548P Diffuse midline gliomas H3K27-altered in adults, the experience of an oncology center

Midline gliomas H3K27-altered (GLM-H3K27) are a rare and very aggressive oncologic entity. Although the clinical, imaging, and prognostic characteristics are better characterized in childhood, studies of this pathology in the adult population are scarce. Demographic, clinical, treatment, and survival characterization of adult patients with GLM-H3K27, followed at an Oncology Hospital between 2012-2022, through a retrospective review of clinical processes. The data cut-off was 31 December 2022. Cohort consisting of 17 patients, 53% male, mean age at diagnosis 38 years (18-63 years). The mean time from symptomatology to diagnosis was 3.9 months. The most common initial symptoms are related to cranial nerve dysfunction (6), headache (6) or motor defect (3). The most frequent anatomical locations were thalamus and trunk, 41% and 35%, respectively. Surgical samples were mostly obtained by biopsy (58%), the remainder by partial excision. The initial histological diagnosis varied, including grade 2 to 4 gliomas. In 88% of the cases (N=15) it was possible to identify the presence of the H3F3A K27M mutation by immunohistochemistry and/or mutational analysis of the gene, with 2 of the cases the diagnosis was made only by the loss of expression of H3K27me3. Most patients (82%) were treated with radiotherapy and chemotherapy; in 11.7%, isolated radiotherapy was performed, and only palliative care was given in one case. So far, 14 patients (70.1%) have died; 3 are alive and 1 patient was lost to follow-up. The median disease-free survival was 11 months (95%CI 8.6-13.4 months) and the median overall survival was 20 months (95%CI 12-28 months). Our study highlights the clinical, anatomical and histological heterogeneity of this new and rare entity, particularly in adulthood. We emphasize the importance of the integrated diagnosis of this population, with the expectation that in the future an effective therapeutic approach can be implemented.