1p/19q co-deleted fibrillary astrocytomas: Not everything that is co-deleted is an oligodendroglioma.

The presence of chromosome 1p/19q co-deletion is one of the hallmark required criteria for the diagnosis of oligodendroglioma, using the 2016 World Health Organization (WHO) Classification of Tumours of the Central Nervous System. Descriptions in the literature of astrocytomas, primarily glioblastomas, demonstrating partial losses on one or the other chromosome have been described. The significance of these small deletions is uncertain. Only rarely have cases of fibrillary astrocytoma been described as having co-deletion, which may potentially cause diagnostic confusion with oligodendroglioma. The goal of this study is to examine a large number of fibrillary astrocytomas to document how often 1p/19q co-deletions are present by Fluorescent In Situ Hybridization (FISH) testing (the testing method of choice in many institutions) and to evaluate what other markers may be helpful in avoiding misdiagnosis.