Background: The standard of care non-surgical approach for locally advanced head and neck squamous cell carcinoma (LA HNSCC) patients (pts) is concurrent chemoradiation with high dose cisplatin or cetuximab in case of contra-indication. Older age is a contra-indication to cisplatin, and cetuximab might not improve survival in older pts. It is therefore urgently needed to develop new treatment options for elderly pts with LA HNSCC. NBTXR3 are hafnium oxide nanoparticles that can enhance the efficacy of radiotherapy (RT) by increasing locally the deposited dose. In this phase I clinical trial we aimed to evaluate the feasibility and safety of NBTXR3 administered as intratumoral (IT) injection prior to RT in LA HNSCC elderly pts. Methods: Pts with stage III-IV LA HNSCC of the oropharynx or oral cavity ineligible for platinum-based chemoradiation received a single IT injection of NBTXR3 into a selected primary tumor and intensity modulated RT (IMRT; 70 Gy/35 fractions/7 weeks) [NCT01946867]. A 3+3 dose escalation design, tested NBTXR3 dose levels equivalent to 5, 10, 15, and 22% of baseline tumor volume, followed by a dose expansion at the Recommended Phase II Dose (RP2D). Primary endpoints included RP2D determination, and early dose limiting toxicities (DLT). NBTXR3 intratumoral bioavailability and anti-tumor activity (RECIST 1.1) were also evaluated. Results: Enrollment was completed at all dose escalation levels: 5% (3 pts), 10% (3 pts), 15% (5 pts), and 22% (8 pts). No early DLT or SAE related to NBTXR3 or injection were observed. The median follow-up from NBTXR3 administration is 7.6 months. One AE (Grade 1) related to NBTXR3 and four AEs (Grade 1-2) related to the injection were observed. RT-related toxicity was as expected with IMRT. CT-scan assessment showed a good dispersion of NBTXR3 throughout the injected tumor and not in surrounding healthy tissues. The RP2D was determined to be 22%. Preliminary efficacy was evaluated in pts who received the intended dose of NBTXR3 and RT. A complete response of the injected lesion was observed in 9/13 (69%) evaluable pts at doses ≥10% (2 unconfirmed) and an overall complete response in 5/13 (38%) evaluable pts at doses ≥10%. Preliminary safety and efficacy data of the dose expansion cohort at the RP2D will also be presented. Conclusions: NBTXR3 activated by RT was well tolerated at all tested doses and demonstrated promising preliminary anti-tumor activity. Recruitment is ongoing in the dose expansion cohort. These results demonstrate that further testing of NBTXR3 in this population is warranted. Clinical trial information: NCT01946867.