RTHP-30. ENHANCED THERAPEUTIC BENEFITS OF TUMOR TREATING FIELDS (TTFIELDs) ON SUPERFICIALLY LOCATED GLIOBLASTOMA MULTIFORME (GBM)

Abstract

BACKGROUND: TTFields are a novel therapeutic modality that induces mitotic arrest in dividing cells via the use of low intensity, intermediate frequency, oscillating electric fields. It recently received FDA approval for the treatment of newly diagnosed GBMs (Stupp et al, JAMA 2015). In vitro studies have demonstrated that TTFields may have an enhanced field strength surrounding the ventricle. There have been no in vivo studies evaluating the effect of tumor depth on the efficacy of TTFields. METHODS: Our institution accrued 17 patients into the Stupp et al trial, 10 of whom were randomized to receive TTFields. The initial planning MRI, performed after surgery and radiotherapy, was reviewed by two neuro-radiologists blinded to the treatment patients received. Tumors were characterized as either 'superficial' or 'deep' based upon whether the bulk of enhancing tumor volume was located greater or less than 50% along a perpendicular axis drawn from the nearest pial (superficial) surface to the ependymal (deep) surface of the proximate ventricle. Tumors crossing margins were classified as 'intermediate'. The patients were then analyzed for progression-free survival (PFS) and overall survival (OS) based upon their tumor depth and the treatment modality. RESULTS: Nine patients had 'superficial' tumors and the rest had 'deep or intermediate' tumors. Median OS and PFS of superficial tumors (30 and 12 months) were superior to deep or intermediate tumors (12 and 6 months) regardless of treatment modality (OS:p=0.05;PFS:p=0.02). Superficial tumors treated with TTF and Temozolomide (n=6) compared to those treated with Temozolomide alone (n=3) had improved PFS (26 vs 6 months, p=0.012) and OS (47 vs. 15 months, p=0.003). CONCLUSION: Tumor depth may be an independent prognostic factor for patients receiving TTField treatment for GBMs, with superficially located GBMs having the greatest improvement in PFS and OS. We plan to confirm these preliminary findings in a larger data set.