Silva, A. I.

Deletion (DL) of chromosome 1p is common in oligodendroglial (OLG) tumors and is prognostically significant. We studied 1p DL (FISH method) and morphologic/ultrastructural (EM)/immunohistochemical (IHC) features of OLG, in 22 tumors. Six of 12 gr II, 6 of 8 gr III and 1 of 2 gr IV tumors were 1p DL. On EM, all tumors contained round cells with lucent to organelle (Golgi, mitochondria, microtubules)‐rich cytoplasm and few or no intermediate filaments (IF); 3 features (myelin figures, whorls of IF, synapses) suggested OLG, astrocytic and neuronal status, respectively. OLG features were seen in 3 DL gr II OLG, 2 nondeleted (NDL) gr II and in 1 NDL gr III tumor. Astrocytic feature was seen in 2 DL gr II, 4 DL gr III, and in all 9 NDL tumors. Neuronal features were seen in 3 DL gr II OLG, 2 NDL gr II and in 1 NDL gr III tumor. Overall, the 3 phenotypes were present in 23 , 46 and 23 % in of DL OLGs and in 44, 100 and 33 % of NDL tumors, respectively. IHC for synaptophysin (SYN), neurofilament protein (NFP), chromogranin, and Neu N were performed. Reactivity(s) were noted in 77% of all tumors, 71% of pure OLG, 100% of mixed tumors, 69% of DL tumors, and 89% of NDL tumors. Neu‐N and SYN were most often seen. Of all tumors with OLG, astrocytic and/or neuronal EM, 86, 71 and 83% showed staining, respectively. In summary, a) ND is common in OLG, both pure and mixed, and b) OLG tumors with and without a glioneuronal EM +/− IHC phenotype may both be 1p DL. Mayo Clinic funded.