The disclosure relates to methods for treating tumors. In particular, the disclosure relates to a method of treating a tumor by ionizing radiations in a subject in need thereof, said method comprising the steps of: (i) injecting a first therapeutically effective amount of high-Z element containing nanoparticles as radiosensitizing agents in said subject in need thereof within a period between 2 and 7 days prior to the first irradiation of the tumor, (ii) injecting a second therapeutically effective amount of the same or different high-Z element containing nanoparticles within a period between 1 hour to 12 hours prior to the first irradiation of the tumor, and, (iii) irradiating the tumor of said subject with a therapeutically efficient dose of radiations; wherein said high-Z element containing nanoparticles are nanoparticles containing an element with an atomic Z number higher than 40 and said nanoparticles have a mean hydrodynamic diameter below 10 nm.