Use of a nanoparticle or nanoparticle aggregate to prepare a pharmaceutical composition intended to alter or destroy target cells in an animal when said cells are exposed to ionizing radiations, wherein the nanoparticle or nanoparticle aggregate consists of a metal oxide (MxOy) selected from the group consisting of CeO2, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3, HfO2, TaO2, Ta2O5, WO2, WO3, ReO2, OsO2, IrO2, PtO, PtO2, HgO, Hg2O, Tl2O3, PbO, Pb2O3, Pb3O4, PbO2, PoO2, Bi2O3, NbO, RuO2, Rh2O3, RhO2, PdO, Ag2O, AgO, CdO, In2O3, wherein the density of said nanoparticle or nanoparticle aggregate is of at least 7 g/cm3 and wherein the nanoparticle or nanoparticle aggregate is covered with a biocompatible coating allowing the nanoparticle stability between pH 6.5 and 7.5 in a physiological fluid. The target cells may be selected form the group consisting of benign cells, pre-malignant cells and malignant cells, wherein said malignant cells are cells from a tumor selected in the group consisting of a haematological tumor and a solid tumor. Preferably the biocompatible coating oxide is a non-biodegradable coating selected from the group consisting of silica, alumina, sugar, phosphate, silane, switterionic compound, lipid, saturated carbon polymer and an inorganic polymer or a biodegradable coating selected from the group consisting of biological polymer, phospholipid, saccharide, oligosaccharide and polysaccharide.
