Precise image-guided proton irradiation of mouse brain sub-volumes

Introduction Due to the beneficial inverse physical depth-dose profile, proton radiotherapy (RT) offers the potential to reduce normal tissue toxicity by depositing the maximum dose within the tumor volume while sparing the surrounding tissue. However, range uncertainties and necessary clinical safety margins in combination with varying relative biological effectiveness (RBE) may result in a critical dose in tumor-surrounding normal tissue. Dedicated preclinical studies have been proposed to assess and better understand potential adverse effects of proton RT using image-guided proton irradiation of mouse brain. Here, we present the entire workflow from pre-treatment imaging, over treatment planning, mouse brain irradiation as established at the University Proton Therapy Center Dresden as well as first results from subsequent DNA damage analysis. Materials & Methods An experimental setup was designed and characterized to shape proton beams with 7 mm range in water and 3 mm diameter allowing for irradiation of the mouse brain´s right hemisphere. To simulate the dose distributions in vivo, a Monte Carlo model of the proton beam was designed in the simulation toolkit TOPAS, experimentally commissioned and validated. Cone-beam computed tomography (CT) and orthogonal X-ray imaging were used to delineate the hippocampus as target and position the mice at the proton beam. Mouse brains of C3H and C57BL/6 mice were irradiated with 4 Gy or 8 Gy in a single fraction and excised at different timepoints after irradiation. The number of remaining DNA double-strand break repair proteins was visualized by staining brain sections for cell nuclei and H2AX. Imaged sections were analyzed with an automated and validated processing pipeline to provide quantitative data on spatially resolved radiation damage distributions. Results Animals were planned and treated for proton irradiation of the right hippocampus with a proton beam stopping in the center of the brain. The analysis of irradia