IDH1 and astrocytoma (excluding glioblastoma): In IDH-mutant astrocytomas, chromosomal instability is an established poor prognostic factor.39 Its prognostic value in AO remains to be fully established, yet it has been associated with more frequent contrast-enhancement, larger tumor volume, and a poorer prognosis.23,38 Chromosome 9p loss and CDKN2A deletion have been shown to be important poor prognostic factors in AO.21 Herein, there was a trend toward more frequent 9p loss in STS compared to CS, but this trend was not statistically significant, possibly because of the small sample size.