PATH-21. INVESTIGATING CHANGES IN IDH MUTATION STATUS AT GLIOMA RECURRENCE

Abstract

Abstract PURPOSE Isocitrate dehydrogenase (IDH) mutation status is a key diagnostic and prognostic feature of gliomas. It is thought to occur early in glioma tumorigenesis and remains stable over time. We investigated our tumour bank to identify patients where IDH mutation status changed after glioma recurrence. METHODS We identified four patients with immunohistochemistry-recorded IDH mutation statuses that conflicted between multiple tumour samples taken over time. Tissue samples from the tumour bank were analyzed using methylation profiling and Sanger sequencing of IDH1 and IDH2 to verify IDH status. RESULTS In all four instances, we found that IDH mutation status in fact remained stable, and there were reasonable explanations as to why mutation status was misinterpreted. One tumour had a non-canonical IDH2 R172K mutation that is not routinely tested for. Two patients had perilesional samples, revealed by methylation profiling to contain a mixture of tumour and normal brain. One patient had a IDH wildtype sample with a methylation profile consistent with reactive brain tissue rather than tumour. CONCLUSIONS All four cases of IDH mutation change at glioma recurrence were due to either misinterpreting perilesional or reactive tissue as tumour or overlooking a non-canonical mutation. This finding supports the concept that IDH mutation is a fixed characteristic of gliomas and highlights the importance of accurate surgical biopsy and molecular testing at initial diagnosis.