Functionalized hafnium oxide nanoparticles (NBTXR3) have been developed as selective radioenhancers, which may represent a breakthrough approach for the local treatment of solid tumors. This is a unique approach where crystalline nanomaterials with high electron density when exposed to radiotherapy, can allow penetrate into the cell and make feasible the absorption/deposition of a high energy dose within the tumor cell. A phase I/II trial was implemented in patients with locally advanced STS.