NIMG-37. IMAGING THE EARLY METABOLIC RESPONSE DURING TUMOR-TREATING FIELDS THERAPY IN RECURRENT GLIOBLASTOMA

Abstract

Tumor-treating fields (TTF) therapy is an innovative treatment approach for newly diagnosed and recurrent glioblastoma. In this study we evaluated whether positron emission tomography with alpha[11C]-methyl-L-tryptophan (AMT-PET) would detect early metabolic responses during the first 3 months of TTF therapy. Five patients with MRI-detected recurrent glioblastoma being treated with temozolomide underwent AMT-PET scanning prior to delivery of TTF. Four of the 5 patients had >75% compliance with the device and underwent a repeat PET scan 1.5 to 3 months later. Interval changes of tumoral tryptophan uptake and PET-based tumor volumes (based on a previously defined threshold) were measured and compared to changes in MRI enhancement. Three patients showed an early metabolic response reflected by interval decrease of tumoral tryptophan uptake and PET-based tumor volumes during the follow-up period. Patients #1 and #2 who received TTF therapy in addition to maintenance temozolomide showed a ≥25% decrease of PET-based tumor volumes 1.5 and 3 months later, respectively, followed by a delayed MRI response 2 months after the second PET scan. In the other two patients, TTF therapy was combined with bevacizumab. Patient #3 showed markedly decreased tumoral AMT uptake and reduced PET-based tumor volume, along with stable MRI contrast enhancement after 3 months, which continued for 3 additional months before progression. Patient #4, with a much larger baseline tumor volume than the other three patients, showed an increase in the PET-based tumor volume during a 2-month follow-up. MRI showed a steady increase in the volume of contrast enhancement both during and after the AMT-PET follow-up period. These preliminary data demonstrate an early metabolic response to TTF therapy in 3 out of 4 the patients with recurrent glioblastoma who maintained good compliance with the device. This metabolic response seems to precede a subsequent MRI response.