Delayed elective nodal irradiation in head-and-neck Cancer radiotherapy reduces acute lymphocyte damage, lymphopenia, and modulates immune signatures

Background Elective nodal irradiation (ENI) for locally advanced head-and-neck squamous cell carcinoma (LA-HNSCC) contributes to hematotoxicity and may impair radiotherapy-immunotherapy synergy. We investigated whether delayed ENI, using an upfront boost to gross disease and involved nodes followed by standard ENI, reduces radiation-induced lymphocyte DNA damage and lymphopenia (RIL) while modulating peripheral immune signaling compared with standard ENI. Materials and Methods Twenty-eight LA-HNSCC patients assigned for adjuvant ( n = 13) or definitive ( n = 15) (chemo)radiotherapy were randomized to standard or delayed ENI (upfront boost: adjuvant: 2 × 2 Gy, n = 6/15; definitive: 5 × 2 Gy, n = 7/13). Blood was collected pre-treatment, 15 min and 24 h after the first fraction, and (definitive only) before fraction six and at treatment end. Primary endpoints: DNA damage in lymphocytes via γH2AX and 53BP1 foci ( n = 25, pre-therapy, post-first and pre-sixth fraction) and dicentric chromosomes ( n = 9, pre-therapy and pre-sixth fraction). Secondary endpoints: exploratory leukocyte transcriptomics ( n = 4; pre-therapy, pre-sixth fraction and end of therapy), lymphocyte counts, and RIL ( n = 28; pre-therapy and weekly during therapy). Results Delayed ENI reduced initial planning target volume and whole-body dose by 9.9- and 4.4-fold (both p < 0.001), resulting in 3–4-fold lower DNA damage marker frequencies ( p < 0.001) and fewer radiation-damaged lymphocytes (~11% vs. ~23%, p < 0.001). Baseline lymphocyte counts were preserved during the first treatment week, delaying RIL onset, whereas standard ENI induced an immediate decline in lymphocyte counts and RIL. Week 1 lymphocyte counts inversely correlated with planning target volume, whole-body dose, and DNA damage (all p < 0.01). Exploratory transcriptomic profiling revealed enhanced interferon and metabolic signaling with delayed ENI, whereas standard ENI preferentially activated inflammatory and damage-associated pathways. Conclusion These findings suggest that delaying elective nodal irradiation may help preserve circulating immune cells during radiotherapy and provide a mechanistic rationale for future prospective studies evaluating whether this strategy can enhance radiotherapy–immunotherapy synergy in LA-HNSCC.