in non-neoplastic brain also differed significantly from oligodendroglioma (percentage cells with deletion: range = 45-100%; mean +/- SD = 82.62 +/- 18.13%; target:control ratio range:0.50-0.78; mean +/- SD = 0.59 +/- 0.09). Using recommended calculation method, for diagnosis of 1p deletion, percentage of cells showing deletion should be >32-33% and/or target:control ratio <0.83. For 19q, percentage of cells showing deletion should be >22% and target:control ratio <0.88. Using these cut-offs all 13 oligodendroglioma demonstrated 1p19q codel.