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. This study is a retrospective evaluation of 359 fibrillary astrocytomas (55 grade II, 62 grade III and 242 grade IV) encountered between June 2016 and June 2019, we identified 11 tumors (3.1%) that had 1p/19q co-deletion by FISH testing. The clinical and pathologic features of these cases were reviewed. The 11 cases with co-deletion included 5 females who ranged in age from 37 to 86 years (median 63 years). Tumors arose in the temporal lobe in 5 patients, frontal lobe in 2, parietal lobe in 2, occipital lobe in 1, and cerebellum in 1. Final diagnoses included glioblastoma in 8 patients, anaplastic astrocytoma in 2, and diffuse astrocytoma in 1. Only 1 case (anaplastic astrocytoma) demonstrated evidence of IDH-1 immunoreactivity; none of the other 10 tumors showed evidence of an IDH1/2 mutation by PCR testing. Four tumors demonstrated p53 immunostaining of 30% or more. ATRX mutation as evidenced by loss of staining was observed in only 2 cases. Evidence of EGFR amplification by FISH testing was noted in 5 cases. Of particular note in the one case that demonstrated both 1p/19q co-deletion and an IDH-1 mutation, LOH testing was done and showed only partial losses on both chromosomes. Additionally, this tumor also demonstrated evidence of ATRX and p53 mutations by immunohistochemistry. In conclusion, co-deletions were noted in a minority of astrocytomas (3.1% of cases in the current study). Only 1 of 11 of these cases also demonstrated evidence of an IDH mutation, potentially raising differential diagnostic confusion with oligodendroglioma. Use of LOH 1p/19q testing, if available, or other markers such as ATRX, p53 and EGFR may be helpful in avoiding misclassification of such tumors as oligodendroglioma.