Hafnium oxide is a radio enhancer; its nanoparticles (NBTXR3) were designed for high dose energy deposition within cancer cells when exposed to ionizing radiation. When activated by radiotherapy (RT), NBTXR3 allows for a higher energy deposit than RT alone, yielding an increased tumoral cell death. Nanocrystals of hafnium oxide are under investigation in phase II/III in patients with locally advanced soft tissue sarcoma (STS) of the extremity and trunk wall. In addition, NBTXR3, nanoparticles are being studied in phase I/II in patients with head and neck cancer; liver cancer; prostate cancer and rectal cancer.