P06.01 Pediatric Non-Pontine Diffuse Midline Glioma H3K27M Mutant: a Monoinstitutional Experience

Abstract BACKGROUND The new entity “diffuse-midline-glioma H3K27M-mutant”(DMG) was defined in the 2016 WHO classification. This tumor subtype, regardless of histological features, is associated with an aggressive clinical behavior and poor prognosis, sharing the same outcome as DIPG. MATERIAL AND METHODS We report our experience from 2016 to 2018 in treating patients with radiotherapy and concomitant Nimotuzumab/Vinorelbine, as already published for DIPG’s patients (DOI10.1007/s11060-014-1428-z). All patients’ parents provided written informed consent for treatment. RESULTS Patients were 9: 7/9 females, median age at diagnosis 13 years (range 1–26); 8 with localized disease, 1 with spinal fluid (CSF) dissemination. Five patients had thalamic disease (1 bithalamic), 2 ponto-cerebellar (one with 2 lesions), 1 pineal gland and one with disseminated CSF disease at diagnosis. 3/9 were biopsed in thalamus, the others had partial resection. Central neuropathological review with confirmed histopathological diagnosis of DMG mutated was necessary for inclusion. At the first pathological evaluation tumors had been classified according to the fourth edition of the WHO 2007 as glioblastoma (n=4), anaplastic astrocytoma grade III (n=1), diffuse astrocytoma grade II (n=1), and 1 glial tumor with oligodendroglial features; 2 patients already had DMG at the first pathological evaluation. Immunohistochemical analysis revealed p53 mutation in 3 patients. All 9 patients were evaluated on serial plasma samples with NGS targeted panel (Pan-Cancer) and 6/9 presented p53 mutation in at least one of the samples. 8/9 expressed MAP2K1 gain of function at liquid biopsy. The small number of patients did not allow correlations with the prognosis. Two patients completed the scheduled 2 years of treatment: one had a local relapse at 37 months after diagnosis, the other is alive without evidence of progression at 30 months. With a median follow up of 14.4 months, PFS and OS were 33% and 77% at 1 year; OS was 22% at 2 years, with 2 patients long survivors at 36 and 40 months after diagnosis. CONCLUSION The molecular and phenotypic pattern of DMG allows to include patients in clinical trials/registry shared with patients suffering from DIPG. Our knowledge is still limited about this entity; new insights into molecular profile should help us to study new drugs directed against the effects of the H3K27M mutations and to define new diagnostic/prognostic approach as liquid biopsy able to correlate treatments and prognosis