1101 - Concomitant Sub-ablative Stereotactic Body Radiation Therapy and Immune Checkpoint Inhibition in Recurrent and Metastatic Head and Neck Cancer

Purpose/Objective(s):

The optimal fractionation strategy for stereotactic body radiotherapy (SBRT) delivered concomitantly with immune checkpoint inhibitors (ICI) remains an area of active investigation and has largely been examined from the perspective of enhancing systemic immune responses and inducing abscopal effects. From a radiation oncology standpoint, we aimed to evaluate whether concurrent sub-ablative SBRT and ICI could enhance local control within the irradiated field. We hypothesized that this combination could achieve high local control in recurrent or metastatic head and neck cancer (R/M HNC).

Materials/Methods:

This study included patients treated with sub-ablative SBRT (3x8 Gy) delivered concomitantly with ICI to all recurrent and metastatic disease foci between January 2019 and November 2025. At our institution, patients with R/M HNC are managed according to a rigorous predefined clinical protocol and eligible patients were included in this analysis. Survival outcomes were estimated using the Kaplan- Meier method, and comparisons were performed with the log-rank test.

Results:

A total of 114 lesions in 34 patients were treated. Median age was 59 years (range, 19–81). At last follow-up, 5 patients (14.7%) were alive with no evidence of disease, 5 (14.7%) were alive with disease, 19 (55.9%) had died with disease, and 5 (14.7%) had died without evidence of disease. Among treated lesions, 2 (1.8%) received first-course radiotherapy to the primary tumor in the setting of metastatic disease, 46 (40.4%) underwent second-course re-irradiation, 11 (9.6%) underwent third-course re-irradiation, and 55 (48.2%) was distant metastatic foci. The median number of treated lesions per patient was 2 (range, 1–11). Median follow-up was 20.8 months (range, 2.6–83.2) from initiation of ICI and 17.1 months (range, 1.3–81.8) from the start of SBRT. The 1-, 2-, and 5-year overall survival rates were 66.7%, 42.5%, and 23.9%, respectively, and corresponding local control rates were 86%, 84%, and 74%. Grade =3 acute toxicity occurred in 1 patient, and grade =3 late toxicity occurred in 1 patient.

Conclusion:

Concomitant sub-ablative SBRT and ICI demonstrated encouraging local control in R/M HNC. Our preliminary findings suggest that dose de-escalation may be feasible when SBRT is delivered concurrently with ICI. This strategy may expand local treatment options in settings where cumulative dose constraints preclude high-dose re-irradiation or when ablative treatment of all lesions is not achievable.