Mice were injected with the fluorescent dye Hoechst 33342 (Tocris Bioscience), 20 mg/kg, 30 minutes after irradiation. Mice were sacrificed 2 minutes after injection and tumors were frozen on dry ice in O.C.T. compound (Scigen), cryosectioned and fixed with 4% paraformaldehyde. Tumor sections were imaged using Pannoramic 250 Flash digital slide scanner (3D Histech, Hungary) and fluorescence was quantified using ImageJ software .