Integration of immunotherapy with radiotherapy in head-and-neck cancer: Synergy or sequencing?

Head and neck squamous cell carcinoma (HNSCC) remains a major cause of cancer death, and despite advances in surgery, radiation, and chemotherapy, five-year survival in locally advanced disease stays below 50%, with most relapses occurring within two years. Immune checkpoint inhibitors and radiotherapy carry a strong preclinical rationale for synergy: radiation drives immunogenic cell death, releases tumor antigens and danger signals, upregulates MHC class I, and can prime systemic anti-tumor immunity, occasionally producing an abscopal effect. The same treatment, however, can be immunosuppressive - elective nodal irradiation, lymphocyte depletion from large radiation fields, regulatory T-cell recruitment, and PD-L1 upregulation all blunt the response. This review examines whether that synergy translates into clinical benefit across three settings. In metastatic or recurrent disease, adding stereotactic body radiotherapy to nivolumab (MSKCC trial) did not improve response or survival, with no meaningful abscopal effect. In unresectable locally advanced disease, two phase III trials adding checkpoint inhibitors to concurrent chemoradiotherapy - JAVELIN Head and Neck 100 (avelumab) and KEYNOTE-412 (pembrolizumab) — failed their primary endpoints, though a PD-L1 CPS ≥20 subgroup appeared to benefit; in cisplatinineligible patients, PembroRad showed pembrolizumab and cetuximab had comparable efficacy with less toxicity from the checkpoint inhibitor. In the postoperative setting, both KEYNOTE-689 (perioperative pembrolizumab) and NIVOPOSTOP (adjuvant nivolumab) significantly improved survival outcomes in high-risk resected patients. The benefit of combining immunotherapy with radiotherapy in HNSCC therefore lies with postoperative chemoradiation patients, not those treated with concurrent chemoradiation for unresectable disease — and progress in unresectable disease will likely depend on biomarker-driven selection and smarter sequencing rather than layering more drugs onto existing chemoradiation.