The disclosure relates to methods for treating tumors. In particular, the disclosure relates to a method of treating a tumor by magnetic resonance image-guided radiation therapy in a subject in need thereof, said method comprising the steps of:



(i) administering an efficient amount of high-Z element containing nanoparticles having, contrast enhancement for magnetic resonance imaging and/or radiosensitizing properties for radiation therapy, in a subject in need thereof, and,
(ii) exposing said subject to magnetic resonance image-guided radiation therapy by means of a Magnetic Resonance Imaging Guided Linear Accelerator (MR-Linac),

wherein said high-Z element containing nanoparticles are nanoparticles containing an element with an atomic Z number higher than 40, preferably higher than 50, and said nanoparticles have a mean hydrodynamic diameter below 20 nm, for example between 1 and 10 nm, preferably between 2 and 8 nm.