The invention relates to the use of nanoparticles having dimensions between 1 and 50 nm, at least one part of which is constituted of at least one oxide and/or one oxyhydroxide of at least lanthanide, said nanoparticles being: - either constituted of at least one oxide and/or one oxyhydroxide of at least one lanthanide, - or in the form of nanoparticles comprising an oxyhydroxide core of at least one lanthanide, and a coating constituted of a polysiloxane, optionally with organic molecules grafted to the surface or included within it, as a radiosensitizing agent in the manufacture of an injectable composition intended for improving the effectiveness of the treatment of a tumour by X-ray or gamma irradiation. It also relates to nanoparticles that are particularly suitable for the above use, constituted of a core constituted of at least one oxide and/or one oxyhydroxide of at least one lanthanide and of a polysiloxane coating comprising 1 to 5 silicons per lanthanide and of which at least 10% of the silicon atoms are linked to hydrophilic organic molecules having molar masses of less than 450 g/mol.Linvention concerne lutilisation de nanoparticules de dimensions comprises entre 1 et 50 nm, dont une partie au moins est constituée dau moins un oxyde et/ou un oxohydroxyde dau moins un lanthanide, lesdites nanoparticules étant : soit constituées dau moins un oxyde et/ou un oxohydroxyde dau moins un lanthanide, soit sous forme de nanoparticules comprenant un cœur oxohydroxyde dau moins un lanthanide, et un enrobage constitué dun polysiloxane, avec éventuellement des molécules organiques greffées en surface ou comprises en son sein, en tant quagent radio-sensibilisant dans la fabrication dune composition injectable destinée à améliorer lefficacité du traitement dune tumeur par irradiations X ou gamma. Elle concerne également des nanoparticules particulièrement adaptées pour lutilisation ci-dessus, constituées dun cœur constitué dau moins un oxyde et/ou un oxohydroxyde dau m
