NIMG-34. DELAYED PSEUDOPROGRESSION IN GLIOBLASTOMA PATIENTS TREATED WITH TTFIELDS: A REPORT OF TWO CASES

INTRODUCTION: Tumor treating fields (TTFields) is an established treatment modality for glioblastoma (GBM) and is administered with the portable Optune system. Although the EF-14 phase 3 trial demonstrated the efficacy of TTFields for newly diagnosed GBM, uncertainty regarding the specific effects of this treatment has prevented its widespread clinical use. Pseudoprogression in response to chemoradiation is a known problem in GBM patients and most commonly occurs within 3 months after radiotherapy. We report 2 cases of TTFields delayed pseudoprogression. CASE REPORT: Two GBM patients being treated with TTFields showed signs of radiographic progression at 5 to 6 months after completing radiotherapy. Patient 1 was a 37-year-old woman with gliosarcoma in the right temporal lobe. Patient 2 was a 70-year-old man with wildtype-IDH GBM in the left temporal lobe. Both patients received TTFields in addition to maintenance temozolomide (TMZ) after radiotherapy (RT). Radiographic progression was noted at 5 and 6 months after RT in Patients 1 and 2, respectively. Second resections were performed, and pathology showed only the treatment effect, which ultimately led to diagnosis of pseudoprogression. DISCUSSION: Both patients had radiographic progression outside the typical pseudoprogression window. Recent studies reported that TTFields increased plasma cell membrane permeability, which could result in additional gadolinium leakage into the extracellular space. CONCLUSIONS: Better characterization of delayed pseudoprogression would improve treatment and could potentially reduce unnecessary surgeries and discontinuation of successful therapies.