In 2016, the World Health Organization (WHO) eliminated “oligoastrocytoma” from their classification of CNS tumors, in favor of an integrated histologic and molecular diagnosis. In the 2021 classification, oligodendrogliomas are defined by mutations in isocitrate dehydrogenase (IDH) with concurrent 1p19q codeletion, while astrocytomas are IDH-mutant tumors, usually with ATRX loss.

Case #1: In 2007, a 24-year-old man presented with focal seizures and a left parietal tumor with pathology histologically described as astrocytoma, but molecular studies consistent with an oligodendroglioma, IDH1-mutant, 1p19q-codeleted, CNS WHO grade 2. Fourteen years later, he underwent epilepsy surgery, and pathology revealed an astrocytoma, IDH-mutant, CNS WHO grade 3, with partial ATRX loss on immunohistochemistry, and mutations of IDH, ATRX, TP53, and the TERT promoter confirmed by NGS sequencing. FISH confirmed 1p19q co-deletion in sections with retained ATRX expression.

Case #2: In 2017, a 36-year-old woman presented with complex partial seizures and a right frontal tumor. She underwent open biopsy and pathology was consistent with an oligodendroglioma, IDH1-mutant, and 1p19q-codeleted, CNS WHO grade 2, with TERT promoter mutation. She completed standard radiation and chemotherapy and two years later underwent epilepsy surgery for continued debilitating seizures. Pathology revealed an astrocytoma, IDH-mutant, with ATRX loss, CNS WHO grade 2.

To our knowledge, ten cases of dual-genotype IDH-mutant glioma (true oligoastrocytoma) have been reported in the literature. We present two cases in which this distinct molecular phenotype is present in a tumor in the same location with surgeries at two points in time, both with 1p19q codeletion at biopsy and ATRX loss at the time of resection. Whether this represents a true “collision tumor” with sampling bias from the original biopsy, or genetic switching over time is not known, but the co-occurrence of ATRX loss and TERT promoter mutations support a diagnosis of dual-genotype IDH-mutant glioma.