Large Tumour Volume Reduction of Idh Mutated Anaplastic Glioma Involving the Insular Region Following Radiation Therapy: an Alternative to Maximal Surgical Resection

Abstract Background: In IDH-mutated anaplastic glioma (IDHmutAG) of the insular cortex there remains uncertainty of benefit with near-total resection compared to limited surgery with radiation therapy (IMRT). This study aimed to assess tumour volume reduction in patients following IMRT and impact of residual post-surgical volume. Methods and Materials: Patients with IDHmutAG involving insular cortex managed with IMRT from 2008-2019 had baseline patient, tumour and treatment factors recorded. Volumetric assessment of residual disease on MRI was performed at baseline, month+3 and month+12 post-IMRT. Potential prognostic factors were analysed for tumour reduction and relapse-free survival, and assessed by log-rank and Cox regression analyses. Results: 32 patients with IDHmutAG of the insular cortex were managed with median follow-up post-IMRT of 67.2 months. Pathology was anaplastic astrocytoma (AAmut) in 20, and anaplastic oligodendroglioma (AOD) in 12 patients. Median pre-IMRT volume on T1 and T2Flair was 24.3cm 3 and 52.2cm 3 . Twenty-seven patients were alive with 5-year relapse-free survival of 80%. There was a median 67% and 64% reduction from baseline occurring at 3 months post-IMRT for T1 and T2Flair respectively; and subsequent median 78% and 73% at 12 months. At 12 months AOD patients had median 83% T1 volume reduction compared to 63% in AAmut (p<0.01). There was no difference on T2Flair volume (p=0.64). No other pathological factors influenced volume reduction at 12 months. No factors were associated with relapse-free survival including baseline T1 (p=0.52) and T2Flair (p=0.93) volume. Conclusion: IMRT provides large tumour volume reduction in IDHmutAG of the insular cortex with no significant negative impact of residual disease volume on relapse-free survival.