Inhibition of glycolysis by decreasing blood glucoses levels and increasing ketone bodies (KB) could force tumor cells to shift their metabolism towards potentially impaired mitochondria. The aim of this study was to explore the combination of six days of calorie restricted ketogenic diet (crKD) and three days of fasting (as a possible radiosensitizer) with a re-irradiation therapy in patients with recurrent glioblastoma. Intracerebral concentrations of KB as well as pHi and ATP were non-invasively monitored using MR-Spectroscopy. We were able to show evidence of intratumoral acetone using 1H-MRS in some patients with malignant glioma at day 6 of crKD/fasting. Changes in pHi and ATP during crKD/fasting will remain subject to preclinical studies.