Radiotherapy-induced neurotoxicity may be life threatening and its characterization is crucial for cancer treatment management, especially in early treatment phases. Diffusion MRI allows to assess non-invasively microstructural changes occurring with radiation treatment, which are of interest in relation to local absorbed dose and cognitive changes. Proton therapy (PT) offers the promise of more focal tumor damage relative to conventional radiotherapy. However, few studies have evaluated how it affects brain microstructure. As part of an ongoing longitudinal study, we present the first available data of 5 acquisitions along PT course showing a possible treatment-related “necrosis-like” effect during treatment.