DDEL-05. IMPRESSIVE RESPONSE TO IDH-INHIBITOR THERAPY IN IDH-MUTANT, H3K27M INTACT BRAINSTEM GLIOMAS

Abstract IDH-mutated brainstem gliomas in adults are a rare entity, with only a handful of cases reported in the literature. Diagnosis and treatment of these gliomas remain challenging. There is significant morbidity associated with a surgical biopsy, thus biopsies are often deferred, and tumors of the brainstem are treated presumptively with radiation and temozolomide. However, in the presence of new targeted therapies, confirming pathology for these patients is becoming increasingly important. We present two cases of IDH-mutant grade 2 astrocytomas of the brainstem that revealed both clinical and radiographic responses to IDH-inhibitor therapy. Interestingly, both patients had the same non-canonical IDH mutation, IDH-R132C, towards which ivosedinib is known to have activity. One patient had tumor progression following radiation therapy and 8 cycles of temozolomide and had subsequently exhibited a dramatic radiographic improvement, including resolution of enhancing disease following initiation of ivosidenib therapy. Clinically, this patient improved from a KPS 50 to KPS 100 after 2.5 years of ivosedinib. The other patient also had an impressive clinical improvement after only 3 months of ivosedinib treatment, with radiographic stability, improvement in balance, and resolution of his previous refractory nausea. These case reports suggest that consideration should be given to brainstem biopsy in adults specifically to evaluate for the presence of a targetable IDH mutation.