Epid-07. distant progression to the posterior fossa in supratentorial glioblastoma: a report of three cases during tumor treating fields therapy

Abstract BACKGROUND Distant progression of supratentorial glioblastoma (GBM) to the posterior fossa is rare. We report three cases in which such distant progression was observed during Tumor Treating Fields (TTFields) therapy. CASES: All three cases involved solitary supratentorial tumors. Biopsy was performed in case 1 (61-year-old male) and partial resection in cases 2 (69-year-old female) and 3 (73-year-old male). The diagnosis was IDH-wildtype GBM in all three cases. TTFields therapy was performed following chemoradiotherapy. In case 1, distant progression was observed in the dorsal side of the medulla oblongata at 7 postoperative months and in the left cerebellum at 23 postoperative months. The patient died 5 months later. In case 2, distant progression was evident in the pons at 41 postoperative months and the patient died 3 months later. In case 3, distant progression was observed in the cerebellum at 5 postoperative months and the patient died 3 months later. Autopsy was performed in case 1 and revealed multiple separate GBM lesions in the supra- and infratentorial space. Although there was histological heterogeneity within lesions, there was no clear histological difference between supra- and infratentorial lesions. DISCUSSION In the EF-14 trial, although there were no cases of GBM progression to the posterior fossa in the control group who did not receive TTFields therapy (0/122 cases), such progression occurred in 3.6% (11/306) of cases in the TTFields-treated group (p< 0.001). Even though TTFields therapy affords good local control of GBM, distant progression might occur in the posterior fossa, where the electric field effect is poor. Further accumulation and analysis of such cases is warranted.